What is Low Blood Pressure?
Blood pressure is the force exerted by circulating blood on the walls of blood vessels, and constitutes one of the principal vital signs of life, which also include heart beat, rate of breathing, and temperature. Blood pressure is generated by the heart pumping blood into the arteries and is regulated by the response by the arteries to the flow of blood.
An individual’s blood pressure is expressed as systolic/diastolic blood pressure, for example, 120/80. The systolic blood pressure (the top number) represents the pressure in the arteries as the muscle of the heart contracts and pumps blood into them. The diastolic blood pressure (the bottom number) represents the pressure in the arteries as the muscle of the heart relaxes after it contracts. Blood pressure always is higher when the heart is pumping (squeezing) than when it is relaxing.
Systolic blood pressure for most healthy adults falls between 90 and 120 millimeters of mercury (mm Hg). Normal diastolic blood pressure falls between 60 and 80 mm Hg. Current guidelines define normal blood pressure as lower than 120/80. Blood pressures over 130/80 are considered high. High blood pressure increases the risk of developing heart disease, kidney disease, hardening of the arteries (atherosclerosis or arteriosclerosis), eye damage, and stroke.
Low blood pressure (hypotension) is pressure so low it causes symptoms or signs due to the low flow of blood through the arteries and veins. When the flow of blood is too low to deliver enough oxygen and nutrients to vital organs such as the brain, heart, and kidney, the organs do not function normally and may be permanently damaged.
Unlike high blood pressure, low blood pressure is defined primarily by signs and symptoms of low blood flow and not by a specific blood pressure number. Some individuals may have a blood pressure of 90/50 with no symptoms of low blood pressure and therefore do not have low blood pressure. However, others who normally have high blood pressure may develop symptoms of low blood pressure if their blood pressure drops to 100/60.
How is blood pressure generated?
During relaxation of the heart (diastole) the left ventricle of the heart fills with blood returning from the lungs. The left ventricle then contracts and pumps blood into the arteries (systole). The blood pressure during contraction of the ventricle (systolic pressure) when blood is being actively ejected into the arteries is higher than during relaxation of the ventricle (diastolic pressure). The pulse that we can feel when we place our fingers over an artery is caused by the contraction of the left ventricle.
Blood pressure is determined by two factors: 1) The amount of blood pumped by the left ventricle of the heart into the arteries, and 2) the resistance to the flow of blood caused by the walls of the arterioles (smaller arteries).
Generally, blood pressure tends to be higher if more blood is pumped into the arteries or if the arterioles are narrow and stiff. (Narrow and stiff arterioles, by resisting the flow of blood, increase blood pressure.) This often happens when older patients develop atherosclerosis.
Blood pressure tends to be lower if less blood is being pumped into the arteries or if the arterioles are larger and more flexible and, therefore, have less resistance to the flow of blood.
How does the body maintain normal blood pressure?
The body has mechanisms to alter or maintain blood pressure and blood flow. There are sensors that sense blood pressure in the walls of the arteries and send signals to the heart, the arterioles, the veins, and the kidneys that cause them to make changes that lower or increase blood pressure. There are several ways in which blood pressure can be adjusted – by adjusting the amount of blood pumped by the heart into the arteries (cardiac output), the amount of blood contained in the veins, the arteriolar resistance, and the volume of blood.
- The heart can speed up and contract more frequently and it can eject more blood with each contraction. Both of these responses increase the flow of blood into the arteries and increase blood pressure.
- The veins can expand and narrow. When veins expand, more blood can be stored in the veins and less blood returns to the heart for pumping into the arteries. As a result, the heart pumps less blood, and blood pressure is lower. On the other hand, when veins narrow, less blood is stored in the veins, more blood returns to the heart for pumping into the arteries, and blood pressure is higher.
- The arterioles can expand and narrow. Expanded arterioles create less resistance to the flow of blood and decrease blood pressure, while narrowed arterioles create more resistance and raise blood pressure.
- The kidney can respond to changes in blood pressure by increasing or decreasing the amount of urine that is produced. Urine is primarily water that is removed from the blood. When the kidney makes more urine, the amount (volume) of blood that fills the arteries and veins decreases, and this lowers blood pressure. If the kidneys make less urine, the amount of blood that fills the arteries and veins increases and this increases blood pressure. Compared with the other mechanisms for adjusting blood pressure, changes in the production of urine affect blood pressure slowly over hours and days. (The other mechanisms are effective in seconds).
For example, low blood volume due to bleeding (such as a bleeding ulcer in your stomach or from a bad laceration from an injury) can cause low blood pressure. The body quickly responds to the low blood volume and pressure by the following adjustments which all increase blood pressure:
- The heart rate increases and the forcefulness of the heart’s contractions increase, thus more blood is pumped through the heart.
- Veins narrow to return more blood to the heart for pumping.
- Blood flow to the kidneys decreases to reduce the formation of urine and thereby increases the volume of blood in the arteries and veins.
- Arterioles narrow to increase resistance to blood flow
These adaptive responses will keep the blood pressure in the normal range unless blood loss becomes so severe that the responses are overwhelmed.
Source: www.medicinenet.com
