Introduction

Low blood pressure (hypotension) is blood pressure low enough to cause symptoms such as dizziness and fainting.

Normally, the body maintains the pressure of blood in the arteries within a narrow range. If blood pressure is too high, it can damage a blood vessel and even rupture it, causing bleeding or other complications. If blood pressure is too low, not enough blood reaches all parts of the body; as a result, cells do not receive enough oxygen and nutrients, and waste products are not adequately removed. Even so, having low blood pressure is generally better than having high blood pressure. Healthy people who have blood pressure that is low but still in the normal range (when measured at rest) tend to live longer than those who have higher normal blood pressure.

The body has several compensatory mechanisms that control blood pressure (see Section 3, Chapter 22). They involve changing the diameter of veins and small arteries (arterioles), the amount of blood pumped from the heart (cardiac output), and the volume of blood in the blood vessels. These mechanisms return blood pressure to normal after it increases or decreases during normal activities, such as exercise or sleep.

Veins can widen (dilate) and narrow (constrict) to change how much blood they can hold (capacity). When veins constrict, their capacity to hold blood is reduced, forcing more blood into the arteries. As a result, blood pressure increases. Conversely, when veins dilate, their capacity to hold blood is increased, forcing less blood into the arteries. As a result, blood pressure decreases.

Arterioles can also dilate and constrict. The more constricted arterioles are, the greater their resistance to blood flow and the higher the blood pressure. Constriction of arterioles (decreasing their diameter) increases blood pressure because more pressure is needed to force blood through the narrower space. Conversely, dilation of arterioles reduces resistance to blood flow, thus reducing blood pressure.

The more blood pumped from the heart per minute (that is, the larger the cardiac output), the higher the blood pressure--as long as resistance to blood flow in the arteries remains constant. The body can change the amount of blood pumped during each heartbeat by making the heart rate slower or faster or by making each contraction weaker or stronger.

The higher the volume of blood in the blood vessels, the higher the blood pressure--as long as resistance to blood flow in the arteries remains constant. To increase or decrease blood volume, the kidneys can vary the amount of fluid excreted in urine.

The compensatory mechanisms are activated by specialized cells that act as sensors, called baroreceptors. Located within arteries, these sensors constantly monitor blood pressure. Those in the neck and chest are particularly important. When sensors detect a change in blood pressure, they trigger a change in one of the compensatory mechanisms and so maintain a steady blood pressure. Nerves carry signals from these sensors and the brain to several key organs, which control the compensatory mechanisms:

The heart is signaled to change the rate and force of heartbeats (thus changing the amount of blood pumped). This change is one of the first; it corrects blood pressure quickly.
The arterioles are signaled to constrict or dilate (thus changing the resistance of blood vessels).
The veins are signaled to constrict or dilate (thus changing their capacity to hold blood).
The kidneys are signaled to change the amount of fluid excreted (thus changing the volume of blood in blood vessels). This change takes a long time to produce results and thus is the slowest mechanism for blood pressure control.

For example, when a person is bleeding, blood volume and thus blood pressure decrease. In such cases, sensors activate the compensatory mechanisms to prevent blood pressure from decreasing too much: The heart rate increases, increasing the amount of blood pumped; the veins constrict, reducing their capacity to hold blood; and the arterioles constrict, increasing their resistance to blood flow. If the bleeding is stopped, fluids from the rest of the body move into the blood vessels to begin restoring blood volume and thus blood pressure. The kidneys decrease their production of urine. Thus, they help the body retain as much fluid as possible to return to the blood vessels. Eventually, the bone marrow and spleen produce new blood cells, and blood volume is fully restored.

Nonetheless, these compensatory mechanisms have limitations. For example, if a person loses a lot of blood quickly, these mechanisms cannot compensate quickly enough, and blood pressure falls.

Causes

Various disorders and drugs can cause the compensatory mechanisms to malfunction, and low blood pressure may result. For example, cardiac output may be reduced as a result of heart disease, such as a heart attack (myocardial infarction), a heart valve disorder, an extremely rapid heartbeat (tachycardia), a very slow heartbeat (bradycardia), or another abnormal heart rhythm (arrhythmia). These disorders impair the heart's pumping ability. Arterioles may be dilated by toxins produced by bacteria during a bacterial infection. Blood volume can be reduced as a result of dehydration, bleeding, or kidney disorders. Some kidney disorders impair the kidneys' ability to return fluid to the blood vessels, resulting in the loss of large amounts of fluid in the urine. (Conversely, kidney failure, in which the kidneys cannot remove fluid from the blood, may result in overhydration leading to high blood pressure.) The ability of the nerves to conduct signals between sensors and the organs that control the compensatory mechanisms may be impaired by neurologic disorders (a condition called autonomic nervous system failure). In addition, as people age, compensatory mechanisms respond to changes in blood pressure more slowly.

Symptoms

When blood pressure is too low, the first organ to malfunction is usually the brain. The brain malfunctions first because it is located at the top of the body and blood has to fight gravity to reach the brain. Consequently, most people with low blood pressure feel dizzy or light-headed when they stand, and some may even faint. People who faint fall to the floor, usually bringing the brain to the level of the heart. As a result, blood can flow to the brain without having to fight gravity, and blood flow to the brain increases, helping protect it from injury. However, if blood pressure is low enough, brain damage can still occur.

Low blood pressure occasionally causes shortness of breath or chest pain due to an inadequate blood supply to the heart muscle (angina).

All organs begin to malfunction if blood pressure becomes sufficiently low and remains low; this condition is called shock (see Section 3, Chapter 24).

The disorder causing low blood pressure may produce many other symptoms, which are not due to low blood pressure itself. For example, an infection may produce a fever.

Some symptoms occur when the body's compensatory mechanisms try to increase blood pressure that is low. For example, when arterioles constrict, blood flow to the skin, feet, and hands decreases. These areas may become cold and turn blue. When the heart beats more quickly and more forcefully, a person may feel palpitations (awareness of heartbeats).

See the table Some Causes of Low Blood Pressure.