Exactly how much pressure is needed varies according to the body’s activities. For example, the
heart does not need to beat as fast or as hard to keep blood circulating when you are resting as it does
when you are exercising. During exercise, however, more blood is needed to carry oxygen to the muscles,
so blood pressure rises to meet increased demand. The heart pumps faster and pushes out more blood
with each beat. In other situations, such as when someone stands up suddenly after lying down, the
body must make an almost instantaneous adjustment in blood pressure in order to ensure a steady supply
of blood to the brain. Blood vessels in the abdomen and legs constrict and the heart speeds up. Sometimes
there may be a slight delay in this adjustment,
and as a result, you may feel dizzy for a few seconds.
This is more common in older people whose blood
vessel reflexes might be impaired. A longer delay can
bring on a fainting spell, which is the body's way of
increasing the flow of blood to the brain (when someone
lies down, blood flow to the brain increases).