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Highest Pressure Human Body Can Survive In?

Highest Pressure Human Body Can Survive In?


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One design for underwater human inhabited environments is to have equal pressure between the surrounding water and the submerged habitat, thus allowing a section of the floor to be open to the water and removing the necessity for strong plating to protect it. This requires the air pressure within the environment to be greater than atmospheric pressure so that it can withstand the pressure of both the atmosphere and the pressure from the overlying water.

So how much pressure can the human body survive in without need for special suits and breathing apparatuses (as this would place a limit on how deep you could build such an environment without special accommodation)?

Edit: So far i've found an article about a man trapped in a sunken boat who survived for days in an air pocket trapped in the bathroom 100 feet down. He couldn't resurface without the use of a diving bell and gradual decompression; but this case provides a lower bound for how high the highest pressure can be.

Calculations: 14.7 psi (atm pressure) + 1200 inches*0.037 psi/in^3 = 60 psi for the underwater pocket (roughly 4 times atmospheric pressure). http://news.nationalgeographic.com/news/2013/12/131204-nigerian-air-bubble-survival-shipwreck-viral-video-science/


The deepest recorded simulated dive is still the on-shore French Hydra 10 experiment at 701 msw (meters salt water); at a pressure of 71 atm. Since breathing a mixture containing helium is required at such depths (to avoid the extreme narcosis produced by nitrogen at such depths), the new problem helium causes is

High-pressure nervous syndrome (HPNS). HPNS, brought on by breathing helium under extreme pressure causes tremors, myoclonic jerking, somnolence, EEG changes, visual disturbance, nausea, dizziness, and decreased mental performance. Symptoms of HPNS are exacerbated by rapid compression, a feature common to ultra-deep "bounce" dives.

Actually that Wikipedia description is a bit misleading, the problem is caused by any gas mixture, not just helium, but other gasses like nitrogen or hydrogen have a narcotic effect that counteracts HPNS. HPNS sets in at about 120m:

The high pressure neurological syndrome (HPNS) begins to show signs at about 1.3 MPa (120 m) and its effects intensify at greater depths. HPNS starts with tremor at the distal extremities, nausea, or moderate psychomotor and cognitive disturbances. More severe consequences are proximal tremor, vomit, hyperreflexia, sleepiness, and psychomotor or cognitive compromise. Fasciculations and myoclonia may occur during severe HPNS. Extreme cases may show psychosis bouts, and focalized or generalized convulsive seizures. Electrophysiological studies during HPNS display an EEG characterized by reduction of high frequency activity (alpha and beta waves) and increased slow activity, modification of evoked potentials of various modalities (auditory, visual, somatosensory), reduced nerve conduction velocity and changes in latency. Studies using experimental animals have shown that these signs and symptoms are progressive and directly dependent on the pressure. HPNS features at neuronal and network levels are depression of synaptic transmission and paradoxical hyperexcitability.

The problem with extreme helim pressure was partially alleviated in the Hydra experiments by mixing/replacing some of it with hydrogen, in the so-called Hydreliox mixture.

For the Hydra VIII mission at 50 atmospheres of ambient pressure, the mixture used was 49% hydrogen, 50.2% helium, and 0.8% oxygen.

The French did this based on prior experiments which found that the narcotic effect of hydrogen reduces HPNS (or even completely eliminates it at lower depths):

A H2-He-O2 mixture with 54 to 56% hydrogen was studied with 6 subjects (professional divers) during 2 dives to 450 m. The 38-h compression was the same as that used with other types of breathing mixtures (He-O2 and He-N2-O2). The results obtained during compression and during the stay at 450 m in H2-He-O2 show that the EEG changes (increase of theta activities in the anterior regions of the skull, decrease of alpha activities) are similar to those found with other respiratory mixtures. On the other hand, the other symptoms of high pressure neurologic syndrome (HPNS) were clearly improved for the same depths. Thus, neurologic symptoms (tremor, dysmetria, myoclonia, drowsiness) are nonexistent, and the performances during psychometric tests remain similar to those of the surface. Hydrogen, with its narcotic potency, suppresses some symptoms of HPNS and seems to open new perspectives for deep diving.

Of course, having hydrogen (in such large proportion) and oxygen in a gas mixture poses dangers of fire, explosion etc. This is avoided by decreasing the oxygen concentration; alas this cannot be a one-step process (for diving):

The major problem with hydrogen-oxygen mixtures is the potential for explosion. Although the concentration of oxygen needed for combustion of oxygen-hydrogen mixes varies a bit with pressure, a general rule of thumb is that hydrogen-oxygen mixes above 5 % O2 are at-risk… So, to avoid nasty fires and explosions, hydrogen is only considered as a breathing gas component at pressures where a less-than 5% oxygen concentration in the breathing gas mix gives a partial pressure of oxygen great enough to sustain life. Perhaps the most common example of a hydrogen-fire related disaster is the destruction of the Hindenburg dirigible. [… ]

In 1944 Arne Zetterstrom discovered a way to breach the transition between compressed air and Hydrox without risking explosion. The technique was to descend to 100 feet and switch to a 4% oxygen / 96 % nitrogen mixture. After breathing this mix for sufficient time to allow the oxygen concentration in the lungs to drop below the "explosion threshold," the diver switched to Hydrox and continued descent. On ascent, the diver again used the Nitrox (4% O2 / 96% N2) as a transition between Hydrox and air. Using this technique, he descended to 363 feet. At that depth, the alteration in voice characteristics, coupled with excitement, made communication impossible and additional dives used a telegraph key. [… ]

During the mid 1960's research into the use of hydrogen in breathing gases resumed with animals breathing Hydrox for up to 24 hours at 70 Ata. One interesting aspect of the animal research was the suggestion that hydrogen reduced the HPNS (high pressure nervous syndrome) often observed with helium based gas mixes on deep dives. Ultimately animals would be taken to 3500 feet on hydrox.

In 1983 COMEX, the French deep diving concern (perhaps more famous in the US as the company providing the submersible used in the recovery of artifacts from the Titanic) began a series of dives to investigate the narcotic potential of hydrogen. Divers including H.G. DeLauze, President of COMEX, descended in open sea to approximately 300 feet for five minutes. The divers could not perceive a difference between Hydox and Heliox at that depth. Chamber dives to 300 m (984 ft) demonstrated that hydrogen possessed a narcotic effect different from nitrogen. Hydrogen narcosis (the "hydrogen effect") had a tendency to be more psychotropic, i.e. more like LSD, while nitrogen narcosis had an effect similar to alcohol. This deeper work suggested that Hydrox as a binary gas mix would not be too useful at depths below about 500 feet.

Based on that COMEX developed a protocol of switching from heliox to hydreliox at 250msw.

A more recent (1994) COMEX paper reported that at 500m (on hydreliox), manual dexterity was about 80% of the surface one, while the arithmetic ability decreased to 60%. So not dying and being able to function as on the surface, aren't the same thing.

Also these experiments are time consuming and costly. In Hydra VIII, the whole thing took one month, of which only 10 days or so were spent at maximum depth. And in case "something bad happens" interventions aren't easy in such settings.

A 1984 American experiment with trimix at 650m was aborted after one of the three subjects (despite negative previous neuropsychiatric screening, as well having substantial commercial diving experience and successfully taking part in the previous experiments of the series) developed hallucinations and then full blown mania at 625-650m.

The Atlantis IV dive was designed to test the ability of 5% N2 trimix breathing mixture (5% N2, 0.5 atmosphere oxygen, with the balance helium) to counteract the [high pressure nervous] syndrome without nitrogen narcosis. [… ]

The dive began with an initial compression rate of 30 m/hour to 300 m (1000 ft) sea water equivalent pressure. By day 3 of the dive, at about 470 m, subject C had developed significant insomnia. On day 4 (540 m), this subject became stressed and complained of mild auditory illusions of music and visual distortions consisting of a “halo effect” around the chamber's apparatus. On day 5 (625 m) of the dive, the subject's irritability, insomnia, and agitation worsened. He was medicated with temazepam, 60 mg, and achieved substantial relief of these symptoms after 15 hours' sleep. However, with further compression to 650 m, subject C experienced a recrudescence of auditory and visual illusions and developed rapid speech, racing thoughts, shortened attention span, and difficulty in focusing on tasks. Ankle clonus, marked hyperreflexia, and myoclonic jerking were documented, and he complained of nausea and weakness. In contrast, the other two subjects performed normally in almost all respects. On day 9 (650 m), the subject stated he felt “paranoid” and that he felt like “I'm going insane.” He began to carry a mirror to look at himself to “remind myself that I'm not a raving maniac.” He stated that colors were enhanced and vivid and he could see “unusual” mosaic patterns on the metalwork within the chamber. He was unable to comply with instructions to enable cognitive function to be tested because of extreme agitation and distractibility.[… ]

Early on day 9 (650 m), the decision was made to halt the dive, but decompression from 650 m is still experimental and could not be accelerated. Because the decompression process was expected to last as long as 30 days, it was necessary to consider medicating the subject more intensively. Since lithium had been shown to exacerbate symptoms of the high pressure nervous syndrome in animals and the effects of phenothiazines at such high pressures were virtually unknown, the decision was made to control the diver's agitation with benzodiazepines. Accordingly, diazepam was used in doses up to 120 mg/day. This treatment only mildly sedated the subject and provided no real control of his behavior. The subject's agitation persisted over the next 14 days and was marked by decrements in his cognitive function and bursts of aggressive paranoia, grandiosity, and irritability. By day 24 of the dive (383 m), the subject's irritable and aggressive behavior was becoming potentially dangerous and led to the decision to medicate with chlorpromazine, up to 300 mg/day. Within 48 hours of the beginning of chlorpromazine treatment there was dramatic improvement in the subject's behavior. He became relatively calm and better organized. He denied having illusions or hallucinations. [… ]

Ten days after exiting the chamber, the subject again exhibited signs of hypomania including hyperactivity, insomnia, and irritability. Lithium carbonate therapy lessened these symptoms and was continued prophylactically for 6 months. It was then discontinued without any recurrence of symptoms.


The Highest Temperature A Human Can Actually Survive

Mankind has a love-hate relationship with heat. Extremely attractive people are called hot. Hope and knowledge are both flames. And when Elvis felt his temperature rising, his brain on fire, and flames "licking [his] body]," he was "just a hunk, a hunk of burning love." At the same time, people burn with hatred, hell is a lake of fire, and if you can't take the heat, you need to get out of the kitchen. As with all things, moderation is key when it comes to heat. Too little and you'll freeze to death — too much, and you'll become a hunk, a hunk of burning corpse. But just how hot can it get before you stop sweating bullets and your body is completely shot to hell?

According to National Geographic, the human body is ill-suited to spend extended periods of time in temperatures higher than its own internal temperature, which on average clocks in at 98.6 degrees Fahrenheit. Once your innards hit 104 degrees, you're in the danger zone, but unfortunately not the one that Kenny Loggins and Archer sang about. At that point you're on the cusp of heat stroke, which officially strikes when you reach 105 degrees. At 107 degrees, you're in terrible peril of exiting the danger zone and entering the dead zone as your blood flow slows and your organs sustain potentially "irreversible damage."

But even a temperature of 107 degrees or above isn't a guaranteed death sentence. Outside Online describes the harrowing case of Willie Jones, who spent 24 days at a hospital after his core temperature reached a scorching 115.7 degrees. It's also important to note that hydration is an important factor in whether you beat the heat or lose your life to it.

Of course, what we've described is the highest internal temperature a person might survive. You might be wondering about how much external heat a person can tolerate. Live Science writes that most humans can endure about 10 minutes in 140-degree heat before suffering from hyperthermia, a lethal form of which is the aforementioned heat stroke. If you're a firefighter, however, you have to battle far higher temperatures.

Chris Armstrong, fire lieutenant of Richmond, Virginia, told WTVR that "a typical house fire gets above 1,000 degrees." Plus, they have to wear about 50 pounds of gear. How do they keep from melting? They stay in the burning house for brief bursts, switching places in quick rotation. So even a hellish amount of heat is survivable in moderation with the right training. That, and firefighters are actual, real-life superheroes.


What is pressure?

Pressure can generally be defined as the force, per unit area, applied to the surface of something. We’re always under a certain amount of pressure, we just don’t notice. We hear about air pressure on the weather channel, but we actually have our own pressure in air-filled spaces of our body like our lungs, stomach, and ears. Our internal pressure is usually equal to the outside air pressure (the weight of the atmosphere pushing down on us.) We become uncomfortable whenever we venture away from sea level our internal pressure is no longer equal to the ambient pressure. This is why our ears hurt when we go up in a plane or when we dive too deep underwater.

There are a few ways pressure changes could spell doom for us humans. One of these is what would happen to us if we were to materialize deep underwater without a pressurized suit — death, in a nutshell, but I can explain.


How Does the Air Pressure Affect the Human Body?

Dr. Matthew Fink says in the New York Times that changes in air pressure can cause physical discomfort. Headaches and joint pain are common in low pressure systems, and uncomfortable ear popping can occur as the body tries to equalize the pressure inside its cavities with the changing atmospheric pressure.

ACS Distance Education explains that air pressure is the force exerted by the weight of air molecules. Atmospheric pressure is determined by the amount of air directly above a person or object. At sea level, the atmospheric pressure is 14.7 pounds per square inch, or PSI. At higher altitudes, the PSI decreases due to lower air pressure and density. Skin adjusts easily to changes in pressure, but the cavities within the body, such as the lungs, ears and sinuses, do not adjust automatically. This is why many people experience a popping in their ears while taking off in an airplane or driving through mountains.

Dr. Fink explains other effects that may be felt by the body under these circumstances or during a low pressure weather system. The difference in pressure between the body's cavities and the atmosphere can result in headaches or distension in the sinuses, which are filled with air. People who suffer from arthritis or bursitis may experience joint pain as their muscles and joints swell in response to the decreased pressure on their bodies.


How Big An Explosion Could You Realistically Survive?

Everyone knows you can’t survive a nuclear blast in a refrigerator (unless you’re Indiana Jones). But what can the human body withstand? We take a look at the damage explosions cause and how humans survive it.

Most of the time, we’re ready to forgive movies for the crimes against science they regularly perpetrate. We don’t pay 20 bucks to see realism. We pay to see someone on a skateboard jump off the seat of an exploding motorcycle, which has just jumped from the top of an exploding bus. And if there’s a way to make the skateboard also explode – get on that. But occasionally movies push credibility just a little too far. People outrun fireballs. People survive explosions by hopping away from them. People sometimes just walk away from explosions without even looking back, because they’re that cool. The reality is a lot grislier.

The Basics of How Explosions Kill

Many movie explosions are pretty flashy, with smouldering slow-motion fireballs and flying pieces of sharp metal twisting through the air – often in three dimensions. Fire and fast-moving objects are two visual ways to show the destruction that many explosions wreak, which is why they’re so lovingly and carefully rendered on screen. But there are invisible ways that explosions kill. When a hero ducks out of the way of a 20m long fireball, or dodges a car door being thrown through the air like a tinkertoy, movies make it look like they would manage to survive the blast.

Although fire and shrapnel do cause many injuries during explosions, perhaps the most major destructive force in an explosion is simply the blast wave. When an explosion goes off, it pushes a great deal of air outward in a small amount of time. Although people are generally unaware of the air they move through, it has a way of making itself known when it’s propelled outwards by TNT. This wave of sudden pressure causes a huge amount of damage.

The PSI of Staying Alive

The air around us already puts pressure on us to the tune of 15kg per square inch. This is the kind of pressure the human body has evolved to deal with, though. We have a great deal of trouble getting by without it, in fact. A sudden variance causes a lot of problems for the body. Explosions exacerbate this problem by varying the pressure two ways. The blast wave is comprised of compressed air being shoved out of the way. Since it is doing this in an even sphere around the explosive, there’s no way for new air to fill the void. What’s left is a partial vacuum – the body goes from a hit of overpressure to a near vacuum in a split second.

Surprisingly, the human body is pretty tough. If the pressure is stacked up slowly, it can survive as 180kg per square inch, if the pressure is gradually increased and decreased to allow the body to adjust. A sudden change of pressure causes damage as far lower levels. Anywhere from 10 to 20 kilograms per square inch can be fatal, depending on the time it takes to wash over the body.

At the high pressure shifts, the body just comes apart. There’s only so much trauma that flesh and bone can take. Although lower-level explosions could be survivable, they do cause seriuos internal injuries. The ears are the first organs to feel the change in pressure. That’s what they’re designed for, after all. Ears are meant to allow people to sense minute changes in air pressure – sound – over short amounts of time. As such, they’re extremely sensitive to pressure, and especially to pressure over time. A wave of pressure that lasts less than .3 milliseconds leaves the eardrum no time to adjust to changes in pressure, and simply tears it. This can happen with pressure change as small as 5 psi. Lungs are the next thing to feel the change. Filled with tiny air sacs, they rupture and bleed when too much pressure is applied too quickly. The bowels are also destroyed. Although they’re not the first thing most people think of being injured in an explosion, they are sensitive to changes in pressure. Filled as they are with liquid and gas, a change in pressure can cause them to expand or shrink suddenly and simply tear themselves open.

The Hidden Dangers of Changes in Pressure

Although it seems like the primary danger to the human body would be the changes in pressure itself, there are other ways that pressure waves kill. The human body can survive blasts of sudden pressure of 20-40 psi, but it’s not the only thing receiving that pressure. The pressure radiates outward from the blast in all directions. When it leaves a vacuum behind it, air from the surrounding atmosphere moves in to fill that vacuum. This means wind. A lot of it.

Pressure changes of 5 psi can cause 260km/h winds. Changes of 20 psi can cause winds of 760km/h. This kind of wind doesn’t knock people over, it lifts them through the air. It drags them over the ground like they were caught behind train, or blows them out of windows to the ground below. It slams them into cars and buildings hard enough to kill. Even if someone were to survive a blast – they would most likely be killed by being knocked into their surroundings.

Jumping Behind an Object Doesn’t Work

Some films show heroes escaping by ducking behind a wall, or into a tunnel, or sometimes (I’m looking at you, Independence Day.) around a corner. Unfortunately, pressure isn’t something that a person can hide from. Most people have felt a building or a car shake when a wave of wind hits it. Although the wave of air is stopped, the force moves on, kicking the structure forward.

As for running down a hallway, or into an enclave, that could actually be worse than facing the onslaught. Many people here have seen a hose dribble water until they put their thumb over the nozzle. Instead of dripping out, the water begins to spray, moving farther and faster than it did before. The water is coming out of the hose with a certain forcer per square inch. Reducing the number of inches it can leave the hose by doesn’t change the force the water is coming with – it just concentrates it. Ducking into smaller and smaller corners – depending on their position, could concentrate the blast wave. (To be fair, though, ducking into a sheltered area is a good way to hide from shrapnel.)

When getting to shelter, it’s also important to consider the durability of the shelter itself. This is probably one thing that movies get right. When a building is blowing up – get out of it. Apparently most buildings are build to withstand snow and wind. Those can be pretty heavy, and buildings are meant to be built solid, but roofs generally support snow loads of 345kg/m walls withstand winds of 160km/h – which works out to .2 pounds per square inch. Structures can crumble under pressure anywhere between 5 and 20 psi. Downright puny compared to what people can live through. The building might provide more danger than the explosion.

So How Does One Survive An Explosion?

Generally, one doesn’t. At least not any movie explosion. Films that show people using missiles or building-destroying dynamite as a ‘diversion’ or a way to propel themselves or their vehicles or their chairs (Looking at you, Long Kiss Goodnight.) are doing the equivalent of running someone over with a car as a way of patting them on the back. Military-grade explosives unleash millions of pounds per square inch of pressure. Anything near it is getting destroyed.

For more modest explosives, the best defence is distance. Since force is applied over area, it decreases by the square of the distance it travels. Run like hell. A good hundred meter dash will put you in the safe range of one kilogram of TNT. A thousand metres will keep you safe from a thousand kilograms of it. Keep moving directly away from the explosive and keep doing it as far as you can. If you can run while covering your head – especially your ears – you’ll decrease incidental injuries but don’t let anyone distract you from distance. Just get away.


The answers are found in the SCUBA diving world. Diving is interesting in this sense in that 10m is roughly an increase in pressure of 1atm.

In general, you wont see humans doing well above 30atm. 300m is a "holy grail" of sorts for deep diving. Only a handful of people have ever gone that far down. And by a handful, I mean it's been done seven times, total.

NCBI provides a short paper with a theoretical limit of 1000m for humans, based on data we have collected from saturation divers to date. That would be 100atm of pressure.

Somewhere in between is the claimed record for deep diving which is roughly 600m.

Slightly higher than that, we find synthetic testing of Hydreliox. This was done in a chamber compressed to a simulated 700m (70atm). It was found that there were issues with hydrogen narcosis at depths below 500m, no matter how they tweaked the mix.

I cannot find the link, but I came across a fascinating link months ago which described the different gases you can breathe and how deep you can go. At somewhere in the 40m range, you start suffering from nitrogen narcosis, so its beneficial to use a mix without nitrogen to go lower, such as heliox. However, at extreme depths, you start running into helium toxicity issues, and adding a little nitrogen in helps with that. Yes, I said helium toxicity. At high enough pressures, the noble gasses start to kill us, and of the noble gasses, we use helium at those depths because it kills us the least!


So how hot is too hot?

A 1958 report by NASA explained that our bodies are made to live in environments that are between 4-35 degrees, however if humidity is lower than 50%, we can withstand slightly hotter temperatures. The higher the humidity, the hotter it feels because it makes it harder for us to sweat and keep ourselves cool.

Live Science used data from the NASA report to put together an infographic that shows just how long your body could survive in the heat and humidity.

Information Source: NASA | Live Science


What's the maximum air pressure humans can survive in?

With a helmet hooked up to a breathing source? Without one?

Its really more a matter of how much time they are given to acclimate to it. Air compresses easily. Water does not. As long as sufficient times is given for the body's air spaces to equalize with the ambient pressure, you're ok. If you're giving the air time, the water content will also have time to equalize because it is affected at a much slower pace, provided you are allowed to intake more air and water during pressurization to provide for the increased density requirement.

That's all theory built off stuff any diver knows though. Iɽ be really interested to understand the other factors I'm not aware of. Does biological matter, independent of air and water, have a failure point? Does air or water change its properties to toxic at a certain pressure? Is electrical conductivity retarded at pressure?

Yeah, the main reason would probably be oxygen and nitrogen becoming toxic at high enough pressures.

At around 8x atmospheric pressure, humans develop hyperoxia and suffer oxidative damage from normal air - damaging the nervous system, lungs, and eyes.

Nitrogen dissolves into body tissues at higher pressures, causing impaired cognition. I don't think it causes any damage (other than the bends when depressurising).

For these reasons, divers use mixes of helium etc to be able to dive deeper without toxicity.

Besides that, I guess the only issues would just be if pressure differentials developed - which would either collapse spaces or cause barotrauma.

Without a helmet/breathing source, so essentially free-diving(?), I guess the issues of chest collapse would become more significant. Presumably they would be holding their breath and the air in the chest would initially just be at atmospheric pressure, so any air spaces would get compressed to a smaller volume under pressure.

Here's a good list of all the bad things that can happen under high pressure.

Without an alternate breathing arrangement, the limiting factor is going to be pulmonary oxygen toxicity. Air contains about 20.9% oxygen, which at normal atmospheric pressure is equivalent to a partial oxygen pressure of 0.209 ata. While the human body can endure elevated PPO2 for some period of time, for sustained (indefinite) exposure, the PPO2 limit is about 0.48 ata, corresponding to an absolute pressure of about 2.3 ata.

If you can control the breathing media, reducing the oxygen content to prevent both acute (central nervous system) and cumulative (pulmonary) oxygen toxicity, reducing (or eliminating) the nitrogen content to prevent inert gas narcosis (typically by replacing nitrogen with helium), reducing the gas density by replacing some of the helium with hydrogen (lowers work of breathing and makes CO2 elimination more effective), and preventing high pressure nervous syndrome (HPNS) from occurring by limiting the rate of compression, and possibly by reintroducing some nitrogen (balancing act between HPNS and narcosis), then you permit the person under pressure to breathe, and the limiting factor to pressure exposure becomes neurological. The limits to such exposure have not been extensively tested. In practice, Comex has deployed saturation divers to depths in excess of 1700 fsw, corresponding to an absolute pressure of about 50 ata. Sustained exposure to high pressure saturation has been shown to precipitate long bone necrosis in sat divers though, so it is unclear as to whether any "limits" exist which don't do any damage at all to the subject.


Narrow Range of Temperature

You have probably seen news stories about athletes who died of heat stroke, or hikers who died of exposure to cold. Such deaths occur because the chemical reactions upon which the body depends can only take place within a narrow range of body temperature, from just below to just above 37°C (98.6°F). When body temperature rises well above or drops well below normal, certain proteins (enzymes) that facilitate chemical reactions lose their normal structure and their ability to function and the chemical reactions of metabolism cannot proceed.

Figure 1. Extreme Heat. Humans adapt to some degree to repeated exposure to high temperatures. (credit: McKay Savage/flickr)

That said, the body can respond effectively to short-term exposure to heat (Figure 1) or cold. One of the body’s responses to heat is, of course, sweating. As sweat evaporates from skin, it removes some thermal energy from the body, cooling it. Adequate water (from the extracellular fluid in the body) is necessary to produce sweat, so adequate fluid intake is essential to balance that loss during the sweat response. Not surprisingly, the sweat response is much less effective in a humid environment because the air is already saturated with water. Thus, the sweat on the skin’s surface is not able to evaporate, and internal body temperature can get dangerously high.

The body can also respond effectively to short-term exposure to cold. One response to cold is shivering, which is random muscle movement that generates heat. Another response is increased breakdown of stored energy to generate heat. When that energy reserve is depleted, however, and the core temperature begins to drop significantly, red blood cells will lose their ability to give up oxygen, denying the brain of this critical component of ATP production. This lack of oxygen can cause confusion, lethargy, and eventually loss of consciousness and death. The body responds to cold by reducing blood circulation to the extremities, the hands and feet, in order to prevent blood from cooling there and so that the body’s core can stay warm. Even when core body temperature remains stable, however, tissues exposed to severe cold, especially the fingers and toes, can develop frostbite when blood flow to the extremities has been much reduced. This form of tissue damage can be permanent and lead to gangrene, requiring amputation of the affected region.

Everyday Connection: Controlled Hypothermia

As you have learned, the body continuously engages in coordinated physiological processes to maintain a stable temperature. In some cases, however, overriding this system can be useful, or even life-saving. Hypothermia is the clinical term for an abnormally low body temperature (hypo– = “below” or “under”). Controlled hypothermia is clinically induced hypothermia performed in order to reduce the metabolic rate of an organ or of a person’s entire body.

Controlled hypothermia often is used, for example, during open-heart surgery because it decreases the metabolic needs of the brain, heart, and other organs, reducing the risk of damage to them. When controlled hypothermia is used clinically, the patient is given medication to prevent shivering. The body is then cooled to 25–32°C (79–89°F). The heart is stopped and an external heart-lung pump maintains circulation to the patient’s body. The heart is cooled further and is maintained at a temperature below 15°C (60°F) for the duration of the surgery. This very cold temperature helps the heart muscle to tolerate its lack of blood supply during the surgery.

Some emergency department physicians use controlled hypothermia to reduce damage to the heart in patients who have suffered a cardiac arrest. In the emergency department, the physician induces coma and lowers the patient’s body temperature to approximately 91 degrees. This condition, which is maintained for 24 hours, slows the patient’s metabolic rate. Because the patient’s organs require less blood to function, the heart’s workload is reduced.


Blood is under pressure in the arteries so that it reaches all parts of the body. Diet, exercise and other factors can affect the risk of heart disease developing.

Arteries carry blood away from the heart. Blood in the arteries is under pressure because of the contractions of the heart muscles. This allows the blood to reach all parts of the body.

This video showcases how this happens

Measuring Blood Pressure

Blood pressure is measured in millimetres of mercury, mmHg. There are two measurements:

  • systolic pressure - the higher measurement when the heart beats, pushing blood through the arteries, and
  • diastolic pressure - the lower measurement when the heart rests between beats

A young, fit person should have a blood pressure of about 120 over 70, which means their systolic pressure is 120 mmHg and their diastolic pressure 70 mmHg.

High and Low Blood Pressure

There are various factors that can increase blood pressure, including:

Example: Smoking and the effect on blood pressure - Smoking increases blood pressure by raising the heart rate. Nicotine itself increases the heart rate and carbon monoxide reduces the oxygen-carrying capacity of the blood. It combines with haemoglobin in red blood cells, preventing oxygen combining with the haemoglobin. This causes an increase in heart rate to compensate for the reduced amount of oxygen carried in the blood.

A balanced diet and regular exercise can reduce high blood pressure.

Extremes of blood pressure can create problems. High blood pressure can cause:

Low blood pressure can cause dizziness, fainting and poor blood circulation.