Gravity Water lessens the effect of gravity on the body and makes it easier for blood to be pumped uphill and back to the heart. This lessens the strain on the heart, resulting in a lower exercising heartrate. The hydrostatic effects of water cause a shift of blood volume from the periphery of the body to the thorax. This increases the central venous pressure, stroke volume, and cardiac output, which leads to a decrease in heart rate. This is evidenced in water that is at chest level.
Gases enter liquids more easily under pressure.
Thus, oxygen is absorbed more easily into the blood during water exercise. More efficient oxygen transfer may reduce the workload of the heart. A primitive reflex associated with a nerve found in the nasal area is called the dive reflex. When the face is submerged, this reflex lowers heartrate and blood pressure. This reflex is stronger in some individuals than others.