Concept

Short entry

[Verified]

Full entry coming in 2027

Encyclopedia definition

Hydrostatic pressure: the pressure that water exerts on an immersed body, which increases with depth; in head-out immersion it pushes blood from the legs toward the chest, raising the heart’s stroke volume and output, compresses the chest so breathing takes more work, and triggers a reflex increase in urine and sodium output (immersion diuresis and natriuresis).

Evidence label: [Verified] that the statements below match the cited physiology reviews and studies, read on October 19, 2026.^1–6 Planning advice is this entry’s own recommendation [Proposed]. Numbers refer to the reference list below.

Credit and origin: Hydrostatic pressure is a principle of physics, not a technique with a founder. Reviews of aquatic physiology describe how the body responds to it.^1,2 [Verified]

Also called: Hydrostatic effects; water as a compressor³; head-out water immersion (HOWI) physiology¹; immersion physiology; cardiovascular effects of immersion; immersion diuresis and natriuresis^1,4

Scope note: this entry covers the pressure of still water and the body’s responses to head-out immersion at rest and during exercise. Buoyancy and water temperature have their own entries, and depth-related unloading will have one. Diving and breath-hold submersion are outside its scope.

Usage note: physiology papers often write “head-out water immersion” (HOWI) for immersion with the head above water.¹ Because the pressure depends on depth,³ results from neck-deep immersion should not be assumed for waist-deep water. [Proposed]

Cite this entry

How to cite

Salzman, A. (Ed.). (2026). Hydrostatic pressure and immersion physiology. In Aquatic Therapy Encyclopedia™. ATU Educational Services, LLC; published by the Aquatic Therapy Foundation. https://aquatictherapyfoundation.org/encyclopedia/hydrostatic-pressure/

Permissions: You may quote and cite this entry with attribution. Reproducing or adapting full entries requires written permission from ATU Educational Services, LLC. Quoted material, trademarks and linked documents belong to their respective owners and are not covered by this permission.

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Longer definition

• The physics. The pressure of water on an immersed body is proportional to the water’s density, gravity and the depth of immersion. It does not depend on the shape or volume of the pool, only on depth; 10 m of water adds as much pressure as the whole atmosphere at sea level.³ [Verified]
• Blood moves toward the chest. Water pressure drives blood and lymph from the limbs toward the chest.³ Blood shifts into the chest and plasma volume rises, which increases the heart’s stroke volume and cardiac output.¹ One review reports that heart volume rises by 30 to 35% with immersion to the neck, citing a 2009 review.^2,3 [Verified]
• Breathing. Compression of the chest wall adds an elastic load and increases the work of breathing;^1,3 pressures in the lung circulation rise and vital capacity falls.¹ In 10 healthy young adults cycling in water and on land at the same oxygen uptake, expiratory reserve volume was lower and inspiratory reserve volume higher in water, and ventilation and breathing rate were higher at moderate intensity.⁵ [Verified]
• Kidneys and hormones. Stretch of the heart’s atria triggers reflex nerve and hormone responses, and plasma volume is brought back down by a reflex increase in urine and sodium output (diuresis and natriuresis).¹ A 1992 review summarised 15 years of research on the kidney effects of head-out immersion.⁴ [Verified]
• Temperature changes the response. In head-out immersion, cooler water does not change cardiac output compared with thermoneutral water, but it changes heart rate and the share of blood flow to muscle and fat, and the lower muscle blood flow reduces maximal oxygen uptake.¹ See Thermal effects of immersion. [Verified]
• In heart failure. A 2025 meta-analysis of 16 studies with 349 people with heart failure found that water exercise training improved peak oxygen uptake (by 2.85 mL/kg/min) and lowered resting heart rate compared with no exercise, and that a single water exercise session increased stroke volume and cardiac output compared with baseline.⁶ [Verified]
• Planning for heart and lung conditions. The rise in lung-circulation pressures carries a potential for pulmonary edema.¹ [Verified] Depth, entry into the water and session length are therefore part of planning for people with heart or lung disease; see Contraindications and precautions. [Proposed]
• Not the same as. Buoyancy, the upward force of water;³ drag and viscosity, which resist movement; or the pressure effects of diving with compressed gas, which can cause gas bubbles in the tissues and blood vessels during decompression.¹ [Verified]

The evidence label after each statement shows how well it is sourced; it is not a grade of clinical effectiveness.

Search the Hot List™

The Evidence Library (our Hot List) is the searchable list of published studies and reviews on aquatic therapy.

The Hot List covers papers published from 2024 to 2026, so the physiology reviews (1992 to 2015) are not in it. Few 2024–2026 papers use the word “hydrostatic,” so this button searches for “immersion.”

Resource links

Professional and research pages. More links will be added as this entry is developed.

• The properties of water and their applications for training (open-access review, 2014)
• Human physiology in an aquatic environment (review, PubMed record)
• MeSH: Immersion (National Library of Medicine)

References

Reference numbers match the evidence labels above.

Related entries

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