Low Blood Pressure
Low blood pressure, hypotension, is often confused with hypertension because they sound similar. However they are complete opposites. Both however do have serious health consequences. Blood pressure is controlled by the autonomic nervous system. This system uses a vast network of hormones, receptors and nerves to balance the effects of the sympathetic nervous system (which raises blood pressure) and the parasympathetic nervous system (which lowers blood pressure). This control occurs on a constant basis and is rapid. This allows people to exercise and perform other activities without a drastic change in blood pressure.
There are three main mechanisms, or causes, of low blood pressure. The most common mechanism is reduced blood volume. This is called hypovolemia and can have many different causes. Hemorrhage, or massive blood loss; not drinking enough fluids such as water or juice; or persistent diarrhea or vomiting can all cause reduced blood volume. Another cause of hypovolemia is excessive diuretics use. This causes the body to produce excessive amounts of urine, which can reduce the amount of fluid in the body very quickly.
Another mechanism that causes low blood pressure is decreased cardiac output. This is usually caused by heart failure of some type. This cause of hypotension can quickly lead to cardiogenic shock, or shock caused by a lack of blood circulation. Arrhythmias, or an irregular heartbeat, can cause cardiogenic shock. A type of medication, beta-blockers, lowers blood pressure by decreasing both the heart rate and the pumping ability of the heart. This results in decreased cardiac output and may even cause hypotension. This is one reason doctors always want to know what medications you take, even the over the counter ones.
The third and final, mechanism of hypotension is an insufficient constriction of the arterioles, also known as excessive vasodilation. There are many reasons this can occur, such as a decrease in the sympathetic nervous system output or an increase in the parasympathetic activity caused by a brain injury, or an abnormality in the autonomic nervous system. Excessive vasodilation can also be caused by certain medical conditions such as sepsis (severe untreated infection) or acidosis (an increase in blood acidity). It can also be caused by some medications, such as nitrate preparations and some anesthetic agents.
The most common form of low blood pressure is called orthostatic hypotension, or postural hypotension. As the second name implies, it occurs when there is a rapid change of body position, usually from a seated position to standing. It is usually temporary. This is the most commonly caused by low blood volume. Taking blood pressure measurements while the person is seated, lying down and standing can confirm this. The second form of hypotension is called neurocardiogenic syncope and commonly results in fainting. A sudden drop in blood pressure while standing characterizes it.
The other form of hypotension is rather rare and is called postprandial hypotension. This form of hypotension occurs about 30 minutes to an hour after eating. It occurs shortly after eating because it is associated with digestion and the intestines need for increased blood flow. It is caused by the body being unable to compensate for this. Some doctors believe this is caused by a faulty or aged autonomic nervous system.
Michael Russell
Your Independent guide to Blood Pressure
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