September 22, 2021

I, Science

The science magazine of Imperial College

At our limits - investigating how far human bodies be pushed ...


The human body is surprisingly adaptable, but everything has its boundaries. Just how deep is too deep? How cold can we go? And at what heights do we truly reach our upper limits?

Under Pressure

The invention of the steam train in the 19th century sparked widespread construction of bridges and tunnels. Hundreds of workers spent their days laying foundations inside huge chambers filled with compressed air to keep the water out. On returning to normal pressure after work, the men would often experience a bizarre range of symptoms, from itchy skin to severe joint pain, paralysis and even death.

We now know this condition as decompression sickness or ‘the bends’, most often experienced today by divers who return to the surface too quickly. This sickness is caused by the formation of air bubbles in the blood. Breathing whilst under great pressure causes more gas to dissolve in the body’s fluids than normal. In solution, this extra gas is harmless, and during a slow ascent the lungs expel it safely.

However, rapid decompression causes the extra gas to come out of solution as bubbles, some of which can be large enough to fatally block small blood vessels in the lungs and brain, and may also trigger blood clotting. Most commonly though, bubbles collect in the joints, causing excruciating pain and preventing the victim from extending their arms and legs – hence the name ‘the bends’.

But the threat of pressure doesn’t end there. Divers must also avoid what has been christened ‘the rapture of the deep’: nitrogen gas (78% of the air we breathe) becomes toxic at high pressure, with effects similar to alcohol, namely euphoria and over-confidence. Intoxicated divers have been known to offer their mouth-pieces to passing fish!

As a result, compressed air can’t be used at depths below 30 metres. Instead, divers breathe an oxygen-helium mixture called ‘heliox’. Unfortunately, when heliox is used at depths below 200‑250m, divers risk the onset of High Pressure Nervous Syndrome, which causes tremors, nausea and dizziness. Fortunately, this condition can be held off by adding a small amount of nitrogen back into to the gas tank. Breathing this ‘trimix’, humans can theoretically endure super depths of up to 450m.

Chilling Out

When our body temperature falls, several coping mechanisms act to delay serious injury. Firstly, we begin to shiver. These muscle contractions require energy production by respiration, which also generates heat. Blood vessels in the skin constrict to reduce surface heat-loss, dilating only occasionally to prevent tissue death by providing oxygen.

If the temperature falls further, the constriction becomes constant and the extremities are sacrificed to preserve core body heat. This causes frostbite to set in as ice crystals form inside cells, puncturing their membranes and killing them one by one.

Hypothermia begins when the body’s core temperature falls below 35°C (from 37°C). As hypothermia progresses, patients become tired and uncooperative. Their speech slurs and they can’t make rational decisions. The body begins to shut down at around 32°C due to lack of energy, and below 30°C the victim loses consciousness. Soon after, they suffer cardiac arrest.

However, hypothermia can be beneficial too. Extreme cold causes the body to enter a state of ‘suspended animation’, where our metabolic (chemical) processes slow to a point where cells can survive on just a fraction of the oxygen needed at 37°C. This can preserve the brain cells of hypothermia victims during long periods without blood flow.

In 1999, a Norwegian skier fell unconscious after tumbling headfirst through ice into fast-flowing water. She arrived at hospital two hours after her heart had stopped with a record-breaking core temperature of just 13.7°C, yet her heartbeat returned on re-warming. Incredibly, she recovered without brain damage.

As they say in emergency medicine, ‘nobody is dead until they’re warm and dead’. Hospitals now use hypothermia therapeutically: surgeons often cool their patients to as low as 10°C, allowing them to cut off the brain’s blood supply for up to 15 minutes without causing brain damage.

Cool Facts:

  • In 2013, specialist London Ambulance teams began administering roadside therapeutic hypothermia to cardiac arrest victims, using a cooling nasal spray.
  • Babies have a built-in heating system: pads of ‘brown fat’, packed with special heat-generating mitochondria, cover their shoulder blades and neck. This disappears before adulthood

Living the High Life

As altitude increases, atmospheric pressure decreases, and we take in less oxygen with each breath. This starts to become problematic at just 2,400m above sea level, where a lack of oxygen in the body can lead to something called ‘Mountain sickness’, causing headaches, nausea and dizziness. At 4000m around 40% of people will be affected, but we don’t yet know why some suffer more than others.

Descent fixes Mountain sickness almost immediately, but ascending further makes matters much worse. Blood vessels in the lungs start to react to the lack of oxygen in the air by constricting. This natural response is helpful at sea level, allowing areas of the lungs with the most airflow to have the best blood supply, but at high altitude, the entire lung experiences low oxygen: blood vessel constriction happens everywhere, not just in patches.

As a result, blood pressure in the lungs shoots up, forcing fluid out of the vessels and into the alveoli (air sacs). This is called pulmonary oedema. Left untreated, the victim effectively drowns in their own fluid.

It’s not all bad news though. The nasty effects of altitude can often be avoided – or at least delayed until higher up – by ascending slowly. Given time, the body can acclimatise to altitude by increasing the breathing rate to bring in more oxygen. This process is gradual, involving chemical changes driven by the kidneys, but eventually the breathing rate at high altitude can reach five to seven times that at sea level.

Acclimatisation also increases the number of circulating red blood cells, boosting the blood’s oxygen-carrying capacity. It’s this adaptation that athletes cash in on by training at altitude, since levels remain high for some time back at sea level, and can improve sporting performance. Acclimatization has its limits though. Above 8000m – known to climbers as the “death zone” – no human body can adapt sufficiently. An extended stay up here without supplementary oxygen would not end well.

Record Highs

  • In 1978, Italian and Australian mountaineers, Reinhold Messner and Peter Habeler became the first climbers to reach the summit of Everest (8848 m) without supplementary oxygen, making a speedy dash into and out of the death zone.
  • The 1968 Mexico City Olympics took place 2,240m above sea level. Sprint events – where breathing is unimportant – saw World Records smashed due to reduced air resistance at high altitude, but endurance athletes underperformed consistently due to the low oxygen levels.

Picture from Mayo Clinic