The American Academy of Orthopedic Surgeons recommends doing cool down exercises similar to the intense workout but at a decreased intensity. For example, if the workout involved walking at a quick pace, the cool down is to walk slowly a few minutes until breathing and heart rate are normal.
Doing cool-down exercises is necessary to give the body time to recover to its pre-exercise condition with normal resting heart rate. According to an article in About.com Sports Medicine, cooling down for a few minutes after maximal efforts mainly prevents the pooling of blood in the extremities and the deprivation of oxygen to the brain, which may result in dizziness and fainting. This explains the passing out of some athletes who immediately stop running instead of continuing to move at a slower pace after crossing the finish line.