Sensors in your body constantly monitor your blood pressure to ensure adequate blood flow to your organs and tissues. Many everyday circumstances affect your blood pressure, not the least of which is your body position. When shifting from lying down, known as the supine position, to standing, rapid adjustments occur to ensure your blood pressure is maintained. A significant difference between standing versus supine blood pressure could indicate a short- or long-term medical problem.
Posture, Gravity and Blood Pressure
The challenge your body faces in maintaining your blood pressure when shifting from lying down to standing boils down to gravity and your brain. Your brain is exquisitely sensitive to blood flow, demanding a constant supply of oxygen-rich blood to function normally. When you're lying flat, your blood flows easily from your heart to your brain as they're both at the same level. But when you stand up, your circulatory system has to work against gravity to pump blood upward to your brain -- and to prevent an excess amount of blood from pooling in your legs. Your body normally makes this adjustment quickly and seamlessly.
Adjustments for Body Position
Your body employs several complex mechanism to regulate your blood pressure. Short-term regulatory mechanism ensure that your blood pressure stays relatively constant as you change from one body position to another. A cardiovascular command center in your brain receives information from your body and responds by sending nerve signals to your heart and blood vessels. For example, when you stand up from a lying position, nerve signals direct your heart to speed up a bit and contract more forcefully. Other signals instruct certain blood vessels to constrict to keep your blood pressure from falling. The opposite short-term effects occur when you shift from standing to lying down.
When you stand up from lying down or sitting, a small momentary dip in blood pressure occurs in the first few seconds. This occurs because gravity causes about 0.5 to 1.0 liters of blood to pool in your lower body. Your body normally corrects for this unnoticeable blood pressure dip rapidly, usually within about 30 seconds. Your doctor may measure your blood pressure in both a standing and lying position to make sure your body responds normally to changes in position. Standing blood pressure is normally only about 3 to 4 mmHg lower than supine blood pressure, with gravity-induced pooling of blood in the lower body accounting for the difference.
Postural hypotension refers to an abnormally large decrease in blood pressure when you move from lying down or sitting to a standing position. This condition, also known as orthostatic hypotension, often causes dizziness or lightheadedness on standing. In severe cases, fainting can occur. These symptoms can last from a few seconds to several minutes, and may lead to injury due to a fall. There are many possible causes of postural hypotension. This condition occurs more commonly in older adults compared to younger adults and children.
Causes of Postural Hypotension
Dehydration is one of the most common causes of postural hypotension. A severe bout of the stomach flu or exercising in hot weather, for example, might lead to dehydration and temporary postural hypotension. Significant blood loss due to injury or anemia can have a similar effect. Medications are another common cause of occasional orthostatic hypotension, especially medicines prescribed to treat high blood pressure or heart disease. A number of diseases and medical conditions can cause persistent bouts of postural hypotension, including:
-- heart attack and stroke
-- heart rhythm and valve abnormalities
-- diabetic or alcohol-related nerve damage
-- Parkinson disease
-- low blood potassium
-- adrenal gland malfunction
See your doctor as soon as possible if you experience repeated bouts of dizziness or lightheadedness on standing. Seek emergency medical care if you experience symptoms of postural hypotension with other symptoms, such as chest or abdominal pain, shortness of breath, weakness, numbness or confusion.
- Cardiovascular Physiology Concepts: Blood Pressure Regulation Tutorial
- American Family Physician: Evaluation and Management of Orthostatic Hypotension
- Marino's ICU Book, 4th Edition; Paul L. Marino, M.D., Ph.D.
- Merck Manual Professional Version: Orthostatic Hypotension