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Low Blood Pressure

(Hypotension)

By Oren Traub, MD, PhD

Low blood pressure is blood pressure low enough to cause symptoms such as dizziness and fainting. Very low blood pressure can cause damage to organs, a process called shock.

  • Various drugs and disorders can affect the body's system for maintaining blood pressure.

  • When blood pressure is too low, the brain malfunctions, and fainting may occur.

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. Very low blood pressure can be life threatening because it can lead to shock. 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 (see High Blood Pressure : The Body's Control of Blood Pressure). They involve

  • Changing the diameter of small arteries (arterioles) and, to a lesser extent, veins

  • Changing the amount of blood pumped from the heart to the body (cardiac output)

  • Changing the volume of blood in the blood vessels

  • Changing the body's position

Changing the diameter of arterioles and veins

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

  • Certain drugs

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.

Changing cardiac output

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

  • How strongly the heart contracts

  • How well the heart valves prevent backflow of blood

  • The amount of blood in the heart at the time of the heart beat

Changing the volume of blood

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 the arteries leak fluid (for example, if protein levels in the blood are very low, fluid will leak from the arteries into the body)

  • How much fluid the kidneys remove from the blood to excrete in the urine

  • Certain drugs, particularly diuretics

Changing the body's position

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. In response, the heart pumps out less blood, and blood pressure may 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.

Monitoring and controlling 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 increasing 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.

Causes

Low blood pressure can result from dilation of arterioles caused by

  • Toxins produced by bacteria during certain severe bacterial infections ( septic shock)

  • Certain drugs

  • Spinal cord injuries, in which the nerves that cause the arterioles to constrict are impaired

  • Allergic reactions ( anaphylaxis)

  • Certain endocrine disorders, such as Addison disease

Various heart disorders can result in low blood pressure. For example, disorders that impair the heart's pumping ability can reduce cardiac output, such as

  • A heart attack (myocardial infarction)

  • A heart valve disorder

  • An extremely rapid heartbeat (tachycardia)

  • A very slow heartbeat (bradycardia)

  • An abnormal heart rhythm (arrhythmia)

Low blood pressure can also result from too little blood volume in the body, which may be caused by

  • Dehydration

  • Bleeding

  • A kidney disorder

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 lightheadedness or faintness. This is called orthostatic hypotension. It can be more pronounced in people who are dehydrated, warm (such as emerging from a hot bath), have certain illnesses, or who have been lying down or sitting for prolonged periods of time and can even cause them 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

What Happens in the Body

Examples of Causes

Blood vessels become dilated (widened)

Alcohol

Some allergic reactions

Some antidepressants, such as amitriptyline

Antihypertensive drugs that dilate blood vessels (such as calcium channel blockers, angiotensin-converting enzyme inhibitors, and angiotensin II receptor blockers)

Nitrates

Severe bacterial infections

Exposure to heat

Nerve damage (such as that due to diabetes, amyloidosis, or spinal cord injuries)

Cardiac output is decreased

Abnormal heart rhythms

Heart muscle damage or malfunction (such as that due to a heart attack or viral infection)

Heart valve disorders

Pulmonary embolism

Blood volume is decreased

Diarrhea

Diuretics (such as furosemide and hydrochlorothiazide)

Excessive bleeding

Excessive sweating

Excessive urination (a common symptom of untreated diabetes or Addison disease)

The autonomic nervous system becomes impaired

Amyloidosis

Diabetes

Multiple system atrophy (Shy-Drager syndrome)

Parkinson disease

Blood flow back to the heart is blocked

During pregnancy, pressure on the inferior vena cava (the main vein that carries blood from the legs) from the uterus when women lie in certain positions

Increased abdominal pressure when straining to move bowels or pass urine or when lifting heavy weights

The brain centers that control blood pressure are inhibited

Alcohol

Antidepressants

Antihypertensive drugs such as methyldopa and clonidine

Barbiturates

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 flow must fight gravity to reach the brain. Consequently, most people with low blood pressure feel dizzy or light-headed, particularly when they stand (see Dizziness or Light-Headedness When Standing Up), 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).

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