What is Low Blood Pressure (Hypotension)

Low blood pressure or hypotension is a condition in which the blood pressure in the arteries is much lower than the normal range.

  • Hypotension may be due to the malfunctioning of the body’s blood pressure controlling mechanism, due to certain disorders or drugs.

The body has a natural mechanism by which it maintains a normal blood pressure reading. Too high a blood pressure can cause damage to the arteries and the smaller arterioles, even causing their rupture, whereas too low a pressure in the arteries affects blood distribution to the different parts of the body. If blood distribution is compromised, tissues become starved of oxygen and essential nutrients and the waste products accumulate in the tissues. Extremely low blood pressure is a potentially fatal condition, which can result in shock and eventually death. At the same time, people with blood pressure nearer to the lower limit within the normal range, seem to have a higher life expectancy than those whose blood pressure is nearer to the upper limit, within the normal range.

The body controls the blood pressure by either changing the size of the smaller veins and arterioles, or by adjusting the amount of circulating blood in the body. It can also increase or decrease the volume of blood being pumped by the heart by changing the cardiac output. By activating any of these mechanisms, the body restores blood pressure to normal levels after an increase or decrease, during exercise or sleep respectively.

Veins dilate to increase their capacity and constrict to decrease their capacity to hold blood. When their capacity is reduced by constriction, more blood returns to the heart and from where it will be pumped into the arteries, increasing the arterial pressure.

When arterioles dilate, the blood pressure within them lowers and when they constrict, the pressure increases within.

The amount of blood pumped by the heart per minute can be controlled, either by increasing or decreasing the number of heart beat per minute, or by changing the force with which each contraction takes place.

The total circulating volume of blood can determine the blood pressure in the arteries, when all other factors such as the size of the vessels and the cardiac output remain the same. By increasing or decreasing the amount of urine excreted by the kidneys, the volume of circulating blood can be controlled.

The baroreceptors are a group of cells within the arteries, which act as pressure sensors, keeping track of the blood pressure within. They play an important role in regulating blood pressure, especially those in the larger neck and chest arteries. Changes in blood pressure are detected by the baroreceptors, and the body’s mechanism to counteract the changes is activated by nerves of the sympathetic system, which carry the required instructions to various organs.

When hypotension develops:

  • The heart increases the rate and force of heartbeats, thus increasing the amount of blood pumped into the arteries, elevating blood pressure quickly.
  • The arterioles constrict to increase the blood pressure within the arteries.
  • The veins constrict to bring in more blood for the heart for pumping into the arteries to increase blood pressure.
  • The kidneys decrease their output of urine, preventing further fluid loss. This is a slow acting blood pressure controlling mechanism.

When there is severe blood loss, blood pressure plummets due to the reduction in the blood volume. The body’s homeostatic mechanism kicks in to arrest the drop in blood pressure, by making the heart to beat faster and more forcefully, thus increasing cardiac output. Urine production by the kidneys is reduced to avoid fluid loss. Blood supply to the bleeding site is reduced by constricting the veins, and the arterioles constrict to increase blood pressure within the arteries. After the bleeding stops, the fluid from the tissues moves into the blood vessels to increase the volume of blood and elevating the blood pressure. Eventually, with the production of blood cells by the bone marrow, the lost blood is fully compensated for. However, if the blood loss is rapid, compensatory mechanisms do not get time to restore blood pressure before shock develops.

Causes of Low Blood Pressure

Hypotension may result from various disorders of the heart such as heart valve disorders, heart attack, fast or slow or irregular arrhythmias. Certain toxins produced during bacterial infections cause the arterioles to dilate and result in low blood pressure. Bleeding and severe dehydration can reduce the circulating volume of blood and cause hypertension. If the kidneys are not able to regulate fluid excretion due to some disorder, excess fluid loss may result in low blood pressure. Neurologic disorders that impair the conduction of stimuli and messages between the sensory receptors and the target organs, interferes with the action of the compensatory mechanisms, thus reducing the body’s capacity to normalize low blood pressure. Slowing down of the body’s blood pressure controlling mechanism due to aging, may make it incapable of meeting the demand, when low blood pressure develops due to some reason.


The brain is the first organ to be affected by low blood pressure, because a certain minimum blood pressure is necessary for the blood to reach the head, which is located much above the heart. The reduced blood supply to the brain results in symptoms usually associated with low blood pressure such as dizziness and fainting. Falling down due to fainting, brings the head at the same level with the heart, helping restore blood flow to the head and minimizing injury to the brain, till the blood pressure returns to normal.

When the supply of blood to the heart muscle decreases due to low blood pressure, angina or shortness of breath may result.

When blood pressure falls below certain minimum levels, the organs of the body shut down and a potentially fatal condition called shock occurs.

If the low blood pressure is a result of an infection, fever may be present, but it is unrelated to the condition itself.

There are certain symptoms which result from the body’s attempts at elevating low blood pressure. When heart beats faster, and with more force, to increase cardiac output, palpitations may be felt. While constriction of arteries helps to increase the blood pressure within the arteries, it also restricts blood flow to the skin and the limbs, which causes them to become cold and blue.

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Yasser Elnahas

MD, PHD, Professor Of CardioVascular Surgery
Dr. Yasser Elnahas, Is an associate Professor of Cardiovascular Surgery. Dr. Elnahas was trained as a fellow At Texas Heart Institute And Mayo Clinic Foundation.Dr. Elnahas is dedicated to educating the general public about different disease conditions and simplifying the medical knowledge in an easy to understand terminology.

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One Response to What is Low Blood Pressure (Hypotension)

  1. Rufus Scarcia says:

    Chronic low blood pressure with no symptoms is almost never serious. But health problems can occur when blood pressure drops suddenly and the brain is deprived of an adequate blood supply. This can lead to dizziness or lightheadedness. Sudden drops in blood pressure most commonly occur in someone who’s rising from a prone or sitting position to standing. This kind of low blood pressure is known as postural hypotension or orthostatic hypotension. Another type of low blood pressure can occur when someone stands for a long period of time. This is called neurally mediated hypotension.

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