How Water Affects the Body: Buoyancy, Hydrostatic Pressure and Temperature

Three Things Change When You Get in Water

Buoyancy is the upward push of water that makes you feel lighter. Hydrostatic pressure is the pressure of the water around you, which rises with depth. Temperature is how warm or cool the water is compared with your body. Most of what is written about "how hydrotherapy works" comes from physiology studies of these three things, mostly in healthy volunteers over short periods. They describe how the body responds, not whether a bath or pool session treats a health problem.

Buoyancy: Feeling Lighter

The NHS says swimming can be good exercise in pregnancy because the water can relieve pressure on your muscles and joints. [1] A 2026 review of rehabilitation after knee replacement describes hydrotherapy as using water buoyancy and resistance, and says evidence of its effectiveness after these surgeries remains limited. [2] In that review, water-based and land-based programs gave similar pain relief, while water-based programs gave greater gains in leg muscle strength. [2]

This is a proposed mechanism: lighter loading on joints may make movement easier for some people. It is not the same as proof that water exercise heals a joint. For what trials found, see aquatic therapy for osteoarthritis and joint replacement.

Hydrostatic Pressure: Circulation and Breathing

When the body is immersed to the neck, physiology reviews describe an immediate shift of blood toward the heart, followed by a slower movement of fluid into the blood vessels. Both changes raise stroke volume and cardiac output (how much blood the heart pumps per beat and per minute). [3] The same reviews describe hormonal and nervous-system responses that bring blood volume and blood pressure back toward their earlier levels, partly by increasing urine production. [4]

Water pressure also acts on the chest. A review of aquatic physiology reports that water pressure on the chest raises the work of breathing, and that higher pressures in the lung blood vessels are linked with a decline in vital capacity (the largest breath you can take), with a potential for fluid in the lungs. [4] These findings are one reason people with heart or lung conditions are advised to check with a clinician before water exercise or deep soaking. A 2026 review also names the abrupt loss of hydrostatic pressure when someone is lifted out of cold water as a main driver of collapse around rescue. [5]

For more detail, see immersion physiology.

Temperature: Warm, Cool and Cold Water

A small laboratory study compared 10 men resting in water at 34.5 °C and at 30 °C. Resting in water raised cardiac output by about 50 percent compared with sitting in air, and the paper describes 34 to 35 °C as thermoneutral temperature. [6] Stroke volume tended to rise more in the 30 °C water, and the authors suggest that narrowing of blood vessels in the cooler water added to the rise. [6] A broader review adds that in cold water, maximal cardiac output falls and skin and muscle vessels narrow, which lowers exercise capacity. [3]

Heat and cold exposure are covered in thermoregulation basics and water temperature ranges. Cold plunge and ice bath evidence is owned by the biohacking pages: see cold-water immersion: evidence summary.

What These Findings Do Not Show

• They do not show that immersion treats, cures or prevents a disease. Evidence for specific uses is on separate pages, such as hydrotherapy evidence by goal.
• Most are short experiments in small groups of healthy people. Responses in older adults, people with heart disease and people taking blood pressure medicines are less well defined. A 2026 review said safety margins for frail older adults and people on beta-blockers remain insufficiently defined. [7]
• A change in a measurement, such as heart output or urine volume, is a physiological finding. It is not the same as feeling better or being healthier, which are clinical outcomes.