Normally, the body maintains the pressure of blood in the arteries within a narrow range. When blood pressure is too high, organs and blood vessels can be damaged. High blood pressure can even cause rupture of a blood vessel and lead to bleeding or other complications.
When 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. Thus, the affected cells and the organs they are in begin to malfunction. Very low blood pressure can be life threatening because it can lead to shock, in which organs are damaged by lack of blood flow.
Healthy people who have blood pressure that is low but still in the normal range (when measured at rest) tend to live longer than people who have blood pressure that is on the high side of normal.
The body has several ways to return blood pressure to normal after it increases or decreases during normal activities, such as exercise or sleep. They involve
Muscle tissue (called smooth muscle) within the walls of arterioles allow these blood vessels to widen (dilate) or narrow (constrict). The more constricted arterioles are, the greater their resistance to blood flow and the higher the blood pressure. Constriction of arterioles 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 degree to which arterioles are constricted or dilated is affected by
Nerves that contract smooth muscle in the arterioles, thus reducing their diameter
Hormones that are primarily made by the kidneys
Veins also play a role in the control of blood pressure, although their effect on blood pressure is much less than that of arterioles. Veins dilate and constrict to change how much blood they can hold (capacity). When veins constrict, their capacity to hold blood is reduced, allowing more blood to return to the heart from which it is pumped into the arteries. As a result, blood pressure increases. Conversely, when veins dilate, their capacity to hold blood is increased, allowing less blood to return to the heart. As a result, blood pressure decreases.
The more blood pumped from the heart per minute (that is, the larger the cardiac output), the higher the blood pressure—as long as the width of the arteries remains constant. The amount of blood pumped during each heartbeat can be affected by
The higher the volume of blood in the arteries, the higher the blood pressure—as long as the width of the arteries remains constant. The volume of blood in the arteries is affected by
How much fluid is in the body (hydration)
Whether very small arteries leak fluid (for example, if protein levels in the blood are very low and/or there is damage to the interior wall of the small arteries, fluid will leak from them into the tissues)
How much fluid the kidneys remove from the blood to excrete in the urine
Certain drugs, particularly diuretics (drugs that help the kidneys remove water from the body)
Blood pressure can vary throughout the body due to the direct action of gravity. When a person is standing, blood pressure is higher in the legs than in the head, much in the way that the water pressure at the bottom of a swimming pool is higher than that at the top. When a person lies down, blood pressure tends to be more equal throughout the body.
When a person stands up, blood from the veins in the legs has a harder time getting back to the heart. As a result, the heart has less blood to pump out, and blood pressure may temporarily drop throughout the body. When a person sits down or lies down, blood can more easily return to the heart, and cardiac output and blood pressure may increase. Elevating the legs above the level of the heart can increase return of blood to the heart, which increases cardiac output and raises blood pressure.
Baroreceptors are specialized cells located within arteries that act as blood pressure sensors. Those in the large arteries of the neck and chest are particularly important. When baroreceptors detect a change in blood pressure, they trigger the body to react to maintain a steady blood pressure. Nerves carry signals from these sensors and the brain to
The heart, which is signaled to change the rate and force of heartbeats (thus changing the amount of blood pumped). This change is one of the first, and it corrects low blood pressure quickly.
The arterioles, which are signaled to constrict or dilate (thus changing the resistance of blood vessels).
The veins, which are signaled to constrict or dilate (thus changing their capacity to hold blood).
The kidneys, which are signaled to change the amount of fluid excreted (thus changing the volume of blood in blood vessels) and to change the amount of hormones that they produce (thus signaling the arterioles to constrict or dilate and changing the volume of blood). This change takes a long time to produce results and thus is the slowest mechanism for how the body controls blood pressure.
For example, when a person is bleeding, blood volume and thus blood pressure decrease. In such cases, sensors activate multiple processes 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, the ways that the body can monitor and control blood pressure have limitations. For example, if a person loses a lot of blood quickly, the body cannot compensate quickly enough, blood pressure falls, and organs may begin to malfunction (shock).
In addition, as people age, the body responds to changes in blood pressure more slowly.
Low blood pressure typically results from one or more of the following:
Dilation of arterioles can be caused by
Various heart disorders that impair the heart's pumping ability and reduce cardiac output include
Too little blood volume in the body may be caused by
Low blood pressure also occurs when the nerves that conduct signals between the brain and the heart and blood vessels are impaired by neurologic disorders called autonomic neuropathies.
When a person quickly moves from a sitting position to a standing position, blood pressure in the blood vessels to the brain decreases, resulting in a temporary sensation of light-headedness or faintness. This is called orthostatic hypotension. It can be more pronounced in people who are dehydrated or warm (such as emerging from a hot bath), have certain illnesses, or have been lying down or sitting for prolonged periods of time. Orthostatic hypotension can even cause people to faint. In most people, the body quickly acts to increase blood pressure and prevent the person from fainting.
Some Causes of Low Blood Pressure
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 flow must fight gravity to reach the brain. Consequently, most people with low blood pressure feel dizzy or light-headed, particularly 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. Also, fainting can result in serious injuries to the head or other parts of the body.
Low blood pressure occasionally causes shortness of breath or chest pain due to an inadequate blood supply to the heart muscle (a condition called angina).
All organs begin to malfunction if blood pressure becomes sufficiently low and remains low. This condition is called shock.
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 tries 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).
The doctor measures blood pressure and pulse while the person is lying down for a few minutes. If the blood pressure is not low and the person feels well, the doctor has the person stand up and rechecks the blood pressure right after standing up, and after a few minutes of standing. Other tests may be done to determine the cause of the low blood pressure, such as:
Doctors treat the cause of low blood pressure. They often also give people intravenous fluids if their heart can handle the extra fluid.
Depending on the cause of the symptoms, doctors may recommend wearing elastic compression stockings that cover the calf and thigh to help push blood out of the veins in the legs and back up to the heart.