Altitude and Diving

7 min read Updated Mar 26, 2026

Everything you have learned about respiratory physiology assumes you are near sea level, breathing normal atmospheric air at 760 mmHg. But what happens when those conditions change? At high altitude, atmospheric pressure drops and oxygen becomes scarce. Underwater, pressure increases and gases dissolve into your tissues in dangerous quantities. Both environments push the respiratory system to its limits and reveal the elegance - and vulnerabilities - of human physiology.

High Altitude Physiology

At high altitude, atmospheric pressure decreases. Since the fraction of oxygen in air remains 21%, the partial pressure of oxygen ( $P_{\text{O}_{2}}$ ) decreases proportionally.

Altitude	Atmospheric Pressure	$P_{\text{O}_{2}}$ (in air)	Approximate Alveolar $P_{\text{O}_{2}}$
Sea level	760 mmHg	160 mmHg	104 mmHg
3,000 m (~10,000 ft)	523 mmHg	110 mmHg	~67 mmHg
5,500 m (~18,000 ft)	379 mmHg	80 mmHg	~45 mmHg
8,848 m (Everest summit)	253 mmHg	53 mmHg	~28 mmHg

Acute Responses to High Altitude

When you first arrive at high altitude, your body responds immediately:

Hyperventilation - peripheral chemoreceptors detect low $P_{\text{O}_{2}}$ and stimulate increased ventilation. This is the most important acute response.
Increased heart rate - sympathetic activation increases cardiac output to deliver more blood per minute.
Hypocapnia - hyperventilation blows off CO2, causing respiratory alkalosis (pH rises).

The respiratory alkalosis from hyperventilation is actually a problem: the elevated pH shifts the O2-Hb curve to the LEFT, making hemoglobin hold onto oxygen more tightly. This partially counteracts the benefit of hyperventilation.

Acclimatization (Chronic Responses)

Over days to weeks at altitude, the body acclimatizes through several mechanisms:

Renal compensation (days): The kidneys excrete excess HCO3- to compensate for the respiratory alkalosis, bringing pH back toward normal. This allows the O2-Hb curve to shift back toward the right.

Increased 2,3-BPG (hours to days): Red blood cells increase production of 2,3-BPG, which binds to hemoglobin and shifts the curve to the RIGHT. This facilitates oxygen delivery to tissues despite the lower arterial $P_{\text{O}_{2}}$ .

Increased EPO and red blood cell production (days to weeks): The kidneys detect hypoxia and release erythropoietin (EPO), which stimulates the bone marrow to produce more red blood cells (polycythemia). More RBCs means more hemoglobin and greater total oxygen-carrying capacity.

Increased capillary density (weeks to months): Tissues develop more capillaries (angiogenesis) to improve oxygen extraction.

Increased mitochondrial density: Muscle cells increase their mitochondrial content to more efficiently utilize available oxygen.

Acute Mountain Sickness (AMS)

Rapid ascent to altitudes above 2,500 m without acclimatization can cause acute mountain sickness, characterized by headache, nausea, fatigue, and dizziness.

Diving Physiology

Underwater, the opposite problem occurs: pressure increases. For every 10 meters (33 feet) of depth, pressure increases by 1 atmosphere (760 mmHg).

Depth	Total Pressure	Gas Partial Pressures
Surface	1 atm (760 mmHg)	$P_{\text{N}_{2}}$ = 593 mmHg, $P_{\text{O}_{2}}$ = 160 mmHg
10 m	2 atm (1520 mmHg)	$P_{\text{N}_{2}}$ = 1186 mmHg, $P_{\text{O}_{2}}$ = 320 mmHg
30 m	4 atm (3040 mmHg)	$P_{\text{N}_{2}}$ = 2372 mmHg, $P_{\text{O}_{2}}$ = 640 mmHg

According to Henry’s Law (C = k x $P_{\text{gas}}$ ), the amount of gas dissolved in a liquid increases proportionally with the partial pressure of that gas. At increased depth, more nitrogen and oxygen dissolve in blood and tissues.

Nitrogen Narcosis

At great depths, the high partial pressure of nitrogen causes nitrogen narcosis - impaired judgment and disorientation similar to alcohol intoxication. The effect reverses upon ascent.

Decompression Sickness (The Bends)

If a diver ascends too quickly, the dissolved nitrogen in their tissues comes out of solution as gas bubbles (like opening a carbonated soda - rapidly reducing pressure causes dissolved gas to form bubbles). These nitrogen bubbles can lodge in joints, blood vessels, and tissues, causing:

Joint and muscle pain (the most common symptom - hence “the bends”)
Skin rash
Neurological symptoms (paralysis, confusion)
Pulmonary embolism (if bubbles reach the lungs)

Prevention: ascend slowly, allowing nitrogen to gradually diffuse out of tissues and be exhaled. The concept to understand is Henry’s Law in reverse - reducing pressure decreases solubility.

Oxygen Toxicity

While O2 is essential, too much is toxic. At very high partial pressures, excess oxygen generates reactive oxygen species (ROS) that damage cells. This is why oxygen concentration must be carefully controlled in clinical settings.

Summary: Altitude vs. Diving

Feature	High Altitude	Diving
Pressure change	Decreased atmospheric pressure	Increased hydrostatic pressure
O2 issue	Hypoxia (low $P_{\text{O}_{2}}$ )	Potential hyperoxia (O2 toxicity at depth)
N2 issue	Not significant	Nitrogen narcosis, decompression sickness
Primary gas law	Dalton’s law ( $P_{\text{O}_{2}}$ = fraction x $P_{\text{total}}$ )	Henry’s law (gas solubility increases with pressure)
Key adaptation	Increased ventilation, EPO, 2,3-BPG	Controlled ascent, gas mixtures
Danger	Acute mountain sickness	Nitrogen narcosis, the bends, O2 toxicity

A mountain climber at 5,500 m has been acclimatizing for two weeks. Name three physiological changes that have occurred.

Click to reveal answer

(1) Increased 2,3-BPG - shifts the O2-Hb curve right, facilitating O2 release to tissues. (2) Increased EPO secretion leading to polycythemia - more red blood cells increase oxygen-carrying capacity. (3) Renal excretion of HCO3- - compensates for respiratory alkalosis from chronic hyperventilation, normalizing pH. Additional changes: increased capillary density and mitochondrial content in tissues.

A scuba diver ascends rapidly from 30 meters. Using Henry's Law, explain why gas bubbles form in the blood.

Click to reveal answer

Henry's Law: C = k x $P_{\text{gas}}$ . At 30 meters (4 atm), the high partial pressure of nitrogen caused large amounts of N2 to dissolve in blood and tissues. Upon rapid ascent, the pressure drops suddenly. The dissolved N2 concentration now exceeds what can stay in solution at the lower pressure, so the excess nitrogen comes out of solution as gas bubbles. These bubbles can lodge in joints, blood vessels, and tissues, causing decompression sickness.