When you stand up, gravity causes blood to pool in your legs. This decreases blood pressure because there's less blood circulating back to your heart to pump. Normally, special cells (baroreceptors) near your heart and neck arteries sense this lower blood pressure and counteract it by triggering your heart to beat faster and pump more blood, which stabilizes blood pressure. In addition, these cells cause blood vessels to narrow, which increases resistance to blood flow and increases blood pressure.

Orthostatic or postural hypotension occurs when something interrupts the body's natural process of counteracting low blood pressure. Orthostatic hypotension can be caused by many different things, including:

  • Dehydration. Fever, vomiting, not drinking enough fluids, severe diarrhea and strenuous exercise with excessive sweating can all lead to dehydration. When you become dehydrated, your body loses blood volume. Even mild dehydration can cause symptoms of orthostatic hypotension, such as weakness, dizziness and fatigue.
  • Heart problems. Some heart conditions that can lead to low blood pressure include extremely low heart rate (bradycardia), heart valve problems, heart attack and heart failure. These conditions may cause orthostatic hypotension because they prevent your body from being able to respond rapidly enough to pump more blood when needed, such as when standing up.
  • Diabetes. Untreated diabetes can cause dehydration by causing frequent urination. This can trigger orthostatic hypotension. In addition, diabetes can damage the nerves that help send signals regulating blood pressure.
  • Nervous system disorders. Some diseases, such as Parkinson's disease, multiple system atrophy (Shy-Drager peripheral neuropathies) and amyloidosis, can disrupt your body's normal blood pressure regulation system.
Jul. 20, 2011