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Is a one ventricle heart feasible?

Is a one ventricle heart feasible?


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So this is derived from a lesson at Khan Academy.

The mind activity assumes that the one ventricle heart pumps blood to the lungs for oxygenation then rest of the body. However, the problem is that there is too much pressure at the lungs leading to bleeding.

This problem, however, can be solved if the lungs come before the one ventricle heart. This way, most of the pressure is used going through the body and pressure would not become a problem.

Thus, is a one ventricle heart that connects to body tissues first then lung feasible? If so, why have organisms not developed this way?


Alternative Heart Morphologies

Amphibians and some reptiles have a three-chambered heart, with 2 atria and a single ventricle. There are still separate circulatory pathways for the lungs and the rest of the body, but the oxygenated and oxygen-depleted blood mix in the ventricle, and are pushed at the same time to the lungs and body.

The disadvantage of this system is that it is less efficient at providing oxygen to the body than a four-chambered system, because the blood pumped to the body is a mixture of blood that has recently been to the lungs and blood that has not.

You seem to be suggesting another system, where there is just one serial pathway from heart to lungs to body, or from lungs to heart to body. This is more like the two-chambered heart of a fish: blood is pumped from the heart through the gills, and then to the body.

Lungs and body in series, and fluid dynamics

So why wouldn't this work for a mammal? It could, but gills and lungs are very different. The vasculature of the lungs requires a lot of pressure to get through, which is generated by the right ventricle.

Pressure is required to push fluids through a constriction (note that even a straight pipe is a constriction). After a fluid passes a constriction, there is always a pressure drop. That pressure drop is a function of the rate of flow and size of the constriction. If you give less pressure at the start, there is less pressure to "drop", and so you get a lower flow rate (note that this is nearly a perfect analogy with electricity, where flow is current, pressure is voltage, and the size of the tube is the conductance).

If you want to pass through a second constriction, you have to use pressure that is "left over" after the first drop: you can't use the pressure you started with because that was lost getting you through the first constriction. In reality, in a closed system, the total flow rate will be determined by the summed series resistance, so adding a second constriction slows the flow rate for the whole system, and you get a partial pressure drop after each.

If you put the lungs and body in series with each other, no matter which comes first, you need to have enough pressure left over when you pass one to get you through the next one, otherwise the flow will slow down. Blood in a human aorta has a mean pressure around 100mmHg, whereas back in the vena cava it is close to 10mmHg. That suggests you need about 90mmHg to get enough flow through the body's circulation. The lungs use a little less, but you still have a pressure drop of around 50-60mmHg. Therefore, you would have to increase the blood pressure by 50%-75% to get circulation through both lungs and body.

Higher pressures lead to more turbulent flow and take more effort to achieve. The four-chambered heart, with 2 ventricles, is simply more efficient.


Heart - Right Atrium and Ventricle

With a single cut through the right side of the heart, the right atrium and right ventricle are revealed. Preserved hearts can be somewhat rigid, you may need to hold the sides of the heart open to see the internal structures.

The right side of the heart is less muscular than the left side this is because they only need to send blood to the lungs and back, which is a much short path than delivering blood to the entire body, which is the job of the left side of the heart.

The tricuspid, or right atrioventricular valve, lies between these two chambers. The chordae tendineae anchor the tricuspid to the papillary muscle. These muscles appear smooth and slightly raised on the interior of the chamber.

The pulmonary artery connects to the right ventricle of the heart. This can be seen by placing a probe in the artery.


Phases of the Cardiac Cycle

At the beginning of the cardiac cycle, both the atria and ventricles are relaxed (diastole). Blood is flowing into the right atrium from the superior and inferior venae cavae and the coronary sinus. Blood flows into the left atrium from the four pulmonary veins. The two atrioventricular valves, the tricuspid and mitral valves, are both open, so blood flows unimpeded from the atria and into the ventricles. Approximately 70–80 percent of ventricular filling occurs by this method. The two semilunar valves, the pulmonary and aortic valves, are closed, preventing backflow of blood into the right and left ventricles from the pulmonary trunk on the right and the aorta on the left.

Atrial Systole and Diastole

Contraction of the atria follows depolarization, represented by the P wave of the ECG. As the atrial muscles contract from the superior portion of the atria toward the atrioventricular septum, pressure rises within the atria and blood is pumped into the ventricles through the open atrioventricular (tricuspid, and mitral or bicuspid) valves. Atrial systole lasts approximately 100 ms and ends prior to ventricular systole, as the atrial muscle returns to diastole.

Ventricular Systole

Ventricular systole (see image below) follows the depolarization of the ventricles and is represented by the QRS complex in the ECG. Initially, as the muscles in the ventricle contract, the pressure of the blood within the chamber rises, but it is not yet high enough to open the semilunar (pulmonary and aortic) valves and be ejected from the heart. However, blood pressure quickly rises above that of the atria that are now relaxed and in diastole. This increase in pressure causes blood to flow back toward the atria, closing the tricuspid and mitral valves. Once the contraction of the ventricular muscle has raised the pressure to the point that it is greater than the pressures in the pulmonary trunk and the aorta blood is pumped from the heart, pushing open the pulmonary and aortic semilunar valves.

Ventricular Diastole

Ventricular relaxation, or diastole, follows repolarization of the ventricles and is represented by the T wave of the ECG. As the ventricular muscle relaxes, pressure on the remaining blood within the ventricle begins to fall. When pressure within the ventricles drops below pressure in both the pulmonary trunk and aorta, blood flows back toward the heart. The semilunar valves close to prevent backflow into the heart. As the ventricular muscle continues to relaxe, pressure on the blood within the ventricles drops even further. Eventually, it drops below the pressure in the atria. When this occurs, blood flows from the atria into the ventricles, pushing open the tricuspid and mitral valves. As pressure drops within the ventricles, blood flows from the major veins into the relaxed atria and from there into the ventricles. Both chambers are in diastole, the atrioventricular valves are open, and the semilunar valves remain closed (see image below). The cardiac cycle is complete. Figure 2 illustrates the relationship between the cardiac cycle and the ECG.

Figure 2. Initially, both the atria and ventricles are relaxed (diastole). The P wave represents depolarization of the atria and is followed by atrial contraction (systole). Atrial systole extends until the QRS complex, at which point, the atria relax. The QRS complex represents depolarization of the ventricles and is followed by ventricular contraction. The T wave represents the repolarization of the ventricles and marks the beginning of ventricular relaxation.


Is a one ventricle heart feasible? - Biology

The basic vertebrate cardiovascular system includes a heart that contracts to propel blood out to the body through arteries, and a series of blood vessels. The blood enters the heart through the upper chamber(s), the atrium (or atria). Passing through a valve, blood enters the lower chamber(s), the ventricle(s). Contraction of the ventricle forces blood from the heart through the arteries.

Fish have a two-chambered heart in which a single-loop circulatory system takes blood from the heart to the gills and then to the body. Amphibians have a three-chambered heart with two atria and one ventricle. A loop from the heart goes to the pulmonary capillary beds, where gas exchange occurs. Blood then is returned to the heart. Blood exiting the ventricle is diverted, some to the pulmonary circuit, some to systemic circuit. The disadvantage of the three-chambered heart is the mixing of oxygenated and deoxygenated blood. Some reptiles have partial separation of the ventricle. Crocodilian reptiles, birds and mammals (including humans), have a four-chambered heart, with complete separation of both systemic and pulmonary circuits.

Below, we describe the main characteristics of different groups of vertebrates:

1. Mammalian (and human) heart

1. Mammalian (and human) heart:

Mammals have a double circulation the pulmonary circulation carries blood to and from the lungs and the systemic circulation carries blood to and from the rest of the body. The heart thus acts as a double pump in which the two sides are completely separated by a wall (septum). The right side pumps deoxygenated blood to the lungs for gas exchange with the alveolar air, while the left side pumps oxygenated blood to the body for gas exchange with the tissues. The heart has four chambers, the right and left atria and the right and left ventricles. The atria pass blood to the ventricles. The right atrium receives deoxygenated blood from the body via the anterior and posterior venae cavae. This blood is pumped by the right ventricle to the lungs via the pulmonary arch and pulmonary arteries. The left atrium receives oxygenated blood from the lungs via the pulmonary veins. This blood is pumped by the left ventricle to the body via the aortic arch and aorta. Human cardiac anatomy is more extensively discussed here .

Birds, like mammals, have a four-chambered heart. Two atria and two ventricles allow for complete separation of oxygenated and de-oxygenated blood. The right ventricle pumps blood to the lungs, while the left ventricle pumps blood to the rest of the body. Because the left ventricle must generate greater pressure to pump blood throughout the body (in contrast to the right ventricle that pumps blood to the lungs), the walls of the left ventricle are much thicker and more muscular. Birds tend to have larger hearts than mammals (relative to body size and mass). The relatively large hearts of birds is probably required to meet the high metabolic demands of flight. Among birds, smaller birds have relatively larger hearts than larger birds. Hummingbirds have the largest hearts (relative to body mass) of all birds, probably because hovering takes so much energy. Cardiac output for birds is typically greater than that for mammals of the same body mass. Cardiac output is influenced by both heart rate and stroke volume (blood pumped with each beat). 'Active' birds increase cardiac output primarily by increasing heart rate.

Crocodilians are the only reptiles which possess four chambered hearts comparable to mammals. Even so, crocodilian cardiac anatomy is quite different from what is seen in birds and mammals. Crocodilians possess two aortas the right arising from the left ventricle and the left from the right ventricle. Both aortas route blood to the systemic circulation. The right and left aortas are connected near the base of the heart by the foramen of Panizza. The foramen allows blood from the right ventricle to bypass the pulmonary circulation when necessary. A valve exists at the opening of the pulmonary artery which has interdigitating muscular projections, hence the commonly used name "cog-wheel valve". When the animal holds its breath, the cog-wheel valve closes and blood that would have normally entered the pulmonary circulation is diverted into the left aorta. It should be noted that most veterinary texts incorrectly report that the location of the foramen of Panizza is in the ventricular septum or atrial septum.

The cardiac structure of non-crocodilian reptiles is significantly different from that of mammals. Most reptiles have three chambered hearts with two atria and one common ventricle. The right atrium receives blood returning from the systemic circulation via the sinus venosus, which is formed by the confluence of the right and left precaval veins and the single postcaval vein. The walls of the sinus venosus contain cardiac muscle and the pacemaker of the heart. The left atrium receives oxygenated blood from the lungs via the pulmonary veins. The atrioventricular valves are bicuspid, membranous structures. Under normal conditions the three chambered heart functions much like a four chambered structure, therefore relatively little mixing of oxygenated and de-oxygenated blood occurs. Three cavities exist within the ventricle and can be functionally separate the cavum venosum, cavum arteriosum and the cavum pulmonale. These cavities are partially separated by two muscular ridges found within the ventricle. These ridges vary in prominence in different species, but are generally well-developed in chelonians (turtles). The muscular ridge divides the cavum pulmonale and the cavum venosum. The vertical ridge divides the cavum venosum and cavum arteriosum. The cavum pulmonale receives blood from the right atrium through the cavum venosum and directs flow into the pulmonary circulation. The cavum arteriosum receives blood from the pulmonary veins and then directs oxygenated blood to the cavum venosum. The paired aortic arches arise from the cavum venosum and lead to the systemic circulation. The right and left aortic arches come together to form a single aorta at variable distances caudal to the heart. Differential blood flow and separation of oxygenated and de-oxygenated blood is maintained by pressure differences of the outflow tracts and the muscular ridges that partially divide the ventricle. In most non-crocodilian reptiles the ventricle function as a single pump, meaning that the same pressures are generated by both the cavum pulmonale and cavum venosum. Due the unique anatomy, both right to left and left to right shunts are possible in the reptilian heart for example during apnea.

The purpose of right to left shunting in reptiles is still under debate and it is proposed to be useful for the conservation of cardiac energy, facilitation of warming, reduction of plasma filtration into the lungs, reduction of carbon dioxide flux into the lungs and the metering of oxygen stores from the lungs during apnea. Theories to explain the purpose of left to right shunting include facilitation of carbon dioxide elimination from the lungs, minimization of ventilation/perfusion mismatches and improvement of systemic oxygen transport. In times of oxygen deprivation (diving in some reptiles, consumption of large prey in snakes), reptiles can shunt blood away from the lungs.

Amphibians have a three-chambered heart consisting on two atria and one ventricle. The two atria receive blood from the two different circuits (the lungs and the systemic circulation). Mixing of the blood in the heart's ventricle, reduces the efficiency of oxygenation. This is partially mitigated by a ridge within the ventricle that diverts oxygen-rich blood through the systemic circulatory system and deoxygenated blood to the pulmocutaneous circuit where gas exchange occurs.

Blood leaves the heart from the ventricle through a single truncus arteriosus which is short and soon branches into two aortic arches which loop left and right and dorsal to the heart to rejoin as a single aorta in the mid dorsal region of the body cavity. Each aortic arch has a branch leading to the lungs and skin where oxygenation occurs. Carotid arteries also branch off the aortic arches and supply the head region. Veins bring blood to the left and right atria. Both atria then empty into the single ventricle.

The pacemaker is the sinus venosus, an enlarged region between the vena cava and the right atrium. This the cells of the pacemaker are termed “leaky”, meaning that calcium ions leak into the cells. Leaking of positive ions causes a slow depolarization to threshold, thus initiating an action potential that quickly spreads throughout the muscle. The atria are very conductive, and the action potential spreads readily across these two chambers. The major route for the transmission of action potentials from the SA node to the ventricle(s) is by way of a set of modified conductive muscle cells that compose the bundle of His embedded in the septum separating the two atria.

The circulatory systems of all vertebrates, are closed. Fish have the simplest circulatory system, consisting of only one circuit, with the blood being pumped through the capillaries of the gills and on to the capillaries of the body tissues. This is known as single cycle circulation.

Fish have a two-chambered heart consisting of one atrium to receive blood and one ventricle to eject it. Entry and exit compartments are often referred as accessory chambers. The four compartments are arranged sequentially. The sinus venosus (first accessory chamber), collects deoxygenated blood through the incoming hepatic and cardinal veins. The atrium, a thicker-walled, muscular chamber sends blood to the ventricle. The ventricle, a thick-walled, muscular chamber that pumps the blood to the fourth part, the outflow tract (second accessory chamber). The outflow tract connects to the ventral aorta, and consists of the tubular conus arteriosus, bulbus arteriosus, or both. The conus arteriosus (in more primitive species), contracts to assist blood flow to the aorta. The bulbus anteriosus does not contract. The ventral aorta delivers blood to the gills where it is oxygenated and flows, through the dorsal aorta, into the rest of the body.

Ostial valves, consisting of flap-like connective tissues, prevent blood from flowing backward through the compartments. The ostial valve between the sinus venosus and atrium is called the sino-atrial valve, which closes during ventricular contraction. Between the atrium and ventricle is an ostial valve called the atrio-ventricular valve, and between the bulbus arteriosus and ventricle is an ostial valve called the bulbo-ventricular valve. The conus arteriosus has a variable number of semilunar valves. In the adult fish, the four compartments are not arranged in a straight row but, instead form an S-shape with the latter two compartments lying above the former two. This relatively simpler pattern is found in cartilaginous fish and in the ray-finned fish. In teleosts, the conus arteriosus is very small and can more accurately be described as part of the aorta rather than of the heart proper.

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The Respiratory System

Take a breath in and hold it. Wait several seconds and then let it out. Humans, when they are not exerting themselves, breathe approximately 15 times per minute on average. This equates to about 900 breaths an hour or 21,600 breaths per day. With every inhalation, air fills the lungs, and with every exhalation, it rushes back out. That air is doing more than just inflating and deflating the lungs in the chest cavity. The air contains oxygen that crosses the lung tissue, enters the bloodstream, and travels to organs and tissues. There, oxygen is exchanged for carbon dioxide, which is a cellular waste material. Carbon dioxide exits the cells, enters the bloodstream, travels back to the lungs, and is expired out of the body during exhalation.

Breathing is both a voluntary and an involuntary event. How often a breath is taken and how much air is inhaled or exhaled is regulated by the respiratory center in the brain in response to signals it receives about the carbon dioxide content of the blood. However, it is possible to override this automatic regulation for activities such as speaking, singing and swimming under water.

During inhalation the diaphragm descends creating a negative pressure around the lungs and they begin to inflate, drawing in air from outside the body. The air enters the body through the nasal cavity located just inside the nose (Figure 1). As the air passes through the nasal cavity, the air is warmed to body temperature and humidified by moisture from mucous membranes. These processes help equilibrate the air to the body conditions, reducing any damage that cold, dry air can cause. Particulate matter that is floating in the air is removed in the nasal passages by hairs, mucus, and cilia. Air is also chemically sampled by the sense of smell.

From the nasal cavity, air passes through the pharynx (throat) and the larynx (voice box) as it makes its way to the trachea (Figure 1). The main function of the trachea is to funnel the inhaled air to the lungs and the exhaled air back out of the body. The human trachea is a cylinder, about 25 to 30 cm (9.8–11.8 in) long, which sits in front of the esophagus and extends from the pharynx into the chest cavity to the lungs. It is made of incomplete rings of cartilage and smooth muscle. The cartilage provides strength and support to the trachea to keep the passage open. The trachea is lined with cells that have cilia and secrete mucus. The mucus catches particles that have been inhaled, and the cilia move the particles toward the pharynx.

The end of the trachea divides into two bronchi that enter the right and left lung. Air enters the lungs through the primary bronchi. The primary bronchus divides, creating smaller and smaller diameter bronchi until the passages are under 1 mm (.03 in) in diameter when they are called bronchioles as they split and spread through the lung. Like the trachea, the bronchus and bronchioles are made of cartilage and smooth muscle. Bronchi are innervated by nerves of both the parasympathetic and sympathetic nervous systems that control muscle contraction (parasympathetic) or relaxation (sympathetic) in the bronchi and bronchioles, depending on the nervous system’s cues. The final bronchioles are the respiratory bronchioles. Alveolar ducts are attached to the end of each respiratory bronchiole. At the end of each duct are alveolar sacs, each containing 20 to 30 alveoli. Gas exchange occurs only in the alveoli. The alveoli are thin-walled and look like tiny bubbles within the sacs. The alveoli are in direct contact with capillaries of the circulatory system. Such intimate contact ensures that oxygen will diffuse from the alveoli into the blood. In addition, carbon dioxide will diffuse from the blood into the alveoli to be exhaled. The anatomical arrangement of capillaries and alveoli emphasizes the structural and functional relationship of the respiratory and circulatory systems. Estimates for the surface area of alveoli in the lungs vary around 100 m 2 . This large area is about the area of half a tennis court. This large surface area, combined with the thin-walled nature of the alveolar cells, allows gases to easily diffuse across the cells.

Figure 1. Air enters the respiratory system through the nasal cavity, and then passes through the pharynx and the trachea into the lungs. (credit: modification of work by NCI)

Which of the following statements about the human respiratory system is false?


Contents

Location and shape

The human heart is situated in the middle mediastinum, at the level of thoracic vertebrae T5-T8. A double-membraned sac called the pericardium surrounds the heart and attaches to the mediastinum. [15] The back surface of the heart lies near the vertebral column, and the front surface sits behind the sternum and rib cartilages. [7] The upper part of the heart is the attachment point for several large blood vessels—the venae cavae, aorta and pulmonary trunk. The upper part of the heart is located at the level of the third costal cartilage. [7] The lower tip of the heart, the apex, lies to the left of the sternum (8 to 9 cm from the midsternal line) between the junction of the fourth and fifth ribs near their articulation with the costal cartilages. [7]

The largest part of the heart is usually slightly offset to the left side of the chest (though occasionally it may be offset to the right) and is felt to be on the left because the left heart is stronger and larger, since it pumps to all body parts. Because the heart is between the lungs, the left lung is smaller than the right lung and has a cardiac notch in its border to accommodate the heart. [7] The heart is cone-shaped, with its base positioned upwards and tapering down to the apex. [7] An adult heart has a mass of 250–350 grams (9–12 oz). [16] The heart is often described as the size of a fist: 12 cm (5 in) in length, 8 cm (3.5 in) wide, and 6 cm (2.5 in) in thickness, [7] although this description is disputed, as the heart is likely to be slightly larger. [17] Well-trained athletes can have much larger hearts due to the effects of exercise on the heart muscle, similar to the response of skeletal muscle. [7]

Chambers

The heart has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers. The atria open into the ventricles via the atrioventricular valves, present in the atrioventricular septum. This distinction is visible also on the surface of the heart as the coronary sulcus. [18] There is an ear-shaped structure in the upper right atrium called the right atrial appendage, or auricle, and another in the upper left atrium, the left atrial appendage. [19] The right atrium and the right ventricle together are sometimes referred to as the right heart. Similarly, the left atrium and the left ventricle together are sometimes referred to as the left heart. [6] The ventricles are separated from each other by the interventricular septum, visible on the surface of the heart as the anterior longitudinal sulcus and the posterior interventricular sulcus. [18]

The cardiac skeleton is made of dense connective tissue and this gives structure to the heart. It forms the atrioventricular septum which separates the atria from the ventricles, and the fibrous rings which serve as bases for the four heart valves. [20] The cardiac skeleton also provides an important boundary in the heart's electrical conduction system since collagen cannot conduct electricity. The interatrial septum separates the atria and the interventricular septum separates the ventricles. [7] The interventricular septum is much thicker than the interatrial septum, since the ventricles need to generate greater pressure when they contract. [7]

Valves

The heart has four valves, which separate its chambers. One valve lies between each atrium and ventricle, and one valve rests at the exit of each ventricle. [7]

The valves between the atria and ventricles are called the atrioventricular valves. Between the right atrium and the right ventricle is the tricuspid valve. The tricuspid valve has three cusps, [21] which connect to chordae tendinae and three papillary muscles named the anterior, posterior, and septal muscles, after their relative positions. [21] The mitral valve lies between the left atrium and left ventricle. It is also known as the bicuspid valve due to its having two cusps, an anterior and a posterior cusp. These cusps are also attached via chordae tendinae to two papillary muscles projecting from the ventricular wall. [22]

The papillary muscles extend from the walls of the heart to valves by cartilaginous connections called chordae tendinae. These muscles prevent the valves from falling too far back when they close. [23] During the relaxation phase of the cardiac cycle, the papillary muscles are also relaxed and the tension on the chordae tendineae is slight. As the heart chambers contract, so do the papillary muscles. This creates tension on the chordae tendineae, helping to hold the cusps of the atrioventricular valves in place and preventing them from being blown back into the atria. [7] [g] [21]

Two additional semilunar valves sit at the exit of each of the ventricles. The pulmonary valve is located at the base of the pulmonary artery. This has three cusps which are not attached to any papillary muscles. When the ventricle relaxes blood flows back into the ventricle from the artery and this flow of blood fills the pocket-like valve, pressing against the cusps which close to seal the valve. The semilunar aortic valve is at the base of the aorta and also is not attached to papillary muscles. This too has three cusps which close with the pressure of the blood flowing back from the aorta. [7]

Right heart

The right heart consists of two chambers, the right atrium and the right ventricle, separated by a valve, the tricuspid valve. [7]

The right atrium receives blood almost continuously from the body's two major veins, the superior and inferior venae cavae. A small amount of blood from the coronary circulation also drains into the right atrium via the coronary sinus, which is immediately above and to the middle of the opening of the inferior vena cava. [7] In the wall of the right atrium is an oval-shaped depression known as the fossa ovalis, which is a remnant of an opening in the fetal heart known as the foramen ovale. [7] Most of the internal surface of the right atrium is smooth, the depression of the fossa ovalis is medial, and the anterior surface has prominent ridges of pectinate muscles, which are also present in the right atrial appendage. [7]

The right atrium is connected to the right ventricle by the tricuspid valve. [7] The walls of the right ventricle are lined with trabeculae carneae, ridges of cardiac muscle covered by endocardium. In addition to these muscular ridges, a band of cardiac muscle, also covered by endocardium, known as the moderator band reinforces the thin walls of the right ventricle and plays a crucial role in cardiac conduction. It arises from the lower part of the interventricular septum and crosses the interior space of the right ventricle to connect with the inferior papillary muscle. [7] The right ventricle tapers into the pulmonary trunk, into which it ejects blood when contracting. The pulmonary trunk branches into the left and right pulmonary arteries that carry the blood to each lung. The pulmonary valve lies between the right heart and the pulmonary trunk. [7]

Left heart

The left heart has two chambers: the left atrium and the left ventricle, separated by the mitral valve. [7]

The left atrium receives oxygenated blood back from the lungs via one of the four pulmonary veins. The left atrium has an outpouching called the left atrial appendage. Like the right atrium, the left atrium is lined by pectinate muscles. [24] The left atrium is connected to the left ventricle by the mitral valve. [7]

The left ventricle is much thicker as compared with the right, due to the greater force needed to pump blood to the entire body. Like the right ventricle, the left also has trabeculae carneae, but there is no moderator band. The left ventricle pumps blood to the body through the aortic valve and into the aorta. Two small openings above the aortic valve carry blood to the heart itself, the left main coronary artery and the right coronary artery. [7]

Heart wall

The heart wall is made up of three layers: the inner endocardium, middle myocardium and outer epicardium. These are surrounded by a double-membraned sac called the pericardium.

The innermost layer of the heart is called the endocardium. It is made up of a lining of simple squamous epithelium and covers heart chambers and valves. It is continuous with the endothelium of the veins and arteries of the heart, and is joined to the myocardium with a thin layer of connective tissue. [7] The endocardium, by secreting endothelins, may also play a role in regulating the contraction of the myocardium. [7]

The middle layer of the heart wall is the myocardium, which is the cardiac muscle—a layer of involuntary striated muscle tissue surrounded by a framework of collagen. The cardiac muscle pattern is elegant and complex, as the muscle cells swirl and spiral around the chambers of the heart, with the outer muscles forming a figure 8 pattern around the atria and around the bases of the great vessels and the inner muscles, forming a figure 8 around the two ventricles and proceeding toward the apex. This complex swirling pattern allows the heart to pump blood more effectively. [7]

There are two types of cells in cardiac muscle: muscle cells which have the ability to contract easily, and pacemaker cells of the conducting system. The muscle cells make up the bulk (99%) of cells in the atria and ventricles. These contractile cells are connected by intercalated discs which allow a rapid response to impulses of action potential from the pacemaker cells. The intercalated discs allow the cells to act as a syncytium and enable the contractions that pump blood through the heart and into the major arteries. [7] The pacemaker cells make up 1% of cells and form the conduction system of the heart. They are generally much smaller than the contractile cells and have few myofibrils which gives them limited contractibility. Their function is similar in many respects to neurons. [7] Cardiac muscle tissue has autorhythmicity, the unique ability to initiate a cardiac action potential at a fixed rate—spreading the impulse rapidly from cell to cell to trigger the contraction of the entire heart. [7]

There are specific proteins expressed in cardiac muscle cells. [25] [26] These are mostly associated with muscle contraction, and bind with actin, myosin, tropomyosin, and troponin. They include MYH6, ACTC1, TNNI3, CDH2 and PKP2. Other proteins expressed are MYH7 and LDB3 that are also expressed in skeletal muscle. [27]

Pericardium

The pericardium is the sac that surrounds the heart. The tough outer surface of the pericardium is called the fibrous membrane. This is lined by a double inner membrane called the serous membrane that produces pericardial fluid to lubricate the surface of the heart. [28] The part of the serous membrane attached to the fibrous membrane is called the parietal pericardium, while the part of the serous membrane attached to the heart is known as the visceral pericardium. The pericardium is present in order to lubricate its movement against other structures within the chest, to keep the heart's position stabilised within the chest, and to protect the heart from infection. [29]

Coronary circulation

Heart tissue, like all cells in the body, needs to be supplied with oxygen, nutrients and a way of removing metabolic wastes. This is achieved by the coronary circulation, which includes arteries, veins, and lymphatic vessels. Blood flow through the coronary vessels occurs in peaks and troughs relating to the heart muscle's relaxation or contraction. [7]

Heart tissue receives blood from two arteries which arise just above the aortic valve. These are the left main coronary artery and the right coronary artery. The left main coronary artery splits shortly after leaving the aorta into two vessels, the left anterior descending and the left circumflex artery. The left anterior descending artery supplies heart tissue and the front, outer side, and the septum of the left ventricle. It does this by branching into smaller arteries—diagonal and septal branches. The left circumflex supplies the back and underneath of the left ventricle. The right coronary artery supplies the right atrium, right ventricle, and lower posterior sections of the left ventricle. The right coronary artery also supplies blood to the atrioventricular node (in about 90% of people) and the sinoatrial node (in about 60% of people). The right coronary artery runs in a groove at the back of the heart and the left anterior descending artery runs in a groove at the front. There is significant variation between people in the anatomy of the arteries that supply the heart [30] The arteries divide at their furthest reaches into smaller branches that join together at the edges of each arterial distribution. [7]

The coronary sinus is a large vein that drains into the right atrium, and receives most of the venous drainage of the heart. It receives blood from the great cardiac vein (receiving the left atrium and both ventricles), the posterior cardiac vein (draining the back of the left ventricle), the middle cardiac vein (draining the bottom of the left and right ventricles), and small cardiac veins. [31] The anterior cardiac veins drain the front of the right ventricle and drain directly into the right atrium. [7]

Small lymphatic networks called plexuses exist beneath each of the three layers of the heart. These networks collect into a main left and a main right trunk, which travel up the groove between the ventricles that exists on the heart's surface, receiving smaller vessels as they travel up. These vessels then travel into the atrioventricular groove, and receive a third vessel which drains the section of the left ventricle sitting on the diaphragm. The left vessel joins with this third vessel, and travels along the pulmonary artery and left atrium, ending in the inferior tracheobronchial node. The right vessel travels along the right atrium and the part of the right ventricle sitting on the diaphragm. It usually then travels in front of the ascending aorta and then ends in a brachiocephalic node. [32]

Nerve supply

The heart receives nerve signals from the vagus nerve and from nerves arising from the sympathetic trunk. These nerves act to influence, but not control, the heart rate. Sympathetic nerves also influence the force of heart contraction. [33] Signals that travel along these nerves arise from two paired cardiovascular centres in the medulla oblongata. The vagus nerve of the parasympathetic nervous system acts to decrease the heart rate, and nerves from the sympathetic trunk act to increase the heart rate. [7] These nerves form a network of nerves that lies over the heart called the cardiac plexus. [7] [32]

The vagus nerve is a long, wandering nerve that emerges from the brainstem and provides parasympathetic stimulation to a large number of organs in the thorax and abdomen, including the heart. [34] The nerves from the sympathetic trunk emerge through the T1-T4 thoracic ganglia and travel to both the sinoatrial and atrioventricular nodes, as well as to the atria and ventricles. The ventricles are more richly innervated by sympathetic fibers than parasympathetic fibers. Sympathetic stimulation causes the release of the neurotransmitter norepinephrine (also known as noradrenaline) at the neuromuscular junction of the cardiac nerves. This shortens the repolarization period, thus speeding the rate of depolarization and contraction, which results in an increased heart rate. It opens chemical or ligand-gated sodium and calcium ion channels, allowing an influx of positively charged ions. [7] Norepinephrine binds to the beta–1 receptor. [7]

The heart is the first functional organ to develop and starts to beat and pump blood at about three weeks into embryogenesis. This early start is crucial for subsequent embryonic and prenatal development.

The heart derives from splanchnopleuric mesenchyme in the neural plate which forms the cardiogenic region. Two endocardial tubes form here that fuse to form a primitive heart tube known as the tubular heart. [35] Between the third and fourth week, the heart tube lengthens, and begins to fold to form an S-shape within the pericardium. This places the chambers and major vessels into the correct alignment for the developed heart. Further development will include the septa and valves formation and remodelling of the heart chambers. By the end of the fifth week the septa are complete and the heart valves are completed by the ninth week. [7]

Before the fifth week, there is an opening in the fetal heart known as the foramen ovale. The foramen ovale allowed blood in the fetal heart to pass directly from the right atrium to the left atrium, allowing some blood to bypass the lungs. Within seconds after birth, a flap of tissue known as the septum primum that previously acted as a valve closes the foramen ovale and establishes the typical cardiac circulation pattern. A depression in the surface of the right atrium remains where the foramen ovale was, called the fossa ovalis. [7]

The embryonic heart begins beating at around 22 days after conception (5 weeks after the last normal menstrual period, LMP). It starts to beat at a rate near to the mother's which is about 75–80 beats per minute (bpm). The embryonic heart rate then accelerates and reaches a peak rate of 165–185 bpm early in the early 7th week (early 9th week after the LMP). [36] [37] After 9 weeks (start of the fetal stage) it starts to decelerate, slowing to around 145 (±25) bpm at birth. There is no difference in female and male heart rates before birth. [38]

Blood flow

The heart functions as a pump in the circulatory system to provide a continuous flow of blood throughout the body. This circulation consists of the systemic circulation to and from the body and the pulmonary circulation to and from the lungs. Blood in the pulmonary circulation exchanges carbon dioxide for oxygen in the lungs through the process of respiration. The systemic circulation then transports oxygen to the body and returns carbon dioxide and relatively deoxygenated blood to the heart for transfer to the lungs. [7]

The right heart collects deoxygenated blood from two large veins, the superior and inferior venae cavae. Blood collects in the right and left atrium continuously. [7] The superior vena cava drains blood from above the diaphragm and empties into the upper back part of the right atrium. The inferior vena cava drains the blood from below the diaphragm and empties into the back part of the atrium below the opening for the superior vena cava. Immediately above and to the middle of the opening of the inferior vena cava is the opening of the thin-walled coronary sinus. [7] Additionally, the coronary sinus returns deoxygenated blood from the myocardium to the right atrium. The blood collects in the right atrium. When the right atrium contracts, the blood is pumped through the tricuspid valve into the right ventricle. As the right ventricle contracts, the tricuspid valve closes and the blood is pumped into the pulmonary trunk through the pulmonary valve. The pulmonary trunk divides into pulmonary arteries and progressively smaller arteries throughout the lungs, until it reaches capillaries. As these pass by alveoli carbon dioxide is exchanged for oxygen. This happens through the passive process of diffusion.

In the left heart, oxygenated blood is returned to the left atrium via the pulmonary veins. It is then pumped into the left ventricle through the mitral valve and into the aorta through the aortic valve for systemic circulation. The aorta is a large artery that branches into many smaller arteries, arterioles, and ultimately capillaries. In the capillaries, oxygen and nutrients from blood are supplied to body cells for metabolism, and exchanged for carbon dioxide and waste products. [7] Capillary blood, now deoxygenated, travels into venules and veins that ultimately collect in the superior and inferior vena cavae, and into the right heart.

Cardiac cycle

The cardiac cycle refers to the sequence of events in which the heart contracts and relaxes with every heartbeat. [9] The period of time during which the ventricles contract, forcing blood out into the aorta and main pulmonary artery, is known as systole, while the period during which the ventricles relax and refill with blood is known as diastole. The atria and ventricles work in concert, so in systole when the ventricles are contracting, the atria are relaxed and collecting blood. When the ventricles are relaxed in diastole, the atria contract to pump blood to the ventricles. This coordination ensures blood is pumped efficiently to the body. [7]

At the beginning of the cardiac cycle, the ventricles are relaxing. As they do so, they are filled by blood passing through the open mitral and tricuspid valves. After the ventricles have completed most of their filling, the atria contract, forcing further blood into the ventricles and priming the pump. Next, the ventricles start to contract. As the pressure rises within the cavities of the ventricles, the mitral and tricuspid valves are forced shut. As the pressure within the ventricles rises further, exceeding the pressure with the aorta and pulmonary arteries, the aortic and pulmonary valves open. Blood is ejected from the heart, causing the pressure within the ventricles to fall. Simultaneously, the atria refill as blood flows into the right atrium through the superior and inferior vena cavae, and into the left atrium through the pulmonary veins. Finally, when the pressure within the ventricles falls below the pressure within the aorta and pulmonary arteries, the aortic and pulmonary valves close. The ventricles start to relax, the mitral and tricuspid valves open, and the cycle begins again. [9]

Cardiac output

Cardiac output (CO) is a measurement of the amount of blood pumped by each ventricle (stroke volume) in one minute. This is calculated by multiplying the stroke volume (SV) by the beats per minute of the heart rate (HR). So that: CO = SV x HR. [7] The cardiac output is normalized to body size through body surface area and is called the cardiac index.

The average cardiac output, using an average stroke volume of about 70mL, is 5.25 L/min, with a normal range of 4.0–8.0 L/min. [7] The stroke volume is normally measured using an echocardiogram and can be influenced by the size of the heart, physical and mental condition of the individual, sex, contractility, duration of contraction, preload and afterload. [7]

Preload refers to the filling pressure of the atria at the end of diastole, when the ventricles are at their fullest. A main factor is how long it takes the ventricles to fill: if the ventricles contract more frequently, then there is less time to fill and the preload will be less. [7] Preload can also be affected by a person's blood volume. The force of each contraction of the heart muscle is proportional to the preload, described as the Frank-Starling mechanism. This states that the force of contraction is directly proportional to the initial length of muscle fiber, meaning a ventricle will contract more forcefully, the more it is stretched. [7] [39]

Afterload, or how much pressure the heart must generate to eject blood at systole, is influenced by vascular resistance. It can be influenced by narrowing of the heart valves (stenosis) or contraction or relaxation of the peripheral blood vessels. [7]

The strength of heart muscle contractions controls the stroke volume. This can be influenced positively or negatively by agents termed inotropes. [40] These agents can be a result of changes within the body, or be given as drugs as part of treatment for a medical disorder, or as a form of life support, particularly in intensive care units. Inotropes that increase the force of contraction are "positive" inotropes, and include sympathetic agents such as adrenaline, noradrenaline and dopamine. [41] "Negative" inotropes decrease the force of contraction and include calcium channel blockers. [40]

Electrical conduction

The normal rhythmical heart beat, called sinus rhythm, is established by the heart's own pacemaker, the sinoatrial node (also known as the sinus node or the SA node). Here an electrical signal is created that travels through the heart, causing the heart muscle to contract. The sinoatrial node is found in the upper part of the right atrium near to the junction with the superior vena cava. [42] The electrical signal generated by the sinoatrial node travels through the right atrium in a radial way that is not completely understood. It travels to the left atrium via Bachmann's bundle, such that the muscles of the left and right atria contract together. [43] [44] [45] The signal then travels to the atrioventricular node. This is found at the bottom of the right atrium in the atrioventricular septum—the boundary between the right atrium and the left ventricle. The septum is part of the cardiac skeleton, tissue within the heart that the electrical signal cannot pass through, which forces the signal to pass through the atrioventricular node only. [7] The signal then travels along the bundle of His to left and right bundle branches through to the ventricles of the heart. In the ventricles the signal is carried by specialized tissue called the Purkinje fibers which then transmit the electric charge to the heart muscle. [46]

Heart rate

The normal resting heart rate is called the sinus rhythm, created and sustained by the sinoatrial node, a group of pacemaking cells found in the wall of the right atrium. Cells in the sinoatrial node do this by creating an action potential. The cardiac action potential is created by the movement of specific electrolytes into and out of the pacemaker cells. The action potential then spreads to nearby cells. [47]

When the sinoatrial cells are resting, they have a negative charge on their membranes. However a rapid influx of sodium ions causes the membrane's charge to become positive. This is called depolarisation and occurs spontaneously. [7] Once the cell has a sufficiently high charge, the sodium channels close and calcium ions then begin to enter the cell, shortly after which potassium begins to leave it. All the ions travel through ion channels in the membrane of the sinoatrial cells. The potassium and calcium start to move out of and into the cell only once it has a sufficiently high charge, and so are called voltage-gated. Shortly after this, the calcium channels close and potassium channels open, allowing potassium to leave the cell. This causes the cell to have a negative resting charge and is called repolarization. When the membrane potential reaches approximately −60 mV, the potassium channels close and the process may begin again. [7]

The ions move from areas where they are concentrated to where they are not. For this reason sodium moves into the cell from outside, and potassium moves from within the cell to outside the cell. Calcium also plays a critical role. Their influx through slow channels means that the sinoatrial cells have a prolonged "plateau" phase when they have a positive charge. A part of this is called the absolute refractory period. Calcium ions also combine with the regulatory protein troponin C in the troponin complex to enable contraction of the cardiac muscle, and separate from the protein to allow relaxation. [48]

The adult resting heart rate ranges from 60 to 100 bpm. The resting heart rate of a newborn can be 129 beats per minute (bpm) and this gradually decreases until maturity. [49] An athlete's heart rate can be lower than 60 bpm. During exercise the rate can be 150 bpm with maximum rates reaching from 200 to 220 bpm. [7]

Influences

The normal sinus rhythm of the heart, giving the resting heart rate, is influenced by a number of factors. The cardiovascular centres in the brainstem that control the sympathetic and parasympathetic influences to the heart through the vagus nerve and sympathetic trunk. [50] These cardiovascular centres receive input from a series of receptors including baroreceptors, sensing stretch the stretching of blood vessels and chemoreceptors, sensing the amount of oxygen and carbon dioxide in the blood and its pH. Through a series of reflexes these help regulate and sustain blood flow. [7]

Baroreceptors are stretch receptors located in the aortic sinus, carotid bodies, the venae cavae, and other locations, including pulmonary vessels and the right side of the heart itself. Baroreceptors fire at a rate determined by how much they are stretched, [51] which is influenced by blood pressure, level of physical activity, and the relative distribution of blood. With increased pressure and stretch, the rate of baroreceptor firing increases, and the cardiac centers decrease sympathetic stimulation and increase parasympathetic stimulation. As pressure and stretch decrease, the rate of baroreceptor firing decreases, and the cardiac centers increase sympathetic stimulation and decrease parasympathetic stimulation. [7] There is a similar reflex, called the atrial reflex or Bainbridge reflex, associated with varying rates of blood flow to the atria. Increased venous return stretches the walls of the atria where specialized baroreceptors are located. However, as the atrial baroreceptors increase their rate of firing and as they stretch due to the increased blood pressure, the cardiac center responds by increasing sympathetic stimulation and inhibiting parasympathetic stimulation to increase heart rate. The opposite is also true. [7] Chemoreceptors present in the carotid body or adjacent to the aorta in an aortic body respond to the blood's oxygen, carbon dioxide levels. Low oxygen or high carbon dioxide will stimulate firing of the receptors. [52]

Exercise and fitness levels, age, body temperature, basal metabolic rate, and even a person's emotional state can all affect the heart rate. High levels of the hormones epinephrine, norepinephrine, and thyroid hormones can increase the heart rate. The levels of electrolytes including calcium, potassium, and sodium can also influence the speed and regularity of the heart rate low blood oxygen, low blood pressure and dehydration may increase it. [7]

Diseases

Cardiovascular diseases, which include diseases of the heart, are the leading cause of death worldwide. [53] The majority of cardiovascular disease is noncommunicable and related to lifestyle and other factors, becoming more prevalent with ageing. [53] Heart disease is a major cause of death, accounting for an average of 30% of all deaths in 2008, globally. [11] This rate varies from a lower 28% to a high 40% in high-income countries. [12] Doctors that specialise in the heart are called cardiologists. Many other medical professionals are involved in treating diseases of the heart, including doctors such as general practitioners, cardiothoracic surgeons and intensivists, and allied health practitioners including physiotherapists and dieticians. [54]

Ischaemic heart disease

Coronary artery disease, also known as ischaemic heart disease, is caused by atherosclerosis—a build-up of fatty material along the inner walls of the arteries. These fatty deposits known as atherosclerotic plaques narrow the coronary arteries, and if severe may reduce blood flow to the heart. [55] If a narrowing (or stenosis) is relatively minor then the patient may not experience any symptoms. Severe narrowings may cause chest pain (angina) or breathlessness during exercise or even at rest. The thin covering of an atherosclerotic plaque can rupture, exposing the fatty centre to the circulating blood. In this case a clot or thrombus can form, blocking the artery, and restricting blood flow to an area of heart muscle causing a myocardial infarction (a heart attack) or unstable angina. [56] In the worst case this may cause cardiac arrest, a sudden and utter loss of output from the heart. [57] Obesity, high blood pressure, uncontrolled diabetes, smoking and high cholesterol can all increase the risk of developing atherosclerosis and coronary artery disease. [53] [55]

Heart failure

Heart failure is defined as a condition in which the heart is unable to pump enough blood to meet the demands of the body. [58] Patients with heart failure may experience breathlessness especially when lying flat, as well as ankle swelling, known as peripheral oedema. Heart failure is the end result of many diseases affecting the heart, but is most commonly associated with ischaemic heart disease, valvular heart disease, or high blood pressure. Less common causes include various cardiomyopathies. Heart failure is frequently associated with weakness of the heart muscle in the ventricles (systolic heart failure), but can also be seen in patients with heart muscle that is strong but stiff (diastolic heart failure). The condition may affect the left ventricle (causing predominantly breathlessness), the right ventricle (causing predominantly swelling of the legs and an elevated jugular venous pressure), or both ventricles. Patients with heart failure are at higher risk of developing dangerous heart rhythm disturbances or arrhythmias. [58]

Cardiomyopathies

Cardiomyopathies are diseases affecting the muscle of the heart. Some cause abnormal thickening of the heart muscle (hypertrophic cardiomyopathy), some cause the heart to abnormally expand and weaken (dilated cardiomyopathy), some cause the heart muscle to become stiff and unable to fully relax between contractions (restrictive cardiomyopathy) and some make the heart prone to abnormal heart rhythms (arrhythmogenic cardiomyopathy). These conditions are often genetic and can be inherited, but some such as dilated cardiomyopathy may be caused by damage from toxins such as alcohol. Some cardiomyopathies such as hypertrophic cardiomopathy are linked to a higher risk of sudden cardiac death, particularly in athletes. [7] Many cardiomyopathies can lead to heart failure in the later stages of the disease. [58]

Valvular heart disease

Healthy heart valves allow blood to flow easily in one direction, but prevent it from flowing in the other direction. Diseased heart valves may have a narrow opening and therefore restrict the flow of blood in the forward direction (referred to as a stenotic valve), or may allow blood to leak in the reverse direction (referred to as valvular regurgitation). Valvular heart disease may cause breathlessness, blackouts, or chest pain, but may be asymptomatic and only detected on a routine examination by hearing abnormal heart sounds or a heart murmur. In the developed world, valvular heart disease is most commonly caused by degeneration secondary to old age, but may also be caused by infection of the heart valves (endocarditis). In some parts of the world rheumatic heart disease is a major cause of valvular heart disease, typically leading to mitral or aortic stenosis and caused by the body's immune system reacting to a streptococcal throat infection. [59] [60]

Cardiac arrhythmias

While in the healthy heart, waves of electrical impulses originate in the sinus node before spreading to the rest of the atria, the atrioventricular node, and finally the ventricles (referred to as a normal sinus rhythm), this normal rhythm can be disrupted. Abnormal heart rhythms or arrhythmias may be asymptomatic or may cause palpitations, blackouts, or breathlessness. Some types of arrhythmia such as atrial fibrillation increase the long term risk of stroke. [61]

Some arrhythmias cause the heart to beat abnormally slowly, referred to as a bradycardia or bradyarrhythmia. This may be caused by an abnormally slow sinus node or damage within the cardiac conduction system (heart block). [62] In other arrhythmias the heart may beat abnormally rapidly, referred to as a tachycardia or tachyarrhythmia. These arrhythmias can take many forms and can originate from different structures within the heart—some arise from the atria (e.g. atrial flutter), some from the atrioventricular node (e.g. AV nodal re-entrant tachycardia) whilst others arise from the ventricles (e.g. ventricular tachycardia). Some tachyarrhythmias are caused by scarring within the heart (e.g. some forms of ventricular tachycardia), others by an irritable focus (e.g. focal atrial tachycardia), while others are caused by additional abnormal conduction tissue that has been present since birth (e.g. Wolff-Parkinson-White syndrome). The most dangerous form of heart racing is ventricular fibrillation, in which the ventricles quiver rather than contract, and which if untreated is rapidly fatal. [63]

Pericardial disease

The sack which surrounds the heart, called the pericardium, can become inflamed in a condition known as pericarditis. This condition typically causes chest pain that may spread to the back, and is often caused by a viral infection (glandular fever, cytomegalovirus, or coxsackievirus). Fluid can build up within the pericardial sack, referred to as a pericardial effusion. Pericardial effusions often occur secondary to pericarditis, kidney failure, or tumours, and frequently do not cause any symptoms. However, large effusions or effusions which accumulate rapidly can compress the heart in a condition known as cardiac tamponade, causing breathlessness and potentially fatal low blood pressure. Fluid can be removed from the pericardial space for diagnosis or to relieve tamponade using a syringe in a procedure called pericardiocentesis. [64]

Congenital heart disease

Some people are born with hearts that are abnormal and these abnormalities are known as congenital heart defects. They may range from the relatively minor (e.g. patent foramen ovale, arguably a variant of normal) to serious life-threatening abnormalities (e.g. hypoplastic left heart syndrome). Common abnormalities include those that affect the heart muscle that separates the two side of the heart (a 'hole in the heart' e.g. ventricular septal defect). Other defects include those affecting the heart valves (e.g. congenital aortic stenosis), or the main blood vessels that lead from the heart (e.g. coarctation of the aorta). More complex syndromes are seen that affect more than one part of the heart (e.g. Tetralogy of Fallot).

Some congenital heart defects allow blood that is low in oxygen that would normally be returned to the lungs to instead be pumped back to the rest of the body. These are known as cyanotic congenital heart defects and are often more serious. Major congenital heart defects are often picked up in childhood, shortly after birth, or even before a child is born (e.g. transposition of the great arteries), causing breathlessness and a lower rate of growth. More minor forms of congenital heart disease may remain undetected for many years and only reveal themselves in adult life (e.g. atrial septal defect). [65] [66]

Diagnosis

Heart disease is diagnosed by the taking of a medical history, a cardiac examination, and further investigations, including blood tests, echocardiograms, ECGs and imaging. Other invasive procedures such as cardiac catheterisation can also play a role. [67]

Examination

The cardiac examination includes inspection, feeling the chest with the hands (palpation) and listening with a stethoscope (auscultation). [68] [69] It involves assessment of signs that may be visible on a person's hands (such as splinter haemorrhages), joints and other areas. A person's pulse is taken, usually at the radial artery near the wrist, in order to assess for the rhythm and strength of the pulse. The blood pressure is taken, using either a manual or automatic sphygmomanometer or using a more invasive measurement from within the artery. Any elevation of the jugular venous pulse is noted. A person's chest is felt for any transmitted vibrations from the heart, and then listened to with a stethoscope.

Heart sounds

Typically, healthy hearts have only two audible heart sounds, called S1 and S2. The first heart sound S1, is the sound created by the closing of the atrioventricular valves during ventricular contraction and is normally described as "lub". The second heart sound, S2, is the sound of the semilunar valves closing during ventricular diastole and is described as "dub". [7] Each sound consists of two components, reflecting the slight difference in time as the two valves close. [70] S2 may split into two distinct sounds, either as a result of inspiration or different valvular or cardiac problems. [70] Additional heart sounds may also be present and these give rise to gallop rhythms. A third heart sound, S3 usually indicates an increase in ventricular blood volume. A fourth heart sound S4 is referred to as an atrial gallop and is produced by the sound of blood being forced into a stiff ventricle. The combined presence of S3 and S4 give a quadruple gallop. [7]

Heart murmurs are abnormal heart sounds which can be either related to disease or benign, and there are several kinds. [71] There are normally two heart sounds, and abnormal heart sounds can either be extra sounds, or "murmurs" related to the flow of blood between the sounds. Murmurs are graded by volume, from 1 (the quietest), to 6 (the loudest), and evaluated by their relationship to the heart sounds, position in the cardiac cycle, and additional features such as their radiation to other sites, changes with a person's position, the frequency of the sound as determined by the side of the stethoscope by which they are heard, and site at which they are heard loudest. [71] Murmurs may be caused by damaged heart valves, congenital heart disease such as ventricular septal defects, or may be heard in normal hearts. A different type of sound, a pericardial friction rub can be heard in cases of pericarditis where the inflamed membranes can rub together.

Blood tests

Blood tests play an important role in the diagnosis and treatment of many cardiovascular conditions.

Troponin is a sensitive biomarker for a heart with insufficient blood supply. It is released 4–6 hours after injury, and usually peaks at about 12–24 hours. [41] Two tests of troponin are often taken—one at the time of initial presentation, and another within 3–6 hours, [72] with either a high level or a significant rise being diagnostic. A test for brain natriuretic peptide (BNP) can be used to evaluate for the presence of heart failure, and rises when there is increased demand on the left ventricle. These tests are considered biomarkers because they are highly specific for cardiac disease. [73] Testing for the MB form of creatine kinase provides information about the heart's blood supply, but is used less frequently because it is less specific and sensitive. [74]

Other blood tests are often taken to help understand a person's general health and risk factors that may contribute to heart disease. These often include a full blood count investigating for anaemia, and basic metabolic panel that may reveal any disturbances in electrolytes. A coagulation screen is often required to ensure that the right level of anticoagulation is given. Fasting lipids and fasting blood glucose (or an HbA1c level) are often ordered to evaluate a person's cholesterol and diabetes status, respectively. [75]

Electrocardiogram

Using surface electrodes on the body, it is possible to record the electrical activity of the heart. This tracing of the electrical signal is the electrocardiogram (ECG) or (EKG). An ECG is a bedside test and involves the placement of ten leads on the body. This produces a "12 lead" ECG (three extra leads are calculated mathematically, and one lead is a ground). [76]

There are five prominent features on the ECG: the P wave (atrial depolarisation), the QRS complex (ventricular depolarisation [h] ) and the T wave (ventricular repolarisation). [7] As the heart cells contract, they create a current that travels through the heart. A downward deflection on the ECG implies cells are becoming more positive in charge ("depolarising") in the direction of that lead, whereas an upward inflection implies cells are becoming more negative ("repolarising") in the direction of the lead. This depends on the position of the lead, so if a wave of depolarising moved from left to right, a lead on the left would show a negative deflection, and a lead on the right would show a positive deflection. The ECG is a useful tool in detecting rhythm disturbances and in detecting insufficient blood supply to the heart. [76] Sometimes abnormalities are suspected, but not immediately visible on the ECG. Testing when exercising can be used to provoke an abnormality, or an ECG can be worn for a longer period such as a 24-hour Holter monitor if a suspected rhythm abnormality is not present at the time of assessment. [76]

Imaging

Several imaging methods can be used to assess the anatomy and function of the heart, including ultrasound (echocardiography), angiography, CT scans, MRI and PET. An echocardiogram is an ultrasound of the heart used to measure the heart's function, assess for valve disease, and look for any abnormalities. Echocardiography can be conducted by a probe on the chest ("transthoracic") or by a probe in the esophagus ("transoesophageal"). A typical echocardiography report will include information about the width of the valves noting any stenosis, whether there is any backflow of blood (regurgitation) and information about the blood volumes at the end of systole and diastole, including an ejection fraction, which describes how much blood is ejected from the left and right ventricles after systole. Ejection fraction can then be obtained by dividing the volume ejected by the heart (stroke volume) by the volume of the filled heart (end-diastolic volume). [77] Echocardiograms can also be conducted under circumstances when the body is more stressed, in order to examine for signs of lack of blood supply. This cardiac stress test involves either direct exercise, or where this is not possible, injection of a drug such as dobutamine. [69]

CT scans, chest X-rays and other forms of imaging can help evaluate the heart's size, evaluate for signs of pulmonary oedema, and indicate whether there is fluid around the heart. They are also useful for evaluating the aorta, the major blood vessel which leaves the heart. [69]

Treatment

Diseases affecting the heart can be treated by a variety of methods including lifestyle modification, drug treatment, and surgery.

Ischaemic heart disease

Narrowings of the coronary arteries (ischaemic heart disease) are treated to relieve symptoms of chest pain caused by a partially narrowed artery (angina pectoris), to minimise heart muscle damage when an artery is completely occluded (myocardial infarction), or to prevent a myocardial infarction from occurring. Medications to improve angina symptoms include nitroglycerin, beta blockers, and calcium channel blockers, while preventative treatments include antiplatelets such as aspirin and statins, lifestyle measures such as stopping smoking and weight loss, and treatment of risk factors such as high blood pressure and diabetes. [78]

In addition to using medications, narrowed heart arteries can be treated by expanding the narrowings or redirecting the flow of blood to bypass an obstruction. This may be performed using a percutaneous coronary intervention, during which narrowings can be expanded by passing small balloon-tipped wires into the coronary arteries, inflating the balloon to expand the narrowing, and sometimes leaving behind a metal scaffold known as a stent to keep the artery open. [79]

If the narrowings in coronary arteries are unsuitable for treatment with a percutaneous coronary intervention, open surgery may be required. A coronary artery bypass graft can be performed, whereby a blood vessel from another part of the body (the saphenous vein, radial artery, or internal mammary artery) is used to redirect blood from a point before the narrowing (typically the aorta) to a point beyond the obstruction. [79] [80]

Valvular heart disease

Diseased heart valves that have become abnormally narrow or abnormally leaky may require surgery. This is traditionally performed as an open surgical procedure to replace the damaged heart valve with a tissue or metallic prosthetic valve. In some circumstances, the tricuspid or mitral valves can be repaired surgically, avoiding the need for a valve replacement. Heart valves can also be treated percutaneously, using techniques that share many similarities with percutaneous coronary intervention. Transcatheter aortic valve replacement is increasingly used for patients consider very high risk for open valve replacement. [59]

Cardiac arrhythmias

Abnormal heart rhythms (arrhythmias) can be treated using antiarrhythmic drugs. These may work by manipulating the flow of electrolytes across the cell membrane (such as calcium channel blockers, sodium channel blockers, amiodarone, or digoxin), or modify the autonomic nervous system's effect on the heart (beta blockers and atropine). In some arrhythmias such as atrial fibrillation which increase the risk of stroke, this risk can be reduced using anticoagulants such as warfarin or novel oral anticoagulants. [61]

If medications fail to control an arrhythmia, another treatment option may be catheter ablation. In these procedures, wires are passed from a vein or artery in the leg to the heart to find the abnormal area of tissue that is causing the arrhythmia. The abnormal tissue can be intentionally damaged, or ablated, by heating or freezing to prevent further heart rhythm disturbances. Whilst the majority of arrhythmias can be treated using minimally invasive catheter techniques, some arrhythmias (particularly atrial fibrillation) can also be treated using open or thoracoscopic surgery, either at the time of other cardiac surgery or as a standalone procedure. A cardioversion, whereby an electric shock is used to stun the heart out of an abnormal rhythm, may also be used.

Cardiac devices in the form of pacemakers or implantable defibrillators may also be required to treat arrhythmias. Pacemakers, comprising a small battery powered generator implanted under the skin and one or more leads that extend to the heart, are most commonly used to treat abnormally slow heart rhythms. [62] Implantable defibrillators are used to treat serious life-threatening rapid heart rhythms. These devices monitor the heart, and if dangerous heart racing is detected can automatically deliver a shock to restore the heart to a normal rhythm. Implantable defibrillators are most commonly used in patients with heart failure, cardiomyopathies, or inherited arrhythmia syndromes.

Heart failure

As well as addressing the underlying cause for a patient's heart failure (most commonly ischaemic heart disease or hypertension), the mainstay of heart failure treatment is with medication. These include drugs to prevent fluid from accumulating in the lungs by increasing the amount of urine a patient produces (diuretics), and drugs that attempt to preserve the pumping function of the heart (beta blockers, ACE inhibitors and mineralocorticoid receptor antagonists). [58]

In some patients with heart failure, a specialised pacemaker known as cardiac resynchronisation therapy can be used to improve the heart's pumping efficiency. [62] These devices are frequently combined with a defibrillator. In very severe cases of heart failure, a small pump called a ventricular assist device may be implanted which supplements the heart's own pumping ability. In the most severe cases, a cardiac transplant may be considered. [58]

Ancient

Humans have known about the heart since ancient times, although its precise function and anatomy were not clearly understood. [81] From the primarily religious views of earlier societies towards the heart, ancient Greeks are considered to have been the primary seat of scientific understanding of the heart in the ancient world. [82] [83] [84] Aristotle considered the heart to be the organ responsible for creating blood Plato considered the heart as the source of circulating blood and Hippocrates noted blood circulating cyclically from the body through the heart to the lungs. [82] [84] Erasistratos (304–250 BCE) noted the heart as a pump, causing dilation of blood vessels, and noted that arteries and veins both radiate from the heart, becoming progressively smaller with distance, although he believed they were filled with air and not blood. He also discovered the heart valves. [82]

The Greek physician Galen (2nd century CE) knew blood vessels carried blood and identified venous (dark red) and arterial (brighter and thinner) blood, each with distinct and separate functions. [82] Galen, noting the heart as the hottest organ in the body, concluded that it provided heat to the body. [84] The heart did not pump blood around, the heart's motion sucked blood in during diastole and the blood moved by the pulsation of the arteries themselves. [84] Galen believed the arterial blood was created by venous blood passing from the left ventricle to the right through 'pores' between the ventricles. [81] Air from the lungs passed from the lungs via the pulmonary artery to the left side of the heart and created arterial blood. [84]

These ideas went unchallenged for almost a thousand years. [81] [84]

Pre-modern

The earliest descriptions of the coronary and pulmonary circulation systems can be found in the Commentary on Anatomy in Avicenna's Canon, published in 1242 by Ibn al-Nafis. [85] In his manuscript, al-Nafis wrote that blood passes through the pulmonary circulation instead of moving from the right to the left ventricle as previously believed by Galen. [86] His work was later translated into Latin by Andrea Alpago. [87]

In Europe, the teachings of Galen continued to dominate the academic community and his doctrines were adopted as the official canon of the Church. Andreas Vesalius questioned some of Galen's beliefs of the heart in De humani corporis fabrica (1543), but his magnum opus was interpreted as a challenge to the authorities and he was subjected to a number of attacks. [88] Michael Servetus wrote in Christianismi Restitutio (1553) that blood flows from one side of the heart to the other via the lungs. [88]

Modern

A breakthrough in understanding the flow of blood through the heart and body came with the publication of De Motu Cordis (1628) by the English physician William Harvey. Harvey's book completely describes the systemic circulation and the mechanical force of the heart, leading to an overhaul of the Galenic doctrines. [84] Otto Frank (1865–1944) was a German physiologist among his many published works are detailed studies of this important heart relationship. Ernest Starling (1866–1927) was an important English physiologist who also studied the heart. Although they worked largely independently, their combined efforts and similar conclusions have been recognized in the name "Frank–Starling mechanism". [7]

Although Purkinje fibers and the bundle of His were discovered as early as the 19th century, their specific role in the electrical conduction system of the heart remained unknown until Sunao Tawara published his monograph, titled Das Reizleitungssystem des Säugetierherzens, in 1906. Tawara's discovery of the atrioventricular node prompted Arthur Keith and Martin Flack to look for similar structures in the heart, leading to their discovery of the sinoatrial node several months later. These structures form the anatomical basis of the electrocardiogram, whose inventor, Willem Einthoven, was awarded the Nobel Prize in Medicine or Physiology in 1924. [89]

The first successful heart transplantation was performed in 1967 by the South African surgeon Christiaan Barnard at Groote Schuur Hospital in Cape Town. This marked an important milestone in cardiac surgery, capturing the attention of both the medical profession and the world at large. However, long-term survival rates of patients were initially very low. Louis Washkansky, the first recipient of a donated heart, died 18 days after the operation while other patients did not survive for more than a few weeks. [90] The American surgeon Norman Shumway has been credited for his efforts to improve transplantation techniques, along with pioneers Richard Lower, Vladimir Demikhov and Adrian Kantrowitz. As of March 2000, more than 55,000 heart transplantations have been performed worldwide. [91]

By the middle of the 20th century, heart disease had surpassed infectious disease as the leading cause of death in the United States, and it is currently the leading cause of deaths worldwide. Since 1948, the ongoing Framingham Heart Study has shed light on the effects of various influences on the heart, including diet, exercise, and common medications such as aspirin. Although the introduction of ACE inhibitors and beta blockers has improved the management of chronic heart failure, the disease continues to be an enormous medical and societal burden, with 30 to 40% of patients dying within a year of receiving the diagnosis. [92]

Symbolism

As one of the vital organs, the heart was long identified as the center of the entire body, the seat of life, or emotion, or reason, will, intellect, purpose or the mind. [93] The heart is an emblematic symbol in many religions, signifying "truth, conscience or moral courage in many religions—the temple or throne of God in Islamic and Judeo-Christian thought the divine centre, or atman, and the third eye of transcendent wisdom in Hinduism the diamond of purity and essence of the Buddha the Taoist centre of understanding." [93]

In the Hebrew Bible, the word for heart, lev, is used in these meanings, as the seat of emotion, the mind, and referring to the anatomical organ. It is also connected in function and symbolism to the stomach. [94]

An important part of the concept of the soul in Ancient Egyptian religion was thought to be the heart, or ib. The ib or metaphysical heart was believed to be formed from one drop of blood from the child's mother's heart, taken at conception. [95] To ancient Egyptians, the heart was the seat of emotion, thought, will, and intention. This is evidenced by Egyptian expressions which incorporate the word ib, such as Awi-ib for "happy" (literally, "long of heart"), Xak-ib for "estranged" (literally, "truncated of heart"). [96] In Egyptian religion, the heart was the key to the afterlife. It was conceived as surviving death in the nether world, where it gave evidence for, or against, its possessor. It was thought that the heart was examined by Anubis and a variety of deities during the Weighing of the Heart ceremony. If the heart weighed more than the feather of Maat, which symbolized the ideal standard of behavior. If the scales balanced, it meant the heart's possessor had lived a just life and could enter the afterlife if the heart was heavier, it would be devoured by the monster Ammit. [97]

The Chinese character for "heart", 心, derives from a comparatively realistic depiction of a heart (indicating the heart chambers) in seal script. [98] The Chinese word xīn also takes the metaphorical meanings of "mind", "intention", or "core". [99] In Chinese medicine, the heart is seen as the center of 神 shén "spirit, consciousness". [100] The heart is associated with the small intestine, tongue, governs the six organs and five viscera, and belongs to fire in the five elements. [101]

The Sanskrit word for heart is hṛd or hṛdaya, found in the oldest surviving Sanskrit text, the Rigveda. In Sanskrit, it may mean both the anatomical object and "mind" or "soul", representing the seat of emotion. Hrd may be a cognate of the word for heart in Greek, Latin, and English. [102] [103]

Many classical philosophers and scientists, including Aristotle, considered the heart the seat of thought, reason, or emotion, often disregarding the brain as contributing to those functions. [104] The identification of the heart as the seat of emotions in particular is due to the Roman physician Galen, who also located the seat of the passions in the liver, and the seat of reason in the brain. [105]

The heart also played a role in the Aztec system of belief. The most common form of human sacrifice practiced by the Aztecs was heart-extraction. The Aztec believed that the heart (tona) was both the seat of the individual and a fragment of the Sun's heat (istli). To this day, the Nahua consider the Sun to be a heart-soul (tona-tiuh): "round, hot, pulsating". [106]

In Catholicism, there has been a long tradition of veneration of the heart, stemming from worship of the wounds of Jesus Christ which gained prominence from the mid sixteenth century. [107] This tradition influenced the development of the medieval Christian devotion to the Sacred Heart of Jesus and the parallel veneration of the Immaculate Heart of Mary, made popular by John Eudes. [108]

The expression of a broken heart is a cross-cultural reference to grief for a lost one or to unfulfilled romantic love.

The notion of "Cupid's arrows" is ancient, due to Ovid, but while Ovid describes Cupid as wounding his victims with his arrows, it is not made explicit that it is the heart that is wounded. The familiar iconography of Cupid shooting little heart symbols is a Renaissance theme that became tied to Valentine's day. [93]

Animal hearts are widely consumed as food. As they are almost entirely muscle, they are high in protein. They are often included in dishes with other offal, for example in the pan-Ottoman kokoretsi.

Chicken hearts are considered to be giblets, and are often grilled on skewers: Japanese hāto yakitori, Brazilian churrasco de coração, Indonesian chicken heart satay. [109] They can also be pan-fried, as in Jerusalem mixed grill. In Egyptian cuisine, they can be used, finely chopped, as part of stuffing for chicken. [110] Many recipes combined them with other giblets, such as the Mexican pollo en menudencias [111] and the Russian ragu iz kurinyikh potrokhov. [112]

The hearts of beef, pork, and mutton can generally be interchanged in recipes. As heart is a hard-working muscle, it makes for "firm and rather dry" meat, [113] so is generally slow-cooked. Another way of dealing with toughness is to julienne the meat, as in Chinese stir-fried heart. [114]

Beef heart may be grilled or braised. [115] In the Peruvian anticuchos de corazón, barbecued beef hearts are grilled after being tenderized through long marination in a spice and vinegar mixture. An Australian recipe for "mock goose" is actually braised stuffed beef heart. [116]

Pig heart is stewed, poached, braised, [117] or made into sausage. The Balinese oret is a sort of blood sausage made with pig heart and blood. A French recipe for cœur de porc à l'orange is made of braised heart with an orange sauce.

Other vertebrates

The size of the heart varies among the different animal groups, with hearts in vertebrates ranging from those of the smallest mice (12 mg) to the blue whale (600 kg). [118] In vertebrates, the heart lies in the middle of the ventral part of the body, surrounded by a pericardium. [119] which in some fish may be connected to the peritoneum. [120]

The SA node is found in all amniotes but not in more primitive vertebrates. In these animals, the muscles of the heart are relatively continuous, and the sinus venosus coordinates the beat, which passes in a wave through the remaining chambers. Indeed, since the sinus venosus is incorporated into the right atrium in amniotes, it is likely homologous with the SA node. In teleosts, with their vestigial sinus venosus, the main centre of coordination is, instead, in the atrium. The rate of heartbeat varies enormously between different species, ranging from around 20 beats per minute in codfish to around 600 in hummingbirds [121] and up to 1200 bpm in the ruby-throated hummingbird. [122]

Double circulatory systems

  1. Pulmonary vein
  2. Left atrium
  3. Right atrium
  4. Ventricle
  5. Conus arteriosus
  6. Sinus venosus

Adult amphibians and most reptiles have a double circulatory system, meaning a circulatory system divided into arterial and venous parts. However, the heart itself is not completely separated into two sides. Instead, it is separated into three chambers—two atria and one ventricle. Blood returning from both the systemic circulation and the lungs is returned, and blood is pumped simultaneously into the systemic circulation and the lungs. The double system allows blood to circulate to and from the lungs which deliver oxygenated blood directly to the heart. [123]

In reptiles, the heart is usually situated around the middle of the thorax, and in snakes, usually between the junction of the upper first and second third. There is a heart with three chambers: two atria and one ventricle. The form and function of these hearts are different than mammalian hearts due to the fact that snakes have an elongated body, and thus are affected by different environmental factors. In particular, the snake's heart relative to the position in their body has been influenced greatly by gravity. Therefore, snakes that are larger in size tend to have a higher blood pressure due to gravitational change. This results in the heart being located in different regions of the body that is relative to the snake's body length. [124] The ventricle is incompletely separated into two-halves by a wall (septum), with a considerable gap near the pulmonary artery and aortic openings. In most reptilian species, there appears to be little, if any, mixing between the bloodstreams, so the aorta receives, essentially, only oxygenated blood. [121] [123] The exception to this rule is crocodiles, which have a four-chambered heart. [125]

In the heart of lungfish, the septum extends part-way into the ventricle. This allows for some degree of separation between the de-oxygenated bloodstream destined for the lungs and the oxygenated stream that is delivered to the rest of the body. The absence of such a division in living amphibian species may be partly due to the amount of respiration that occurs through the skin thus, the blood returned to the heart through the venae cavae is already partially oxygenated. As a result, there may be less need for a finer division between the two bloodstreams than in lungfish or other tetrapods. Nonetheless, in at least some species of amphibian, the spongy nature of the ventricle does seem to maintain more of a separation between the bloodstreams. Also, the original valves of the conus arteriosus have been replaced by a spiral valve that divides it into two parallel parts, thereby helping to keep the two bloodstreams separate. [121]

The fully divided heart

Archosaurs (crocodilians and birds) and mammals show complete separation of the heart into two pumps for a total of four heart chambers it is thought that the four-chambered heart of archosaurs evolved independently from that of mammals. In crocodilians, there is a small opening, the foramen of Panizza, at the base of the arterial trunks and there is some degree of mixing between the blood in each side of the heart, during a dive underwater [126] [127] thus, only in birds and mammals are the two streams of blood—those to the pulmonary and systemic circulations—permanently kept entirely separate by a physical barrier. [121]

Fish have what is often described as a two-chambered heart, [128] consisting of one atrium to receive blood and one ventricle to pump it. [129] However, the fish heart has entry and exit compartments that may be called chambers, so it is also sometimes described as three-chambered [129] or four-chambered, [130] depending on what is counted as a chamber. The atrium and ventricle are sometimes considered "true chambers", while the others are considered "accessory chambers". [131]

Primitive fish have a four-chambered heart, but the chambers are arranged sequentially so that this primitive heart is quite unlike the four-chambered hearts of mammals and birds. The first chamber is the sinus venosus, which collects deoxygenated blood from the body through the hepatic and cardinal veins. From here, blood flows into the atrium and then to the powerful muscular ventricle where the main pumping action will take place. The fourth and final chamber is the conus arteriosus, which contains several valves and sends blood to the ventral aorta. The ventral aorta delivers blood to the gills where it is oxygenated and flows, through the dorsal aorta, into the rest of the body. (In tetrapods, the ventral aorta has divided in two one half forms the ascending aorta, while the other forms the pulmonary artery). [121]

In the adult fish, the four chambers are not arranged in a straight row but instead form an S-shape, with the latter two chambers lying above the former two. This relatively simple pattern is found in cartilaginous fish and in the ray-finned fish. In teleosts, the conus arteriosus is very small and can more accurately be described as part of the aorta rather than of the heart proper. The conus arteriosus is not present in any amniotes, presumably having been absorbed into the ventricles over the course of evolution. Similarly, while the sinus venosus is present as a vestigial structure in some reptiles and birds, it is otherwise absorbed into the right atrium and is no longer distinguishable. [121]

Invertebrates

Arthropods and most mollusks have an open circulatory system. In this system, deoxygenated blood collects around the heart in cavities (sinuses). This blood slowly permeates the heart through many small one-way channels. The heart then pumps the blood into the hemocoel, a cavity between the organs. The heart in arthropods is typically a muscular tube that runs the length of the body, under the back and from the base of the head. Instead of blood the circulatory fluid is haemolymph which carries the most commonly used respiratory pigment, copper-based haemocyanin as the oxygen transporter. Haemoglobin is only used by a few arthropods. [132]

In some other invertebrates such as earthworms, the circulatory system is not used to transport oxygen and so is much reduced, having no veins or arteries and consisting of two connected tubes. Oxygen travels by diffusion and there are five small muscular vessels that connect these vessels that contract at the front of the animals that can be thought of as "hearts". [132]

Squids and other cephalopods have two "gill hearts" also known as branchial hearts, and one "systemic heart". The branchial hearts have two atria and one ventricle each, and pump to the gills, whereas the systemic heart pumps to the body. [133] [134]


New Understanding of the Heart's Evolution

Humans, like other warm-blooded animals, expend a lot of energy and need a lot of oxygen. Our four-chambered hearts make this possible. It gives us an evolutionary advantage: We're able to roam, hunt and hide even in the cold of night, or the chill of winter.

Now scientists have a better understanding how the complex heart evolved.

The story starts with frogs, which have a three-chambered heart that consists of two atria and one ventricle. As the right side of a frog's heart receives deoxygenated blood from the body, and the left side receives freshly oxygenated blood from the lungs, the two streams of blood mix together in the ventricle, sending out a concoction that is not fully oxygenated to the rest of the frog's body.

Turtles are a curious transition &mdash they still have three chambers, but a wall, or septum is beginning to form in the single ventricle. This change affords the turtle's body blood that is slightly richer in oxygen than the frog's.

Birds and mammals, however, have a fully septated ventricle &mdash a bona fide four-chambered heart. This configuration ensures the separation of low-pressure circulation to the lungs, and high-pressure pumping into the rest of the body.

But not all humans are so lucky to have an intact, four-chambered heart. At one or two percent, congenital heart disease is the most common birth defect. And a large portion of that is due to VSD, or ventricular septum defects. The condition is frequently correctable with surgery.

Benoit Bruneau of the Gladstone Institute of Cardiovascular Disease has honed into the molecular forces at work. In particular, he studies the transcription factor, Tbx5, in early stages of embryological development. He calls Tbx5 "a master regulator of the heart."

Scott Gilbert of Swarthmore College and Juli Wade of Michigan State University study evolutionary developmental biology of turtles and anole lizards respectively. When Bruneau teamed up with them, he was able to examine a wide evolutionary spectrum of animals. He found that in the cold-blooded, Tbx5 is expressed uniformly throughout the forming heart's wall. In contrast, warm-blooded embryos show the protein very clearly restricted to the left side of the ventricle. It is this restriction that allows for the separation between right and left ventricle.

Interestingly, in the turtle, a transitional animal anatomically &mdash with a three-chambered, incompletely septated heart, the molecular signature is transitional as well. A higher concentration of Tbx5 is found on the left side of the heart, gradually dissipating towards the right.

"The great thing about looking backwards like we've done with reptilian evolution is that it gives us a really good handle on how we can now look forward and try to understand how a protein like Tbx5 is involved in forming the heart and how in the case of congenital heart disease its function is impaired," Bruneau said.


Ventricular pathology

The ultimate function of the heart is to get blood ejected from the ventricles into either systemic or pulmonary circuits. Unfortunately, both congenital and acquired disorders can directly affect the ventricles, resulting in suboptimal activity. While some of the pathologies are amenable to surgical intervention, other disorders are irreversible and are associated with life-long morbidity and mortality. Below is a list of congenital and acquired disorders that affect the ventricles. Not all of the disorders listed below will be addressed in this article.

Classifications of ventricular disorders
Congenital ventricular disorders Ventricular septal defect
Double outlet right ventricle
Hypoplastic right ventricle
Acquired ventricular disorders Ventricular hypertrophy (left and right)
Ventricular pseudoaneurysm
Ventricular arrhythmias - Ventricular tachycardia, Ventricular flutter, Ventricular fibrillation, Torsade de Pointe
Bundle branch block

Congenital ventricular disorders

Congenital ventricular disorders – much like other forms of congenital cardiac anomalies – occur early in development. They can arise either in isolation (hypoplastic ventricles) or as a part of a syndrome (tetralogy of Fallot). Furthermore, the cause may be a random genetic mutation or one precipitated by exposure to a teratogen (a drug that can cause a mutation).

Ventricular septal defect

A ventricular septal defect is an abnormal communication between the two ventricles via a perforation in the interventricular septum. It can be observed as a stand-alone defect or in association with syndromic disorders such as tetralogy of Fallot, transposition of the great arteries, or other concomitant atrioventricular septal defects. The major concern with ventricular septal defects is the fact that there is mixing of oxygenated and deoxygenated blood. This concern is minimal as long as blood flow is predominantly from the left side to the right (left to right shunt). However, chronic exposure of the right ventricle to the high pressures of the left ventricle results in subsequent right ventricular hypertrophy and eventually reversal of the shunt (right to left). This leads to predominantly deoxygenated blood entering systemic circulation. The patient may develop symptoms of hypoxia and their sequelae.

Ventricular septal defects rarely occur following an acquired etiology. One example of this acquired disorder is following a myocardial infarction (heart attack). Irrespective of the location, the walls of the heart are susceptible to rupture following an infarction. This phenomenon is based on the fact that as the myocardium heals, dead muscle cells are replaced with fibrous tissue which is unable to adequately accommodate the fluctuations in pressure. Therefore the affected walls are at risk of rupture.

Acquired ventricular disorders

There is a limited list of pathologies that can affect the ventricles of the heart. However, when they do occur, they can have a lasting effect on the heart due to the poor healing capability of cardiomyocytes (heart muscle cells).

Ventricular hypertrophy

Like all muscle cells, cardiomyocytes increase in size following an increasing workload. In the case of the heart, the increased workload comes in the form of increased resistance to blood flow. In the systemic circulatory pathway, this is commonly due to vascular narrowing as a result of atherosclerosis, where blood vessels become less compliant due to the excess plaque build-up in the arterial walls. Therefore higher pressures are needed to overcome the systemic resistance in order to maintain adequate blood flow. The left ventricle increases in size in response to the increased systemic resistance (left ventricular hypertrophy). Another cause of left ventricular hypertrophy includes aortic valvulopathy. Unfortunately, as the left ventricle continues to increase in size, it becomes less efficient. Eventually left ventricular output becomes suboptimal and the patient begins to experience symptoms of left ventricular failure (left heart failure).

Similarly, the right ventricle can undergo hypertrophic changes following an increase in its workload. Pulmonary hypertension, pulmonary valve stenosis, tricuspid valve regurgitation, and left heart failure can all increase the demand on the right ventricle, resulting in ventricular hypertrophy.

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Is a one ventricle heart feasible? - Biology

Left ventricular volumes (LVVs) and ejection fraction (LVEF) are key elements in the evaluation and follow-up of patients with heart failure with reduced ejection fraction (HFrEF). Therefore, a feasible and reproducible imaging method to be used by both experienced and in-training echocardiographers is mandatory. Our aim was to establish if, in a large echo lab, echocardiographers in-training provide feasible and more reproducible results for the evaluation of patients with HFrEF when using 3-dimensional echocardiography (3-DE) versus 2-dimensional echocardiography (2-DE). Sixty patients with HFrEF (46 males, age: 58 ± 17 y) underwent standard transthoracic 2-D acquisitions and 3-D multibeat full volumes of the left ventricle. One expert user in echocardiography (expert) and three echocardiographers with different levels of training in 2-DE (beginner, medium and advanced) measured the 2-D LVVs and LVEFs on the same consecutive images of patients with HFrEF. Afterward, the expert performed a 1-mo training in 3-DE analysis of the users, and both the expert and trainees measured the 3-D LVVs and LVEF of the same patients. Measurements provided by the expert and all trainees in echo were compared. Six patients were excluded from the study because of poor image quality. The mean end-diastolic LVV of the remaining 54 patients was 214 ± 75 mL with 2-DE and 233 ± 77 mL with 3-DE. Mean LVEF was 35 ± 10% with 2-DE and 33 ± 10% with 3-DE. Our analysis revealed that, compared with the expert user, the trainees had acceptable reproducibility for the 2-DE measurements, according to their level of expertise in 2-DE (intra-class coefficients [ICCs] ranging from 0.75 to 0.94). However, after the short training in 3-DE, they provided feasible and more reproducible measurements of the 3-D LVVs and LVEF (ICCs ranging from 0.89–0.97) than they had with 2-DE. 3-DE is a feasible, rapidly learned and more reproducible method for the assessment of LVVs and LVEF than 2-DE, regardless of the basic level of expertise in 2-DE of the trainees in echocardiography. In echo labs with a wide range of staff experience, 3-DE might be a more accurate method for the follow-up of patients with HFrEF.