Control of Blood Pressure - Essay Example

Control of Blood Pressure: Which organ systems are involved, how do they interact and how is blood pressure regulated short and long term? Abstract: Various forms of blood pressure regulation exists within the body and in this discussion, this would be divided into short and long term regulation. Short term control of blood pressure is via cardiovascular autonomic reflexes which can change blood pressure rapidly. This is done via baroreptors, chemoreceptors and cardiopulmonary baroreceptors, all of which send impulses to the brain to facilitate changes to the strength and number of sympathetic impulses. Hormonal control is also involved in short term blood pressure regulation via norepinephrine, epinephrine, renin, angiotensin, vasopressin and aldosterone. On the other hand, long term regulation of blood pressure is achieved via the kidneys regulation of both salt and water levels which directly affect the blood volume and blood pressure. Hormonal factors similarly cause changes to the blood pressure via the renin-angiotensin and aldosterone system. Basically, blood pressure (BP) is defined as the force per unit area exerted on the wall of a blood vessel by its contained blood and the measurement of BP is expressed in terms of millimeters of mercury (mm Hg). This means that a BP of 120 mm Hg is equal to the pressure exerted by a column of mercury 120 mm high. Whenever the term BP is used, it is meant to relate to that of systemic arterial blood pressure in the largest arteries near the heart and it is the pressure gradient that provides the driving force that keeps blood moving through the body. The study of blood pressure and how it is being regulated within the body is very important as today, it has been found that the prevalence of hypertension globally is on the rise. Thus, national and international health policies have to be carried out to prevent and control this disease. As noted by Kearney et al (2005), the estimated total number of adults with hypertension in 2000 was 972 million in which 333 million were from economically developed countries and 639 million from economically developing countries. In the year 2025, the number of adults with hypertension was predicted to rise by about 60% to a total of 1.56 billion. Thus, by understanding how blood pressure is regulated, various modalities of intervention which can serve to prevent damage to the target organs which are involved in regulating blood pressure may lower the global rise of hypertension. In this context, the discussion would center on short and long term control of systemic arterial blood pressure with emphasis towards the heart, the autonomic nervous system, the kidneys as well as hormonal regulation. The short term control of systemic blood pressure is basically regulated by that of the cardiovascular autonomic reflexes which are a powerful and rapid acting control mechanism. There are baroreceptors located in the walls of the internal carotid arteries, the aorta and also in other parts of the body which can detect changes in pressure (Van Hooft et al, 1993). When the blood pressure within these organs become too high, these receptors are stimulated and they send impulses to the brain to stop the sympathetic nervous system. When this occurs, there is a reduction in the heart rate, the strength of the contraction of the heart as well as a decrease in the peripheral vascular resistance. Similarly, when the receptors detect a drop in blood pressure, the impulses from the baroreceptors would be decreased and thus, would no longer hinder the sympathetic nervous system so that the blood pressure then returns to normal. There are also stretch receptors located in other parts of the body, for example the atria and ventricles of the heart called cardiopulmonary baroreceptors. It functions similarly to the arterial baroreceptors (Korner, Bobik and Angus ,1992) . Chemoreceptors are also present in the brain and the peripheral blood circulation. For example, when carbon dioxide concentration increases, the neurons of the vasomotor centers of the brain stem are stimulated which causes an increase in blood pressure. This ensures that there is sufficient blood pressure during physical stress as the body’s metabolism and carbon dioxide levels increase. Additionally, there are small structures which are the carotid and aortic bodies located at the aorta which respond to changes in arterial oxygen levels. When oxygen levels drop, these chemoreceptors would automatically activate the sympathetic nervous system and increase the blood pressure. This functions to maintain adequate delivery of oxygen to organs of the body (Nichols and Rourke, 1998). Hormones are also involved in the short term control of blood pressure. According to Laragh (1973), when the sympathetic system is stimulated via the adrenal medulla, there is a release of norepinephrine and epinephrine which have an effect on the constricting effects of an increased sympathetic stimulation. Another hormonal mechanism which not only affects the short term but also the long term regulation of blood pressure would be the renin-angiotensin system. A basic description of this system would be when there is a decrease in blood pressure, the juxtaglomerular cells of the kidneys would secrete renin and renin causes the conversion of angiotensinogen into angiotensin I. Angiotensin I is then converted into angiotensin II by an enzyme. Angiotensin II is a primary active component which causes the vessels to constrict and this would raise the blood pressure within the arteries. Later on, it would be shown that angiotensin II also regulates the kidneys in regards to water and sodium, another mechanism of blood pressure regulation. It also stimulates the secretion of aldosterone which also decreases renal sodium and water excretion which leads to the expansion of blood volume. Another hormone involved in the regulation of blood pressure would be vasopressin, also known as the antidiuretic hormone (ADH). Vasopressin has a direct constricting effect on blood vessels and it also reduces the excretion of water by the kidneys. These various short term mechanisms of blood pressure regulation work together with the cardiovascular modalities to maintain a normal level of blood pressure under various conditions (Williams, 1994). In the context of long term control of blood pressure, the kidneys are the dominant organ. The most important aspect of long term control of blood pressure is linked to the control of the circulation of the body’s volume by the kidneys or better known as the renal-body fluid feedback mechanism (Guyton and Hall, 1994). When the blood pressure rises above the normal level, the kidneys begin to excrete a higher amount of both water and sodium. Thus, the fluid volume and the blood volume would both decrease and continue to do so until the blood pressure returns to normal and the kidneys would then resume excreting normal amounts of water and sodium. When the blood pressure become lower than normal, the kidneys would reduce the rate of water and sodium excretion and hopefully, through adequate consumption of both fluids and solids over a period of time to increase the blood volume, the blood pressure would then return to normal. As this is a form of long term regulation, the process of normalizing the blood pressure via this mechanism takes a long time, even more than a week. Thus, it is not involved in rapid control of blood pressure but offers a very effective form of blood pressure control in regards to long term control (Bankir, Bouby and Trinh-Trang, 1989). Hormonal factors also play a role in the long term regulation of blood pressure. According to Carretero and Scicli (1991), hormones increase the effectiveness of renal-body fluid feedback control. In this form of control, the control of blood volume and pressure within the arteries by the kidneys is enhanced by the hormonal and nervous mechanisms. For example, when there is a higher amount of salt being taken by the body, blood pressure would increase and this would then cause an increase in the excretion of salt and water by the body. The increase in excretion of salt via the kidneys would cause a relatively small change in blood volume and blood pressure as long as the renin-angiotensin and aldosterone system function normally. Most people are able to eliminate extra salt in their diet with small changes to the blood pressure and blood volume because an increase in salt intake also reduces the formation of angiotensin II and aldosterone which in turn eliminates the extra sodium. Thus, there are certain drugs which can block the renin-angiotensin system called angiotensin-converting enzyme (ACE) inhibitors that prevent the formation of angiotensin II, thus reducing blood pressure (Yusuf, 2000). There are also other forms of blood pressure regulation within the body, for example the endothelial cells as well as genetic factors. Endothelial cells within the vascular system of the body play a role in blood pressure regulation by producing a number of chemicals such as nitric oxide molecules which causes the dilation of blood vessels (thus reducing blood pressure) as well as endothelin which causes the constriction of blood vessels (thus increasing blood pressure) (Lowenstein, Dinerman and Snyder, 1994). In the context of genetic factors, Kurtz (1994) has found that multiple genes are most likely to contribute to the control of blood pressure as well as the development of hypertension in certain individuals. It has been shown that hypertension would be twice as common in someone who have either one or two parents who are hypertensive and epidemiological studies have suggested that genetic factors account for approximately 30% of the variation in blood pressure in various populations around the world (Harrap, 1994). In summary, we can clearly see that there are various forms of blood pressure regulation within the body involving different organs which work together to maintain blood pressure in regards to both short and long term regulation. As a normal blood pressure is important to maintain the body’s homeostasis, blood pressure has to be kept relatively constant but it also has to adapt to certain situations which may necessitate a higher level of blood pressure for example stressful fight-or-flight conditions. Understanding how these mechanisms work is vital in how we overcome issues such as hypertension as drugs or other forms of intervention would then specifically target these organs or systems. For example, the simple act of lowering salt intake or more complex forms of therapy such as ACE inhibitors. References: Bankir L, Bouby N, Trinh-Trang-Tan M-M (1989) The role of the kidney in the maintenance of water balance. In Bailliere’s Clinical Endocrinology and Metabolism. Water and Salt Homeostasis in Health and Disease. Edited by Baylis PH. London: Bailliere;:249–311. Carretero OA, Scicli AG (1991) Local hormonal factors (intracrine, autocrine, and paracrine) in hypertension. Hypertension, 18 (suppl I):I-58–I-69. Guyton AC, Hall JE. (1994) Integration of renal mechanisms for control of blood volume and extracellular fluid volume. In Textbook of Medical Physiology, edn 9. Philadelphia: WB Saunders;:367–383 Harrap SB. (1994) Hypertension: genes versus environment. Lancet;344:169-71. 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