Exercise causes an increase in cardiac output. This increase in cardiac output is necessitated by the active muscles' need for oxygenated blood, and lots of it. For large amounts of blood to be circulated to the active tissue, an equally large amount of blood must be returned. Blood pooling can occur with rapid cessation of exercise due to an inadequate amount of blood returning to the heart.
Venous Return & the Muscle Pump
During exercise, your muscles aid the amount of blood returned to the heart by contracting with more force around the blood vessels. This causes the blood to easily resist the forces of gravity and return quickly to the heart for re-oxygenation and re-circulation. When you stop exercising quickly, the muscles are no longer contracting against your blood vessels – gravity causes the blood to pool in the lower extremities. When this occurs, you may feel faint or dizzy or experience a loss of consciousness.
The purpose of a brief cool-down after cardiovascular exercise is to slowly return your heart to its resting state. By slowly bringing your heart rate back down, you can avoid blood pooling in the lower extremities because the muscles of your legs are still contracting and contributing to venous return. A cool-down also helps to avoid rapid changes in blood pressure. Always engage in a five- to 10-minute cool-down of light cardiovascular exercise such as walking or cycling on a stationary bike.