Does low blood pressure cause low oxygen

• 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.

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.

• 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

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

• 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.

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 fast the heart is beating

• How strongly the heart contracts

• How much blood comes into the heart from the veins

• The pressure in the arteries that the heart has to pump against

• How well the heart valves let blood out and prevent the backflow 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 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. The receptors 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 and the heart beats more forcefully with each contraction, increasing the amount of blood pumped

• The veins constrict, reducing their capacity to hold blood in less important parts of the body

• 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.

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 small arteries (arterioles)

• Certain heart disorders

• Too little blood volume

Dilation of arterioles can be caused by

• Certain drugs

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

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

• Dehydration

• Bleeding

• A kidney disorder

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 Dizziness or Light-Headedness When Standing Up In some people, particularly older people, blood pressure drops excessively when they sit or stand up (a condition called orthostatic or postural hypotension). Symptoms of faintness, light-headedness... read more or light-headed, particularly when they stand, and some may even faint Fainting Light-headedness (near syncope) is a sense that one is about to faint. Fainting (syncope) is a sudden, brief loss of consciousness during which the person falls to the ground or slumps in a... read more . 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.

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.

• Measuring blood pressure

• Tests to determine cause

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:

• Blood tests

• Treatment of the cause

• Fluids given by vein (intravenously)

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.

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