8.6 — Altitude, Diving and Holding Your Breath

The human respiratory system is designed for one atmosphere of pressure at rest. It turns out to work over a far wider range than that — people live permanently at 5,000 metres where the oxygen pressure is half sea level, free divers reach depths where their lungs are compressed to a fraction of their volume, and the record for a static breath-hold on air is over eleven minutes.

This chapter is about the edges of the system, and about a reflex you share with seals that you have almost certainly triggered without knowing.

Living high

Around 140 million people live permanently above 2,500 metres, in the Andes, the Tibetan plateau and the Ethiopian highlands.

And the three populations have adapted differently, which is a natural experiment in evolution.

Andeans have adapted by raising haemoglobin concentration — more red cells, more oxygen per litre of blood.

Tibetans have not. Their haemoglobin is close to sea-level values. Instead they breathe faster, have higher nitric oxide levels, and have wider blood vessels with greater blood flow. This is the EPAS1 variant inherited from Denisovans described in Chapter 3.5.

Ethiopian highlanders show yet another pattern, with normal haemoglobin and mechanisms that are still being worked out.

The Tibetan strategy appears to be the better one, and the reason is instructive: raising haemoglobin raises blood viscosity, which increases the work of the heart and the risk of clotting. Andeans have a higher rate of chronic mountain sickness — an excessive polycythaemia with headache, fatigue and eventual heart failure — than Tibetans do.

Tibetans have lived at altitude for far longer, perhaps 25,000 years or more against a few thousand for the Andean populations, and the difference in adaptation quality tracks that.

The practical acclimatisation rules are in Chapter 8.5 and are worth repeating in one line, because they are the whole of altitude safety: ascend slowly, sleep low, and if someone is confused or cannot walk in a straight line, take them down.

Diving

Pressure increases by one atmosphere for every 10 metres of seawater. At 10 metres you are at 2 atmospheres, at 30 metres at 4.

Two gas laws do all the work.

Boyle's law — at constant temperature, volume is inversely proportional to pressure. So a gas space halves in volume at 10 metres, and is one quarter at 30 metres.

Henry's law — the amount of a gas dissolved in a liquid is proportional to its partial pressure. So at depth, far more nitrogen dissolves in your tissues.

Everything that goes wrong in diving comes from one of those two.

Breath-hold diving

The lungs are compressed. A diver descending with 6 litres in their lungs has 3 litres at 10 metres and 1.2 litres at 40 metres.

This should be a hard limit at the residual volume — around 1.2 litres, below which the lung would be crushed. Early physiologists predicted a maximum depth of about 30 to 40 metres for this reason.

Free divers now exceed 200 metres. The prediction was wrong because blood shifts into the chest — the pulmonary vessels engorge, filling the space that air used to occupy, so the chest cavity does not have to collapse. Up to a litre of blood moves in.

The blood shift is part of a broader reflex.

The mammalian dive reflex

Put your face in cold water and three things happen automatically within seconds.

Bradycardia — the heart slows, by 10 to 25 percent in an ordinary person and by up to 50 percent in trained divers. Triggered by cold receptors around the nose and eyes, acting through the trigeminal nerve on the vagus.

Peripheral vasoconstriction — blood is shunted away from the limbs and skin toward the heart and brain.

Blood shift — as above, protecting the chest at depth.

All three conserve oxygen for the organs that cannot do without it, and the reflex is present in all mammals, most powerfully in seals and whales.

It is stronger in infants, which is part of why very young children are sometimes resuscitated successfully after long immersion.

And it is the reason for one of the most striking facts in emergency medicine: cold water drowning can be survivable after remarkably long submersion. The dive reflex plus rapid cooling — which reduces metabolic demand — has allowed neurologically intact survival after submersions of 40 minutes or more in cold water, mostly in children.

Hence the rule: "nobody is dead until they are warm and dead." Resuscitation in hypothermic drowning is continued far longer than in ordinary cardiac arrest, and rewarming is attempted before a decision is made. Chapter 23.10.

The dive reflex is also used deliberately in medicine. Immersing the face in cold water is one of the vagal manoeuvres used to terminate a fast supraventricular tachycardia (Chapter 7.4), and it is particularly effective in infants, where an ice pack to the face is a standard first step.

Scuba diving

Breathing compressed gas at depth means breathing gas at ambient pressure, which introduces problems a breath-hold diver never has.

Nitrogen narcosis. At depth, dissolved nitrogen has an anaesthetic effect on the nervous system, producing impaired judgement, euphoria and slowed reactions. It begins around 30 metres and becomes serious beyond 40, and the traditional description is "one martini per 10 metres beyond 20". It reverses completely on ascent, which makes it uniquely dangerous — the diver feels fine and makes bad decisions.

Oxygen toxicity. At high partial pressures oxygen becomes toxic, causing convulsions. This is why pure oxygen is never used below about 6 metres, and why technical divers use gas mixtures with reduced oxygen for deep work.

Decompression sickness — "the bends". Nitrogen dissolves into tissues at depth. If the diver ascends too quickly, it comes out of solution as bubbles inside the body — exactly like opening a fizzy drink.

The bubbles cause joint pain (the classic "bends"), skin rash, neurological symptoms, and in severe cases paralysis or death.

Prevention is controlled ascent with decompression stops, allowing the nitrogen to be exhaled gradually.

Treatment is recompression in a hyperbaric chamber, which shrinks the bubbles back into solution, followed by slow controlled decompression. Oxygen is given, which both treats the tissue and speeds nitrogen washout by maximising the gradient.

And divers should not fly within 12 to 24 hours of diving, because cabin pressure is equivalent to about 2,000 metres of altitude and can trigger bubble formation from residual nitrogen.

Pulmonary barotrauma is the one that kills fastest, and the rule preventing it is the first thing any diver is taught.

If a diver holds their breath while ascending, the expanding gas has nowhere to go. From 10 metres to the surface the volume doubles. The lungs over-expand and rupture, and gas can enter the bloodstream — an arterial gas embolism — reaching the brain within seconds.

This can happen from a depth of only a few metres, and it is why "never hold your breath while ascending" is the single most important rule in scuba diving.

Ear and sinus squeeze — the air spaces of the middle ear and sinuses must be equalised as pressure rises, by swallowing, yawning or the Valsalva manoeuvre. A blocked Eustachian tube from a cold means the ear cannot equalise, and the eardrum can rupture, which is why diving with a cold is avoided.

Holding your breath

An untrained person manages 30 to 60 seconds before the urge becomes overwhelming. Trained free divers exceed 8 minutes, and the static apnoea record on air is over 11 minutes.

The limit is not oxygen — it is carbon dioxide (Chapter 8.5). The urge to breathe is driven by rising CO₂ and by contractions of the diaphragm that begin as CO₂ climbs.

The break point comes when arterial CO₂ reaches about 50 mmHg in an untrained person, while oxygen is still perfectly adequate.

Three things extend it, and only two are safe.

Training the tolerance to CO₂ — free divers habituate to higher levels and learn to suppress the involuntary contractions. Safe.

The dive reflex — face immersion in cool water slows the heart and reduces consumption. Safe, and it is why free divers train in water rather than on land.

Pre-breathing pure oxygen — this genuinely increases stores, and it is how the extreme records are set (over 24 minutes with prior oxygen breathing). Safe in controlled conditions.

And the unsafe one, which needs stating clearly: hyperventilating first. It lowers CO₂ so the urge is delayed, without adding any meaningful oxygen, because haemoglobin was already almost fully saturated. So the swimmer exhausts their oxygen before the warning arrives, and blacks out with no sensation of breathlessness at all.

Shallow water blackout kills strong swimmers, often in supervised pools, precisely because nothing looks wrong until the person is unconscious underwater. Do not hyperventilate before breath-holding, and do not practise breath-holding alone. Chapter 23.10.

What the respiratory system does well

It is worth ending this Part on what the system achieves rather than only on how it fails, because the numbers are genuinely impressive.

You breathe about 20,000 times a day and move around 10,000 litres of air, and you never think about it once.

The reserve is enormous. Ventilation can increase 20-fold, oxygen uptake 15-fold, and the diffusion capacity of the lung has a threefold safety margin at rest. You can lose an entire lung and live a normal life — people who have had a lung removed for cancer return to full activity, and single-lung transplant recipients function well.

It repairs. The airway lining regenerates, and cilia recover within weeks of stopping smoking. Ten years after quitting, lung cancer risk is roughly halved; the excess risk of heart disease falls much faster than that, within a few years. Lung function decline returns to the normal age-related rate.

It adapts. Weeks at altitude change your blood, your capillary density and your mitochondria. Training increases respiratory muscle endurance and efficiency.

And it protects itself continuously — filtering, warming, humidifying, sweeping, coughing, and sterilising 10,000 litres of unfiltered outdoor air a day, mostly successfully, for a lifetime.

The practical summary for a reader's own lungs is short and it is almost entirely positive. Do not smoke, and if you do, stopping helps at any age and at any stage. Keep active, because exercise improves respiratory muscle function and the efficiency of gas exchange. Get vaccinated against influenza and pneumococcus if you are in a risk group, because most serious lung damage in later life follows infection. And take breathlessness seriously rather than attributing it to age or unfitness — new or progressive breathlessness has a cause, and most of the causes are treatable when found early.

What Part 9 does next

You have now covered how the body is supplied with oxygen. Part 9 covers how it is supplied with everything else: the digestive system, from the first bite to the last, including the liver — the most chemically versatile organ you have — and the hundred trillion bacteria that turn out to be doing more for you than anyone expected.