Safe ascent: decompression schedules, SCUBA and submarine escape
- Concept
- For completeness
Divers rise slowly and pause on the way up so dissolved nitrogen leaves the body through the lungs instead of forming bubbles.
Also called: Decompression tables, Safety stop, Pulmonary barotrauma
See it in 3D: Eardrum · Maxillary sinus
What it is
Nitrogen dissolves in tissues in proportion to depth and time. On ascent it must leave by the blood and lungs; if pressure falls faster than tissues can unload, it comes out of solution as bubbles (decompression sickness). Decompression schedules (tables and dive computers) model tissues as compartments with different half-times and keep supersaturation within limits by a slow ascent and staged stops. Self-contained underwater breathing apparatus (SCUBA) supplies gas at ambient pressure, which creates its own hazards, and submarine escape adds the danger of expanding lung gas.
How it works, step by step
- At depth the lungs load blood with N₂ at high partial pressure; tissues equilibrate according to their blood flow and fat content.
- On ascent ambient pressure falls; tissue N₂ tension now exceeds alveolar N₂ and the gradient drives N₂ back to the lungs.
- If the ratio of tissue gas tension to ambient pressure becomes too high, bubbles form in tissues and veins.
- Stops at set depths hold the gradient high enough for washout but low enough to avoid bubbles; slow tissues dictate the length of the last stops.
- The SCUBA demand regulator delivers gas at the surrounding water pressure on each inhalation, so lung gas expands as the diver rises.
- If the diver rises without exhaling, lung volume can exceed its limit, alveoli tear and gas enters the pleura, mediastinum or pulmonary veins.
Hazards of SCUBA and submarine escape
| Hazard | Physical basis | Result |
|---|---|---|
| Ear and sinus squeeze | Gas spaces compress on descent if not equalized | Pain, tympanic membrane rupture, sinus bleeding |
| Pulmonary barotrauma | Gas expands on ascent with closed glottis | Pneumothorax, mediastinal air, arterial gas embolism |
| Decompression sickness | Dissolved N₂ forms bubbles if ascent is too fast | Joint pain, skin, spinal cord and lung involvement |
| Gas supply problems | Empty tank, contaminated air | Panic ascent, hypoxia, carbon monoxide poisoning |
| Submarine escape | Rising from depth with lungs full of compressed gas | Air embolism unless the escaper exhales throughout the ascent |
| Submarine atmosphere | Sealed air: O₂ falls, CO₂ and contaminants rise | Hypoxia, hypercapnia, poisoning |
Clinical notes
- Sudden confusion, weakness or loss of consciousness within minutes of surfacing suggests arterial gas embolism; symptoms hours later favour decompression sickness. Both are treated by recompression.
- Divers are examined for conditions that trap gas (bullae, active asthma) or that risk sudden incapacity.
Test yourself
Think of the answer, then tap the question to see it.
Why are the final decompression stops the longest?
Slow tissues (fat, joints) unload N₂ slowly and reach dangerous supersaturation near the surface, where the relative fall in pressure per metre is largest.
A diver holds their breath while ascending quickly from 10 m. What can happen and why?
Lung gas volume would double from 10 m to the surface (Boyle's law); alveoli can rupture, causing pneumothorax or arterial gas embolism.
How does a SCUBA demand regulator let a diver breathe at depth?
It senses the pressure drop of inhalation and delivers gas at the surrounding water pressure, so the chest can expand against the water.
Why is flying soon after a dive discouraged?
Cabin pressure is lower than sea level, so residual tissue N₂ becomes more supersaturated and can form bubbles.
Connections
See it in 3D2
The structures of Durgical's 3D anatomy where it happens.
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: Scuba Diving: Decompression Illness and Other Dive-Related Injuries (CDC Yellow Book, 2025); Ear barotrauma: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2024); Acute Pneumothorax Evaluation and Treatment (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.