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# 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.

Updated Nov 13, 2026

Also called: Decompression tables, Safety stop, Pulmonary barotrauma

**See it in 3D:** [Eardrum](https://durgical.com/en/anatomy/tympanic-membrane/) · [Maxillary sinus](https://durgical.com/en/anatomy/maxillary-sinus/)

## Key points

- Tissues take up and release N₂ exponentially; fast tissues (blood, brain) saturate in minutes, slow tissues (fat, joints) over hours.
- Haldane's principle: tissues tolerate a limited supersaturation; ascent is paced so no compartment exceeds it.
- No-decompression limits shorten with depth; beyond them, mandatory stops are required before surfacing.
- Breath-holding on ascent with compressed gas in the lungs can rupture alveoli (Boyle's law): pneumothorax, mediastinal emphysema or arterial gas embolism with stroke-like signs within minutes.
- Flying soon after diving lowers ambient pressure further and can trigger decompression sickness.
- Repeated deep dives with little surface interval, as in some artisanal fishing, carry a high risk of decompression sickness.

## 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.

## Memory aid

Up slow, pause, keep breathing out: let the gas leave as breath, not as bubbles.

## 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.

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## Connections

## See it in 3D 2

The structures of Durgical's 3D anatomy where it happens.

- [Eardrum](https://durgical.com/en/anatomy/tympanic-membrane/)
- [Maxillary sinus](https://durgical.com/en/anatomy/maxillary-sinus/)

## Related topics 3

- [Diving, decompression and hyperbaric oxygen](https://durgical.com/en/physiology/diving-decompression-and-hyperbaric-oxygen/)
- [Dead space and alveolar ventilation](https://durgical.com/en/physiology/dead-space-and-alveolar-ventilation/)
- [Mechanics of breathing: pressures and muscles](https://durgical.com/en/physiology/mechanics-of-breathing-pressures-and-muscles/)

## Related conditions 4

- [Decompression sickness and diving injuries](https://durgical.com/en/conditions/decompression-sickness-and-diving-injuries/)
- [Pneumothorax](https://durgical.com/en/conditions/pneumothorax/)
- [Drowning](https://durgical.com/en/conditions/drowning/)
- [Carbon monoxide poisoning](https://durgical.com/en/conditions/carbon-monoxide-poisoning/)

[← All of physiology](https://durgical.com/en/physiology/)

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](https://www.cdc.gov/yellow-book/hcp/environmental-hazards-risks/scuba-diving-decompression-illness-and-other-dive-related-injuries.html) (CDC Yellow Book, 2025); [Ear barotrauma: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/001064.htm) (MedlinePlus, US National Library of Medicine, 2024); [Acute Pneumothorax Evaluation and Treatment](https://www.ncbi.nlm.nih.gov/books/NBK538316/) (StatPearls (NCBI Bookshelf), 2025).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/physiology/safe-ascent-decompression-schedules-scuba-and-submarine-escape/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/safe-ascent-decompression-schedules-scuba-and-submarine-escape/