Cooling down, also called warming down, is the term used to describe an easy exercise that will allow the body to gradually transition from an exertional state to a resting or near-resting state. Depending on the intensity of the exercise, cooling down can involve a slow jog or walk, or with lower intensities, stretching can be used. Cooling down helps remove lactic acid

Lactic acid

Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...

 which can cause cramps and stiffness

Joint stiffness

Joint stiffness may be either the symptom of pain on moving a joint, the symptom of loss of range of motion or the physical sign of reduced range of motion. Doctors prefer the latter two uses but patients often use the first meaning....

 and allows the heart rate to return to its resting rate. Contrary to popular belief, cool down does not appear to reduce delayed-onset muscle soreness

Delayed onset muscle soreness

Delayed onset muscle soreness , also called muscle fever, is the pain and stiffness felt in muscles several hours to days after unaccustomed or strenuous exercise. The soreness is felt most strongly 24 to 72 hours after the exercise. It is caused by eccentric exercise...

.

Procedure

Cool downs should involve a gradual yet continuous decrease in exercise intensity (i.e. from a hard run to an easy jog to a brisk walk), stretching, and rehydration. Durations can vary for different people, but 5–10 minutes is considered adequate.

Heart rate

During aerobic exercise, peripheral veins, particularly those within muscle, dilate to accommodate the increased blood flow through exercising muscle. The skeletal-muscle pump

Skeletal-muscle pump

The skeletal-muscle pump is a collection of skeletal muscles that aid the heart in the circulation of blood. It is especially important in increasing venous return to the heart, but may also play a role in arterial blood flow.-Arterial blood pumping:...

 assists in returning blood to the heart and maintaining cardiac output. A sudden cessation of strenuous exercise may cause blood to pool in peripheral dilated veins and the heart must beat faster and harder to adequately oxygenate the body and maintain blood pressure

Blood pressure

Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

. A cool-down period allows a more gradual return to venous tone, and allows a gradual decline in heart rate

Heart rate

Heart rate is the number of heartbeats per unit of time, typically expressed as beats per minute . Heart rate can vary as the body's need to absorb oxygen and excrete carbon dioxide changes, such as during exercise or sleep....

that reduces stress on the organ.
Besides, as the heart rate increases, there is more oxygen supply to the body, which helps the breaking down of lactic acid and hence prevent it from causing harm to the body.