Blood pressure is a measurement of the force exerted by the flow of blood against a vessel wall, as the heart beats. As blood journeys through the body, it may move rapidly through arteries, make its way slowly uphill, squeeze through capillaries or pool in the liver. Blood pressure is influenced by the strength of the heartbeat, the elasticity of the arteries, the resistance of the capillaries, and the amount of blood in circulation. A significant or sudden decrease in blood pressure may cause symptoms of nausea, vomiting, dizziness, fainting, clammy skin, confusion and thirst.
Those who are physically fit have lower blood pressure and a slower pulse than people who aren't athletic. This is because a well-conditioned heart beats efficiently. Nonsmokers and those who eat a plant-based, high fiber, low fat diet and have a body mass index (BMI) between 18.5 and 24.9, are likely to have blood pressure lower than 120/80, which is considered normal by the American Heart Association. BMI is an indicator of the percentage of body fat for a person's height and weight.
Normally, blood volume expands during pregnancy to meet the physiological needs of the fetus. This means that more blood, which is more dilute, has more places to go. By the second trimester, blood pressure is expected to fall from 5 to 10 points systolic, and 10 to 20 points diastolic, according to the Mayo Clinic. Systolic blood pressure is the top number, resulting from the heart contracting, and diastolic is the bottom number, measured when the heart relaxes.
Low Blood Volume
Shock and hemorrhage can cause a precipitous drop in blood pressure due to hypovolemia, or low blood volume. Dehydration may also cause low blood volume. Water loss can result from the use of diuretic medicines or herbs, vomiting, diarrhea, strenuous exercise and excessive perspiration. Signs of dehydration are dizziness, weakness, and fatigue. A sudden decrease in blood pressure due to severe dehydration may be life-threatening.
Depending on the type of medication used to treat hypertension, various factors are influenced to decrease blood pressure. Diuretics (water pills) lower fluid volume. Beta-blockers interfere with the effects of adrenalin on the heart. Calcium channel blockers alter electrolytes and slow heart rate. Angiotensin-converting enzyme (ACE) inhibitors prevent the constriction of blood vessels. Other drugs can also decrease blood pressure, such as antidepressants, those used to treat Parkinson's disease, and medications that remedy erectile dysfunction. Drug-induced postural, or orthostatic, hypotension causes a sudden decrease in blood pressure upon standing, because these medications interfere with blood vessel constriction and increased heart rate which are needed to quickly move blood from the lower limbs to the brain when moving from lying down to an upright position. The drop usually occurs within three minutes of standing and may involve a systolic decrease of 20 points or more, or a ten-point decrease in diastolic blood pressure, according to the American Autonomic Society and the American Academy of Neurology.
Is This an Emergency?
- Mayo Clinic: Low Blood Pressure
- NHLBI: Types of Blood Pressure Medications
- "American Family Physician", Orthostatic Hypotension, Bradley J, M.D., and Davis K, R.N., Dec. 15, 2003, 68(12)
- "Neurology", Consensus statement on the definition of orthostatic hypotension, pure autonomic failure, and multiple system atrophy. The Consensus Committee of the American Autonomic Society and the American Academy of Neurology, no authors listed, May, 1996, 46(5)
- American Heart Association: Low Blood Pressure
- Teen Health: Dehydration
- CDC: Adult BMI Calculator