How Do Our Bodies Control Blood Pressure
In this article i will try to focus on understanding blood pressure and the role of our bodies in controlling it. Our body has its own defense mechanisms to maintain the blood pressure within range. By changing the total circulatory volume of blood in the body, by controlling the volume of blood pumped by the heart or by adjusting the diameter of arteries, the body can control blood pressure. When the rate of contractions as well as the force of each contraction is lowered, it results in reducing the amount of blood pumped into the arteries, lowering the pressure in them. By removing some of the fluid from the body through excretion, the blood pressure can be lowered. Vasodilatation or the widening of the smaller arterioles and veins can reduce the blood pressure in the arteries, even when the volume of blood remains the same. On the other hand, by increasing the rate of heart beat as well as the force with which the heart pumps blood, the body can raise the blood pressure in the arteries. Extra fluid released into the blood stream can increase the total circulating volume of blood, thus raising blood pressure. Constriction or narrowing of the arterioles (smaller arteries) and veins can also raise the blood pressure without the addition of fluid into the blood stream.
The sympathetic nervous system and parasympathetic nervous system which are parts of the autonomous nervous system, are responsible for the body’s maintenance of normality or homeostasis by reacting to stress in a variety of ways. Adrenal glands are stimulated by the sympathetic system, to release the fight or flight hormones adrenaline, also called epinephrine and its counterpart noradrenaline or norepinephrine. These hormones increase the heart rate, to help it pump more blood, when required. They also cause the constriction of arterioles in most parts of the body and at the same time, dilating the vessels in those parts which are involved in the fight or flight response, such as the musculoskeletal system. The regulation of salt movement across cell membranes help to keep the chemical balance of the cells, as excess salt inside the cells make them more vulnerable to overstimulation by the sympathetic nervous system. The kidneys are also stimulated by the sympathetic nervous system, to reduce the removal of water from the body so as to maintain extra volume of blood circulating in the body.
The kidneys have a direct role to play in the regulation of blood pressure. When blood pressure is raised, kidneys react to it by removing a portion of the fluid from the blood, by increasing the production of urine and eliminating salt and water from the body. The decrease in the volume of blood brings down the blood pressure in the arteries. When the blood pressure is lowered below the normal limits, the kidneys reduce urine production and hold salt and water within the body, causing the blood volume to increase till blood pressure is restored to normal levels. An enzyme called renin is secreted by the kidneys to help with elevation of blood pressure. The enzyme rennin induces the production of angiotensin II, a hormone that triggers the sympathetic nervous system, to increase the blood pressure by the constriction of the smaller arteries (arterioles). The arterioles in the kidneys dilate to compensate for the constriction of the arterioles elsewhere in the body. Angiotensin II also causes the release of hormones aldosterone and vasopressin, acting on the kidneys to retain water in the body.
Any temporary change in the blood pressure caused by stress or exertion is immediately counteracted by the body’s defense mechanisms to bring it back to normal. When the heart is forced to pump extra blood, to keep up with the demand of the body, during additional physical activity, the blood pressure temporarily increases in the arteries. This elevated pressure is brought down by reducing the volume of blood through the increased production of urine by the kidneys. If the blood vessels dilate to accommodate the extra amount of blood pumped by the heart, then also the blood pressure is brought back to normal level.
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Yasser Elnahas
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