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Why do you die if you cannot breathe?

Why do you die if you cannot breathe?


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I was wondering what the actual reason for death by suffocation is. Obviously it is related to oxygen deprivation. But what is the underlying cause of death?

  • Is it due to insufficient oxygen for aerobic respiration, and the resulting lack of ATP (as anaerobic respiration forms less ATP)?
  • Or is it because of the build-up of lactic acid, causing enzymes to denature?
  • Or is it the lowering of the pH of the blood because of a build-up of carbon dioxide which forms carbonic acid and subsequent denaturation of enzymes?

Short answer
This is a difficult question to answer. As far as I am aware, asphyxiation results in excitotoxicity, which causes unconsciousness, brain damage and eventually, death.

Background
Asphyxia is a condition of the body that occurs from severely inadequate oxygen supply, or because of excessive carbon dioxide in the body (First Aid and CPR courses). The brain is the organ most sensitive to hypoxia (Medscape). Nerve cells in the brain can survive only up to four minutes without oxygen (First Aid and CPR courses). Consciousness is lost after approximately three minutes (Forensic Pathology). Permanent brain damage begins after approximately 4 minutes without oxygen, and death can occur as soon as 4 to 6 minutes later (Medline). Asphyxial deaths typically involve respiratory arrest with bradycardia / asystole (low heart rate / cardiac arrest) because of the hypoxia-induced dysfunction of the respiratory centers in the brainstem (Forensic Pathology).

Despite the small size of the brain (2% of body weight), it is, however, the largest consumer of total body oxygen (20%) and glucose (25%), which are delivered by 15% of the total cardiac output (Schur & Rigor, 1998).

A lack of oxygen may result in so called hypoxic-ischemic encephalopathy (HIE), i.e., neuronal cell death due to hypoxia. Its pathophysiology is related to the lack of energy because cellular respiration diminishes. This initially causes neurons to stop firing, and eventually in an arrest of cellular functions and cell death. Even sublethal HIE can set in motion a series of toxic reactions that kills injured neurons and even neurons that have not been damaged during the initial insult. Thus, following global brain ischemia, neurons do not die suddenly or all at once. In some of them, damage develops hours or days after the insult. Most neurons undergo necrosis. In some neurons, HIE triggers apoptosis (Neuropathology).

Specifically, energy depletion is believed to result in a failure of the Na+,K+ ATPase, leading to depolarization of the neuronal membrane (Fig. 1). Synaptic function and conductivity cease at this point. Depolarization causes neurons to release glutamate (Glu) into the synaptic cleft. Glu is the most common excitatory neurotransmitter. In small amounts, it is indispensable for neuronal function. In excessive amounts, however, it is neurotoxic. Some Glu receptors, such as the NMDA and AMPA receptors, are non-selective cation-permeable ion channels. Initially, over-activation of these channels causes a passive influx of Cl- (and Na+) into cells causing osmotic (cytotoxic) edema and rapid death (Neuropathology).

Additional structural damage develops hours or even days later as a result of Ca2+ influx into neurons through NMDA and AMPA receptors (Fig. 1). This delayed cell death is caused by an over-activation of NMDA and AMPA receptors by the excessive release of glutamate, which causes massive influx of Ca2+ into neurons. Ca2+ activates catabolic enzymes (proteases, phospholipases, endonucleases), and also NO synthase. NO is a free radical and other free radicals are generated due to the impairment of oxidative phosphorylation. Free radicals and activated catabolic enzymes destroy structural proteins, membrane lipids, nucleic acids, and other cellular contents, causing neuronal necrosis. DNA damage from endonucleases and mitochondrial injury from free radicals trigger apoptosis (Neuropathology). Collectively, these effects are referred to as excitotoxicity (Choi, 1992). When enough brain cells die, the person perishes with them.


Fig. 1. Schematic showing the excititoxic effects of excess glutamate in the brain. Source: Neuropathology.

References
- Choi, J Neurobiol (1992); 23(9): 1261-76
- Schur & Rigor, Dev Neurosci (1998); 20: 348-357


Laryngospasm may be associated with different triggers, such as asthma, allergies, exercise, irritants (smoke, dust, fumes), stress, anxiety or commonly gastroesophageal reflux disease, or GERD. GERD is a condition that occurs when the ring-like muscle that normally closes to keep the stomach's contents from backing up doesn't work right. With reflux, harsh acids from the stomach rise up into the esophagus and cause irritation.

Regular exposure to stomach acids can damage and inflame the delicate lining of the esophagus. This damage can lead to momentary spasms of the vocal cords, which close the airway and prevent air and oxygen from getting into the lungs.

Continued

When stomach acids reach the larynx, the condition is called laryngopharyngeal reflux or LPR. The tissues of the larynx are even more delicate and prone to injury than the esophagus. Coughs from a cold can push more acid into the larynx, so a recent or current upper respiratory infection may increase the likelihood of developing laryngospasm.

Laryngospasm may also be a complication of surgery. Anesthesia used during the surgery can irritate the vocal cords, especially in children. Laryngospasm caused by anesthesia can be life-threatening.


Causes of Breathing Stopping During Sleep

Sleep-related breathing disturbances are fairly common. The most familiar one to most people is snoring. The characteristic sound is caused by vibration in the tissues of your upper airway while you breathe.

It is also possible for you to completely stop breathing for a while. These breathing pauses are called sleep apnea, from the Greek for “no breath.” By definition, apnea events last at least 10 seconds, but they can stretch on for several minutes.

The most common cause of apnea is the sleep disorder known as obstructive sleep apnea (OSA). OSA occurs when the tissues of the upper airway—the tongue, soft palate, and uvula—collapse into the throat and block the normal airflow.

Your body may still make an effort to breathe, with the chest and abdomen moving, but the air can't get past the obstruction. As a result, airflow through your nose and mouth is reduced or cut off during these periods.

Other potential causes of disturbed breathing during sleep are less common. They include:

    : Pauses in breathing are caused by the brain temporarily failing to tell the respiratory muscles to work. This can be caused by a problem with the brainstem, severe obesity, and medications including opioid painkillers.
  • Cheyne-Stokes respiration: Alternating heavy and shallow breathing and pauses in breathing are associated with severe heart failure and neurological disorders including dementia.
  • Congenital central hypoventilation syndrome (Ondine's curse): Shallow breathing, especially during sleep, leads to an oxygen shortage and excess carbon dioxide in the blood. The condition is usually due to nervous system impairment.

What Is a Ventilator?

A ventilator is a machine that helps a person breathe.

Patients who have a medical problem that makes it hard for them to breathe well on their own or are undergoing anesthesia for surgery may be connected to a ventilator. Often, a person who is on a ventilator will receive medicine that makes them sleepy so the ventilator does the work of breathing. It allows the body to rest so it can heal.

A ventilator works similar to the lungs. It pushes a pulse of air into the lungs, as air would enter the lungs during an inhale. The ventilator can give more oxygen to the lungs than when a person breathes air. The ventilator also allows the air to come out of the lungs, as the lungs would do during exhalation. In this way, the person can receive the oxygen needed to keep all their organs alive, when their lungs are injured and not working properly.

People can remain conscious while on a ventilator. However, they may experience discomfort and may need medication to help them be more comfortable. Also, people usually cannot eat while on a ventilator, but they can receive nutrition from a tube that goes from their nose to their stomach.


If water is made up of hydrogen and oxygen, why can't we breathe underwater?

One thing about chemicals is that, once they react in certain ways, they form compounds that are nothing like the original elements. For example, if you react carbon, hydrogen and oxygen together one way you get glucose (C6H12O6) (see How Food Works). If you react them together another way you get vinegar (C2H4O2). If you react them another way you get fat (see How Fat Works). If you react them another way you get ethanol (C2H5OH). Glucose, fat, ethanol and vinegar are nothing like each other, but they are all made from the same elements.

In the case of hydrogen and oxygen gas, if you react them together one way you get liquid water (H2O). The reason we cannot breathe liquid water is because the oxygen used to make the water is bound to two hydrogen atoms, and we cannot breathe the resulting liquid. The oxygen is useless to our lungs in this form.

The oxygen that fish breathe is not the oxygen in H2O. Instead, the fish are breathing O2 (oxygen gas) that is dissolved in the water. Many different gases dissolve in liquids, and we see an example all the time in carbonated beverages. In these beverages, there is so much carbon dioxide gas dissolved in water that it rushes out in the form of bubbles.

Fish "breathe" the dissolved oxygen out of the water using their gills. It turns out that extracting the oxygen is not very easy -- air has something like 20 times more oxygen in it than the same volume of water. Plus water is a lot heavier and thicker than air, so it takes a lot more work to move it around. The main reason why gills work for fish is the fact that fish are cold-blooded, which reduces their oxygen demands. Warm-blooded animals like whales breath air like people do because it would be hard to extract enough oxygen using gills.

Humans cannot breathe underwater because our lungs do not have enough surface area to absorb enough oxygen from water, and the lining in our lungs is adapted to handle air rather than water. However, there have been experiments with humans breathing other liquids, like fluorocarbons. Fluorocarbons can dissolve enough oxygen and our lungs can draw the oxygen out -- see the last link below for some fascinating details!


Respiratory muscles

The lungs have no skeletal muscles of their own. The work of breathing is done by the diaphragm, the muscles between the ribs (intercostal muscles), the muscles in the neck, and the abdominal muscles.

The diaphragm, a dome-shaped sheet of muscle that separates the chest cavity from the abdomen, is the most important muscle used for breathing in (called inhalation or inspiration). The diaphragm is attached to the base of the sternum, the lower parts of the rib cage, and the spine. As the diaphragm contracts, it increases the length and diameter of the chest cavity and thus expands the lungs. The intercostal muscles help move the rib cage and thus assist in breathing.

The process of breathing out (called exhalation or expiration) is usually passive when a person is not exercising. The elasticity of the lungs and chest wall, which are actively stretched during inhalation, causes them to return to their resting shape and to expel air out of the lungs when inspiratory muscles are relaxed. Therefore, when a person is at rest, no effort is needed to breathe out. During vigorous exercise, however, a number of muscles participate in exhalation. The abdominal muscles are the most important of these. Abdominal muscles contract, raise abdominal pressure, and push a relaxed diaphragm against the lungs, causing air to be pushed out.

The muscles used in breathing can contract only if the nerves connecting them to the brain are intact. In some neck and back injuries, the spinal cord can be severed, which breaks the nervous system connection between the brain and the muscles, and the person will die unless artificially ventilated.

Diaphragm’s Role in Breathing

When the diaphragm contracts and moves lower, the chest cavity enlarges, reducing the pressure inside the lungs. To equalize the pressure, air enters the lungs. When the diaphragm relaxes and moves back up, the elasticity of the lungs and chest wall pushes air out of the lungs.


How Do You Die When You’re Buried Alive?

Beatrix Kiddo emerged with just a few bloody knuckles after punching her way out of her own grave in Kill Bill 2. That degree of strength and perseverance, though useful six feet under, unfortunately exists solely in the realm of vengeful Tarantino heroines. The run-of-the-mill vivisepulture victim dies, but the process of slipping the mortal coil isn’t as bad as you might think.

Imagining how a person succumbs to death when they’re buried alive has provided fodder for depraved imaginations throughout the centuries. Poe, the OG sicko, wrote The Premature Burial during the height of the 19th-century cholera epidemic, which birthed a general phobia of being buried alive. In response, “safety coffins,” equipped with little bells for the mistakenly buried to alert the living, rose in popularity. While gravedigger error has been greatly reduced since then, our taphephobia has not, thanks in part to our morbid fascination with how premature burial actually kills us. In the spirit of Halloween, Inverse dug into three live burial scenarios.

Buried in a Coffin

In the classic buried alive scenario, asphyxiation is most likely to be the cause of death. The trick to slowing down the suffocation process is taking slow and shallow breaths, but that won’t be easy once you start to panic. Conserving the air you haven’t displaced with your body is key. On average, a person’s volume is 66 L, and the average casket holds 886 L: The leftover 820 L of air, 164 L of which is oxygen, is yours to ration.

If, despite being pretty stressed out, the grounded party manages to breathe like an average resting adult, their body will convert oxygen at a rate of about 550 L per day day, or 23 L an hour. That means someone in a coffin has seven hours to make a move.

Of course, the next move is likely no move at all, because there really isn’t any escape. As carbon dioxide replaces the last sips of life-bringing oxygen, blackout and coma ensue. The heart stop beatings not too long after.

Just Plain Buried

Maybe your murderers are in a rush. Or maybe they’re just cheap. Tossing a body into a grave without a coffin still counts as being buried alive. Assuming you’re in a pit meant for a funeral — six feet deep and coffin-sized — you’ll be buried with about 2,775 L of soil on top of you — a sweet 3697 lbs of dirt.

Unless all of the soil is replaced at once, the victim is unlikely to break any bones as the grave is refilled. What will happen is that the weight of the dirt will slowly constrict the chest, making it harder to breathe. As things start to go fuzzy — oxygen is in short supply — the mouth and nostrils will fill with soil, making breathing the air available between particulates impossible.

Avalanche

Because hypothermia is a serious issue, few avalanche victims survive past 25 minutes.

Brain damage sets in about 10 minutes after snow encompasses the body so thinking clearly in an avalanche situation is nigh impossible — and that doesn’t even matter much because as extremities freeze they become unresponsive. Avalanche beacons and teamwork really are the only way out. It’s only a matter of minutes until consciousness slips away. Brain activity soon follows.


Biological View: What Do Dying People Feel?

Have you ever wondered what a dying person feels? It sounds morbid, but science and accounts from lucky survivors have helped us make an idea about it. Still, only the dead know exactly how it feels and the sensations you experience at that moment.

A report released by New Scientist earlier this month answered to some questions related to the feelings and sensations a person experiences in various ways of dying. But what's the medical/biological support for all these aspects? Read on!

Generally, a person is considered dead when blood circulation (translated in heart activity) has stopped. This is the clinical death, but in many cases, modern technology has permitted the restart / recovery of heart activity - such methods include cardiopulmonary resuscitation (CPR), defibrillation, epinephrine shots. So perhaps it would be more biologically correct to say that the person has died when the brain is dead.

Still, the only way to instantly kill the brain is by shooting a bullet into someone's head. There are other ways in which the brain is killed more slowly, mainly by stopping its blood supply. The brain can function for a while without nutrients, but the brain cells (neurons) die in a matter of seconds when they lack oxygen. In fact, this is called a stroke: the death of some brain portions.

In most cases of death, the victim's brain is killed ? 'indirectly' (so to say), by stopping the heart (the so-called heart attack). The heart attack can be spontaneous, caused by heart coronary disease (when the heart muscle is deprived of its circulation, and portions of it die). The main symptom is angina pectoris, the famous chest pain, a sensation of pain and squeezing radiating mainly to the left arm but also to all the surrounding areas, accompanied by massive sweating, nausea and palpitations. The asphyxiated brain loses consciousness in a maximum of 10 seconds, and minutes later the brain dies.

Decapitation was believed to be rapid and painless, but it takes about 7 seconds for the brain to lose consciousness. Eye and facial expressions (grimaces) still persist for 30 seconds after the head has been cut off, which leads us to probably the most famous such case in world history, that of the French queen Marie Antoinette - what could the grimace on the face of this unlucky queen (guillotined during the French revolution) mean? That famous smile was surely not one of happiness.

Lethal injection is considered a 'humane' alternative to the electric chair and attempts to stop the heart while stopping the functions of the brain through three active principles: an anesthetic (like sodium thiopental), a paralytic agent (like pancuronium bromide) and potassium chloride (ultrashort-acting sedative). A mixture of these should be redundant: if one chemical does not kill the inmate, one of the other two will do it. Many people believe that the potassium chloride injection causes burning pain, but the paralyzed convicted cannot show what he/she feels. If the dosage is not the right one, or if the one performing the injection misses the vein, the victim will not only feel a burning sensation caused by the chloride, but it will take him up to 9 minutes to die of asphyxiation due to the blocked breath muscles.

Hanging should be an unconsciousness asphyxiation, occurring in 10 seconds: the convicted should be left unconscious and paralyzed by choking, having the neck broken from the first cervical vertebra, and death should come in a matter of seconds, maximum 2 minutes. The reality is more shocking: over three quarters of the convicted die after a torture of many minutes, during which they desperately try to breathe.

Judicial hangings, opposed to suicides, cause a significant damage to the spinal cord. When the fall is longer than predicted, the victim may even be decapitated. Sometimes, intense fear can induce a cardiac arrest to the convicted. It was quite an offence for royalties to be hanged, as the victim loses control of its sphincters (anal and urethral).

Drowning is in the end also a type of asphyxiation. The panicked victim tries to hold his breath, but they generally live up to 90 seconds without breathing, and even diving champions cannot resist over 6 minutes. The human brain is highly intolerant to the accumulation of carbon dioxide (a result of the cell respiration) in the blood irrigating the brain, and the urge for breathing cannot be stopped. Drowning survivors described a "tearing and burning" sensation when water floods the lungs, followed by a feeling of tranquility, as the brain can no longer sustain high activity. What's next is the same scenario: the lack of oxygen leads to consciousness loss, heart attack, and brain death.

Blood loss, called hemorrhagic shock in medicine, produces in the end the same effect: no oxygen for the cells. That's why victims that are massively bleeding breathe so heavily: the lungs are trying to send more oxygen to the oxygen-hungry body.

A 70 kg (180 pounds) man has about 5.6 liters of blood (8% of the body weight). Losing 1.5 liters of blood makes the individual feel weak, since all the blood has gone from the organs (including muscles) towards the lungs and heart, which try to compensate for the oxygen deficit. The adrenalin released to increase blood pressure in the arteries causes anxiety. Arginine vasopressin released to keep the water in the kidneys (the organism loses a lot of water through blood) makes the individual feel thirsty.

If the blood loss is not stopped, and the body has already lost 2 liters of blood, these compensating mechanisms fail. The ion balance is destroyed, and the blood flow through capillaries is constricted. The cells can no longer function, and fluid and protein leakage out of the cells occurs. At this stage, even if the victim receives blood, the prolonged vasoconstiction has caused irremediable damage. The unoxygenated brain falls into dizziness and confusion, followed by unconsciousness.

If the aortic or pulmonary artery dissection has taken place, pain similar to heart attack can occur. Electrocution through a household device can paralyze the heart (heart attack), interfering with its own electric signal coordinating the contraction of the heart muscle. Much powerful shocks go beyond electrical impairment of the body, destroying tissues. Damage to the heart tissue means heart attack, but to the brain it causes rapid unconsciousness, if not instant death. Those executed on the electric chair can even experience tissue burning and paralysis of the breath muscles, of course, through asphyxiation. When falling from a height, the heart does not stop, it breaks into pieces. Usually, the ribs break in many pieces, which stab all the organs around. The thoracic shock, causing instant death, prevails amongst suicide jumpers. Survivors of such incidents describe the feeling of time slowing down.

Burning the heretics was believed to be the worst kind of death during the Medieval Ages. But a sufficiently big fire kills the victims before them being touched by the flames, as the smoke gases, especially carbon monoxide, combine with the hemoglobin - the blood's red pigment carrying oxygen to tissues. The hemoglobin can no longer transport the oxygen to the tissues, and even if the victim is taken out of the burning building, or saved by firemen, he/she will die since the brain cannot be oxygenated.

If the flames touch the skin of the conscious victim, they will induce tremendous pain, which slowly decreases with the destruction of the skin nerves, but persists, as inner tissues have pain sensors too. Carbon monoxide is also the main culprit for sleep asphyxiation death cases caused by broken stoves (the gas is the result of incomplete burning of wood or any other fuel). This is perhaps the less painful death, also employed by suicidal people using car smoke generated in a closed room.

Explosive decompression means a sudden (less than 0.1 seconds) air pressure drop, caused by violent explosion, like in the case of a contained system (inner airplane) exposed in a moment to outer atmosphere or explosions caused by gas accumulation.

No, people do not ?explode? in such cases. The exposure to low pressure causes swelling, but our skin is elastic and resistant and can cope with a drop of one atm. (the required drop for killing a person is of about 8 atmospheres). Survivors of such events reported chest pain, similar to the one you feel when someone hits you hard (due to the swelling of the lungs), and even the feeling of air going out of the lungs. If oxygenation is impeded, the individual loses consciousness in 15 seconds and dies of asphyxiation.


What muscles do you use to breathe?

Your main breathing muscle is the diaphragm. This divides your chest from your abdomen.

Your diaphragm contracts when you breathe in, pulling the lungs down, stretching and expanding them. It then relaxes back into a dome position when you breathe out, reducing the amount of air in your lungs.

When you exercise, your abdominal muscles are used to push air out of the lungs when you breathe out. This is called forced expiration.

There are also muscles in between the ribs, which keep the ribcage stiff and help with breathing. These are called intercostal muscles.

Breathing in:

Healthy lung tissue is springy and elastic, so your muscles need to work to expand your chest and draw air into your lungs.

Signals from the respiratory centre in your brain travel down nerves to your diaphragm and other muscles. The diaphragm is pulled flat, pushing out the lower ribcage and abdomen. At the same time, the muscles between your ribs pull your rib cage up and out. This expands the chest and draws air into the lungs.

Air is pulled into your nose or mouth, and into your windpipe. This divides into airways supplying the left and right lungs.

The air passes down the airways, which divide another 15 to 25 times, and finally into thousands of smaller airways until the air reaches the air sacs.

Breathing out:

At rest, breathing out is mostly a passive process. The muscles you use to breathe in now relax and your elastic lungs push air out. When you exercise and your body needs to move air more quickly, your abdominal muscles provide the main drive for exhaling. The intercostal muscles also help.

The system works so that you breathe in and out comfortably at rest where the least effort is required to move air – and you’re probably not conscious of your breathing. When you exercise, you need to move more air. To do this you can take bigger breaths or breathe more quickly – usually both.

Although breathing is usually automatic, you can control it if you want to - when you talk or sing for example.

Last medically reviewed: February 2021. Due for review: February 2024

This information uses the best available medical evidence and was produced with the support of people living with lung conditions. Find out how we produce our information. If you’d like to see our references get in touch.

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On this page we explain the importance of your lungs, what can damage them and what you can do to protect your lungs.


Why do we die (so quickly) due to lack of oxygen? Can all our cells not respire anaerobically like our muscles can?

If our cells can respire anaerobically, why do we die (so quickly) when suffocated? Wouldn't the cells just start respiring anaerobically until oxygen was re-supplied so the lactic acid could be broken down?

I'm assuming here that all cells can respire anaerobically like muscle cells can.

I think this is the best answer so far. The point is that not all of your cells die quickly without oxygen. In surgeries they cut off blood supplies for up to hours and tissues don't die.

Your brain is really what needs the oxygen. It has a huge metabolic requirement, and it needs a constant supply of oxygen for cellular respiration. Your other tissues are fine after a couple minutes, but it doesn't matter because once your brain shuts down, you die.

Is there a reason they don't just "sleep"? What causes them to die so fast?

What about other nerve cells? Do they die just as fast?

Initially, energy in cells is provided by available ATP. After this is depleted (1-2 seconds) anaerboic energy systems take over. First, the phosphogen system (ATP-CP or PC system) provides ATP for 10-15 more seconds by borrowing a phosphate group from a phosphocreatine molecule. After this energy system is depleted the the lactic acid system (anaerobic glycolysis) provides ATP, it is not as efficient as aerobic glycolysis and only provides energy for

2 minutes. So, after 2-3 minutes cellular currency (ATP) is all used up, normal cell processes stop working, and then the body dies.

EDIT: Can someone w/ more knowledge clarify the phosphogen system. I know muscle cells possess a degree of stored PC, do other types of cells also utilize this system? Thanks in advance.

So they do respire anaerobically, however it only lasts for a short while due to the inefficiency. Thank-you very much, that's been bugging me for a while.

Phosphocreatine can anaerobically donate a phosphate group to ADP to form ATP during the first 2 to 7 seconds following an intense muscular or neuronal effort. Conversely, excess ATP can be used during a period of low effort to convert creatine to phosphocreatine. The reversible phosphorylation of creatine (i.e., both the forward and backward reaction) is catalyzed by several creatine kinases. The presence of creatine kinase (CK-MB, MB for muscle/brain) in plasma is indicative of tissue damage and is used in the diagnosis of myocardial infarction.[1] The cell's ability to generate phosphocreatine from excess ATP during rest, as well as its use of phosphocreatine for quick regeneration of ATP during intense activity, provides a spatial and temporal buffer of ATP concentration. In other words, phosphocreatine acts as high-energy reserve in a coupled reaction the energy given off from donating the phosphate group is used to regenerate the other compound - in this case, ATP. Phosphocreatine plays a particularly important role in tissues that have high, fluctuating energy demands such as muscle and brain.

What's going on when people hold their breath for up to 10 minutes like some record breakers have? Also many free divers can hold it for 4-5 minutes or more. Do their cells work differently/slower to prevent them from dying, or do they just remain conscious while cells are still dying?

Isn't the buildup of lactic acid also harmful? I thought that would kill you first.

Others already commented on the crucial dependency of brain cells on oxygen. I'll add a short quote that lists some of the (very rare) exceptions to this rule: vertebrate species that can survive anoxia (total lack of oxygen). The quote is from the introduction to the book The Brain Without Oxygen (Lutz, Nilsson & Prentice):

"the dependence of the brain on an uninterrupted supply of oxygen is not just a human phenomenon, it is, in fact, common to all vertebrates – including fish, reptiles and birds comparatively few species can withstand severe hypoxia and almost none can survive chronic anoxia lasting more than minutes. There are however exceptions. The epaulette shark (Hemiscyllium ocellatum), and common frog (Rana sp) can tolerate anoxia for several hours. A few species are truly anoxia tolerant in that they can survive anoxia from days to months. Among the fishes these include the crucian carp (Carassius carassius) and goldfish (C. auratus), and in the reptiles the freshwater turtles Chrysemys picta and Trachemys scripta. The crucian carp can live in anoxic water for months at temperatures close to freezing. Similarly the turtle, Trachemys scripta, can withstand anoxia for 48 h at 20 °C and at least 3 months at 3 °C."

Most of the book is in fact devoted to a detailed look at the adaptions that allow these species to survive without oxygen for a long time. They all reduce their metabolic rate drastically some of them go into a comatose state, while others are able to maintain limited physical activity.

You are correct, not all cells have equal ability to survive in low oxygen. All cells have inbuilt mechanisms to restore homeostasis when subject to an insult or injury (such as hypoxia). When compensatory mechanisms cannot keep up with insult, you start to get injured.

In hypoxia there are a series of changes that at first are reversible (cell swelling,mitochondrial permeability change, fatty depsoition). But as hypoxia persists these changes become irreversible (loss of membrane integrity, mitochondrial death, protease activation). The root of these changes is that you have less ATP to maintain many critical systems in the cell: transport activities, ion flux across membranes, osmotic balance, and so forth. Dips in ATP levels of

5-10% begin to have the effects just described.

Some tissues have strong mechanisms to fight hypoxia, such as your liver. Other tissues, such as brain and heart, are highly aerobic and are dependent on minute to minute oxygen flow.The time to irreversible injury is long in liver and very low in heart and brain. Global loss of bloodflow to the brain for 10 seconds will cause unconsciousness, and after 3-5 minutes (depending on what book you read) injury becomes irreversible. It's worth noting that the brain has some special changes in hypoxia that make life even more difficult (release of excitatory neurotransmitters has its own toxicity, and probably some more).

Your whole body is geared to giving adequate bloodflow and oxygenation to the brain, but if your airway is lost it doesn't matter because there is no oxygen to be had. This is why airway is the first thing you check in emergency situations, because without it all other things are futile.


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