Diving Physiology
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Direct answer
Seawater adds roughly one atmosphere of pressure for every 10 metres of descent, so at 30 m a diver breathes gas at 4 atmospheres — and the two gas laws govern everything that follows. Boyle's law (volume inversely proportional to pressure) explains barotrauma on descent and the exploded-lung injury of breath-holding on ascent; Henry's law (gas solubility proportional to pressure) explains the nitrogen loading that emerges as bubbles in decompression sickness. Add the anaesthetic effect of nitrogen under pressure and the toxicity of high-pressure oxygen, and the entire hazard list of diving is accounted for.
What you must remember
- Pressure: 1 additional atmosphere per 10 m of seawater; gas volumes shrink accordingly — half at 10 m, a quarter at 30 m.
- Descent barotrauma: middle-ear and sinus squeeze unless equalised (Valsalva, swallowing); never dive with a blocked nose or upper respiratory infection.
- Ascent barotrauma: a breath-holding ascent expands gas and can rupture alveoli — arterial gas embolism, pneumothorax or surgical emphysema; the rule is never hold your breath on scuba.
- Nitrogen narcosis ("rapture of the deep", the martini effect): below about 30 m, dissolved nitrogen anaesthetises — euphoria, impaired judgement, clumsiness; deeper it becomes lethal, resolving completely on ascent.
- Oxygen toxicity: central nervous system type (Paul Bert) above about 2 atmospheres of oxygen — facial twitching, convulsions; pulmonary type (Lorrain Smith) with prolonged exposure above about half an atmosphere — substernal burning, cough.
- Decompression sickness: nitrogen bubbles in tissues and blood after too-rapid ascent — joint pain (the bends), skin mottling, the chokes (pulmonary), the staggers (spinal and cerebral); a patent foramen ovale allows paradoxical arterial bubbles.
- Treatment: 100% oxygen and recompression in a hyperbaric chamber on standard tables; prevention by dive tables or computers, controlled ascent rates, a safety stop of 3–5 minutes at 5 m, and no flying for 12–24 hours after diving.
- Helium mixtures (heliox, trimix) substitute for nitrogen — less narcotic and less dense (easier breathing at depth), but highly thermally conductive, so divers chill.
One dive profile walked through
A diver descends to 30 m — 4 atmospheres absolute. Gas spaces shrink with Boyle's law, so she equalises her middle ears every few metres by pinching and blowing. Breathing gas at 4 atmospheres is dense; respiratory work rises and carbon dioxide retention adds to the load — around this depth on compressed air, nitrogen narcosis begins: she feels calm, even amused, and sloppy. Her tissues quietly load nitrogen all the while, dissolving four times the surface amount per Henry's law, with fat and slow tissues taking it up longest.
Bottom time ends at 20 minutes. Ascent is the exam: she must rise no faster than about 9–10 m per minute, exhale continuously, and pause at 5 m for a safety stop so dissolved nitrogen leaves through the lungs instead of nucleating as bubbles in her joints and spinal cord. Had she bolted to the surface, half an hour later she might notice mottled skin, then an aching shoulder — the bends — and 100% oxygen plus urgent recompression would be the treatment. If she had instead held her breath from 30 m, her lungs would expand toward four times breath-hold volume on ascent — pulmonary barotrauma with arterial gas embolism within minutes. Every diving rule is one of these two gas laws wearing a safety instruction.
Where students slip
The examiner separates candidates who can time-stamp the hazards. Nitrogen narcosis happens at depth during the dive and clears on ascent; decompression sickness happens after ascent, classically 15 minutes to 12 hours later, even from a perfect-appearing dive. Oxygen toxicity splits by organ and pressure: convulsions (Paul Bert) at high partial pressures over minutes to hours, pulmonary injury (Lorrain Smith) at lower partial pressures over days. The free (breath-hold) diver triggers a different physiology — the diving reflex of bradycardia and peripheral vasoconstriction shared with seals. Finally, remember why pure oxygen is not used deep: at 10 m it is already 2 atmospheres of oxygen, squarely in the convulsive range.
Frequently asked questions
Why must a scuba diver never hold the breath during ascent?
Boyle's law expands the trapped gas as pressure falls, over-distending alveoli and causing rupture with pneumothorax or arterial gas embolism.
What causes nitrogen narcosis and at what depth does it begin?
The anaesthetic effect of dissolved nitrogen under pressure, typically beginning around 30 m on compressed air and deepening with depth.
What is the difference between the bends and the chokes?
The bends is bubble-related joint and limb pain; the chokes is pulmonary decompression sickness with cough, chest pain and dyspnoea from bubbles occluding pulmonary vessels.
Why is helium substituted for nitrogen in deep diving mixtures?
Helium is far less narcotic, less dense (reducing breathing work), and diffuses faster — though its thermal conductivity makes heat loss a problem.
How is decompression sickness treated?
100% oxygen with recompression in a hyperbaric chamber, which compresses the bubbles and drives nitrogen back into solution for gradual elimination.