Always remember to cool down after exercising. This gives your muscles a chance to relax and prevents your blood pressure from dropping too rapidly, which can happen if your blood is allowed to pool in your extremities.
Exercising activates the sympathetic nervous system, the part of the nervous system that is responsible for your body's "flight or fight" response. This is your body's physiologic response to challenges such as running from a saber tooth tiger, defending your home against an intruder, or summoning the nerve to ask someone for a date. Your eyes dilate, heart rate increases, blood pressure rises (in fact, your blood can pump 400 - 600 percent more than when at rest), and your arteries redirect your blood flow away from your abdomen and to your heart, brain, and extremities (if they are active). When you exercise using your arms and legs, the arteries in your extremities dilate to allow blood to flow to them. When you stop exercising, your sympathetic nervous system turns off and your parasympathetic nervous system turns on.
The parasympathetic nervous system takes over when you are at rest, such as immediately after a large meal. Your blood pressure drops, your blood vessels relax and dilate, blood flows to your abdomen, and your heart rate slows. The blood that only moments ago was being powerfully pumped by your sympathetically charged heart no longer has that strong push, and has a tendency to pool in your extremities. The blood does not get to your head, creating the potential for fainting. This can be avoided by cooling down after you exercise.