After standing, a fall in blood pressure

Known as orthostatic hypotension, this condition can leave people dizzy and lightheaded. For some, it may also be a harbinger of heart disease.

Have you ever felt a bit woozy just after standing up? When you transition from sitting to standing, gravity causes blood to pool in your legs. Sometimes — especially if you're dehydrated or overheated — it takes your body a moment to push blood upward, causing a brief delay in blood flow to the brain.

For most people, this phenomenon occurs just once in a while and usually isn't a serious problem. But age, some medications, and certain medical conditions can interfere with the body's normal mechanism for regulating blood pressure (see "Responding to pressure: The role of baroreceptors"). If that happens, your blood pressure may drop dramatically when you stand up. Known as orthostatic hypotension (OH), the problem affects about 5% of people younger than 50. But up to 20% of people ages 70 and older may have OH.

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