When you stand up, gravity causes blood to pool in your legs and abdomen. This decreases blood pressure because there's less blood circulating back to your heart.
Normally, special cells (baroreceptors) near your heart and neck arteries sense this lower blood pressure. The baroreceptors send signals to centers in your brain, which signals your heart to beat faster and pump more blood, which stabilizes blood pressure. These cells also narrow the blood vessels and increase blood pressure.
Orthostatic hypotension occurs when something interrupts the body's natural process of counteracting low blood pressure. Many different conditions can cause orthostatic hypotension, including:
- Dehydration. Fever, vomiting, not drinking enough fluids, severe diarrhea and strenuous exercise with excessive sweating can all lead to dehydration, which decreases blood volume. 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 prevent your body from responding rapidly enough to pump more blood when standing up.
- Endocrine problems. Thyroid conditions, adrenal insufficiency (Addison's disease) and low blood sugar (hypoglycemia) can cause orthostatic hypotension, as can diabetes — which can damage the nerves that help send signals regulating blood pressure.
- Nervous system disorders. Some nervous system disorders, such as Parkinson's disease, multiple system atrophy, Lewy body dementia, pure autonomic failure and amyloidosis, can disrupt your body's normal blood pressure regulation system.
- After eating meals. Some people experience low blood pressure after eating meals (postprandial hypotension). This condition is more common in older adults.