Control of Blood Pressure _ Boundless Anatomy and Physiology.pdf

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Unformatted text preview: 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology Boundless Anatomy and Physiology Cardiovascular System: Blood Vessels Control of Blood Pressure 1/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology Role of the Cardiovascular Center The cardiovascular system plays a role in body maintenance by transporting hormones and nutrients and removing waste products. LEARNING OBJECTIVES Explain the role of the cardiovascular center in controlling blood pressure KEY TAKEAWAYS Key Points The cardiovascular center is a part of the human brain found in the medulla oblongata, responsible for regulation of cardiac output. Numerous receptors in the circulatory system can detect changes in pH or stretch and signal these changes to the cardiovascular center. The cardiovascular center can alter heart rate and stroke volume to increase blood pressure and ow. Key Terms cardiovascular centre: A region of the brain responsible for nervous control of cardiac output. The cardiovascular center forms part of the autonomic nervous system and is responsible for regulation of cardiac output. Located in the medulla oblongata, the cardiovascular center contains three distinct 2/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology components: the cardioaccelerator center, the cardioinhibitor center, and the vasomotor center. The cardioaccelerator center stimulates cardiac function by regulating heart rate and stroke volume via sympathetic stimulation from the cardiac accelerator nerve. The cardioinhibitor center slows cardiac function by decreasing heart rate and stroke volume via parasympathetic stimulation from the vagus nerve. The vasomotor center controls vessel tone or contraction of the smooth muscle in the tunica media. Changes in diameter a ect peripheral resistance, pressure, and ow, which in turn a ect cardiac output. The majority of these neurons act via the release of the neurotransmitter norepinephrine from sympathetic neurons. Although each center functions independently, they are not anatomically distinct. The cardiovascular center can respond to numerous stimuli. Hormones such as epinephrine and norepinephrine or changes in pH such as acidi cation due to carbon dioxide accumulation in a tissue during exercise are detected by chemoreceptors. Baroreceptors that detect stretch can also signal to the cardiovascular center to alter heart rate. 3/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology Human circulatory system: The cardiovascular system is composed largely of the circulatory system, or the system of blood vessels that distributes oxygen from the lungs throughout the body. Short-Term Neural Control Neural regulation of blood pressure is achieved through the role of cardiovascular centers and baroreceptor stimulation. 4/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology LEARNING OBJECTIVES Describe the role of baroreceptors and cardiovascular centers (ANS) in blood pressure control KEY TAKEAWAYS Key Points The cardioaccelerator center, the cardioinhibitor center, and the vasomotor center form the cardiovascular center, a cluster of neurons that function independently to regulate blood pressure and ow. The release of the neurotransmitter norepinephrine from sympathetic neurons directs the majority of neurons associated with the cardiovascular center. Baroreceptors respond to the degree of stretch caused by the presence of blood; this stimulates impulses to be sent to the cardiovascular center to regulate blood pressure to achieve homeostasis when needed. Key Terms autonomic nervous system: The part of the nervous system that regulates the involuntary activity of the heart, intestines, and glands. These activities include digestion, respiration, perspiration, metabolism, and blood pressure modulation. norepinephrine: A catecholamine with multiple roles including as a hormone and neurotransmitter. Areas of the body that produce or are a ected by this substance are described as noradrenergic. sympathetic: Of or related to the part of the autonomic nervous system that under stress raises blood pressure 5/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology and heart rate, constricts blood vessels, and dilates the pupils. baroreceptor: A nerve ending that is sensitive to changes in blood pressure. parasympathetic: Of or relating to the part of the autonomic nervous system that inhibits or opposes the e ects of the sympathetic nervous system. The autonomic nervous system plays a critical role in the regulation of vascular homeostasis. The primary regulatory sites include the cardiovascular centers in the brain that control both cardiac and vascular functions. Neurological regulation of blood pressure and ow depends on the cardiovascular centers located in the medulla oblongata. This cluster of neurons responds to changes in blood pressure as well as blood concentrations of oxygen, carbon dioxide, and other factors such as pH. Baroreceptor Function Baroreceptors are specialized stretch receptors located within thin areas of blood vessels and heart chambers that respond to the degree of stretch caused by the presence of blood. They send impulses to the cardiovascular center to regulate blood pressure. Vascular baroreceptors are found primarily in sinuses (small cavities) within the aorta and carotid arteries. The aortic sinuses are found in the walls of the ascending aorta just superior to the aortic valve, whereas the carotid sinuses are located in the base of the internal carotid arteries. There are also low-pressure baroreceptors located in the walls of the venae cavae and right atrium. When blood pressure increases, the baroreceptors are stretched more tightly and initiate action potentials at a higher rate. At lower blood pressures, the degree of stretch is lower and the rate of ring is slower. When the cardiovascular center in the medulla oblongata receives this input, it triggers a re ex that maintains homeostasis. 6/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology Baroreceptor Re exes: Increased blood pressure results in increased rates of baroreceptor ring, whereas decreased blood pressure results in slower rates of re, both initiating the homeostatic mechanism to restore blood pressure. Baroreceptor Re exes When blood pressure rises too high, baroreceptors re at a higher rate and trigger parasympathetic stimulation of the heart. As a result, cardiac output falls. Sympathetic stimulation of the peripheral arterioles will also decrease, resulting in vasodilation. Combined, these activities cause blood pressure to fall. When blood pressure drops too low, the rate of baroreceptor ring decreases. This triggers an increase in sympathetic stimulation of the heart, causing cardiac output to increase. It also triggers sympathetic stimulation of the peripheral vessels, resulting in vasoconstriction. Combined, these activities cause blood pressure to rise. The baroreceptors in the venae cavae and right atrium monitor blood pressure as the blood returns to the heart from the systemic circulation. Normally, blood ow into the aorta is the same as blood ow back into the right atrium. If blood is returning to the right atrium more rapidly than 7/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology it is being ejected from the left ventricle, the atrial receptors will stimulate the cardiovascular centers to increase sympathetic ring and cardiac output until homeostasis is achieved. The opposite is also true. This mechanism is referred to as the atrial re ex. Other neural mechanisms can also have a signi cant impact on cardiovascular function. These include the limbic system, which links physiological responses to psychological stimuli, chemoreceptor re exes, generalized sympathetic stimulation, and parasympathetic stimulation. Short-Term Chemical Control Blood pressure is controlled chemically through dilation or constriction of the blood vessels by vasodilators and vasocontrictors. LEARNING OBJECTIVES Discuss chemical substances to control blood pressure KEY TAKEAWAYS Key Points Constriction or dilation of blood vessels alters resistance, increasing or decreasing blood pressure respectively. Generalized vasoconstriction usually results in an increase in systemic blood pressure, but it may also occur in speci c tissues, causing a localized reduction in blood ow. Vasoconstriction results from increased concentration of calcium (Ca2+) ions within vascular smooth muscle. 8/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology When blood vessels dilate, the ow of blood is increased due to a decrease in vascular resistance. Therefore, dilation of arterial blood vessels (mainly the arterioles ) causes a decrease in blood pressure. Localized tissues increase blood ow in multiple ways, including releasing vasodilators, primarily adenosine, into the local intersitial uid, which di uses to capillary beds provoking local vasodilation. Key Terms vasodilation: The dilation (widening) of a blood vessel. vasoconstriction: The constriction (narrowing) of a blood vessel. Many physical factors that in uence arterial pressure. Each may in turn be in uenced by physiological factors such as diet, exercise, disease, drugs or alcohol, stress, and obesity. In practice, each individual’s autonomic nervous system responds to and regulates all of these interacting factors so that the actual arterial pressure response varies widely because of both split-second and slow-moving responses of the nervous system and end organs. These responses are very e ective in changing the variables and resulting blood pressure from moment to moment. Chemical Vasoconstriction Vasoconstriction is the narrowing of blood vessels resulting from contraction of the muscular wall of the vessels, particularly the large arteries and small arterioles. Generalized vasoconstriction usually results in an increase in systemic blood pressure, but may also occur in speci c tissues, causing a localized reduction in blood ow. The mechanism that leads to vasoconstriction results from the increased concentration of calcium (Ca2+ ions) and phosphorylated myosin within vascular smooth muscle cells. When stimulated, a signal transduction cascade leads to increased intracellular calcium from the sarcoplasmic 9/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology reticulum through IP3 mediated calcium release, as well as enhanced calcium entry across the sarcolemma through calcium channels. The rise in intracellular calcium interacts with calmodulin, which in turn activates myosin light chain kinase. This enzyme is responsible for phosphorylating the light chain of myosin to stimulate cross-bridge cycling. Once elevated, the intracellular calcium concentration is returned to its basal level through a variety of protein pumps and calcium exchanges located on the plasma membrane and sarcoplasmic reticulum. This reduction in calcium removes the stimulus necessary for contraction allowing for a return to baseline. Endogenous vasoconstrictors include ATP, epinephrine, and angiotensin II. Vasoconstriction: Vasoconstriction of a microvessel by pericytes and endothelial cells that encircle an erythrocyte (E). 10/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology Chemical Vasodilation Vasodilation is the widening of blood vessels resulting from relaxation of smooth muscle cells within the vessel walls, particularly in the large veins, large arteries, and smaller arterioles. Generalized vasodilation usually results in a decrease in systemic blood pressure, but may also occur in speci c tissues causing a localized increase in blood ow. The primary function of vasodilation is to increase blood ow in the body to tissues that need it most. This is often in response to a localized need for oxygen, but can occur when the tissue in question is not receiving enough glucose, lipids, or other nutrients. Localized tissues increase blood ow by several methods, including the release of vasodilators, primarily adenosine, into the local interstitial uid, which di uses to capillary beds provoking local vasodilation. Some physiologists have suggested the lack of oxygen itself causes capillary beds to vasodilate by the smooth muscle hypoxia of the vessels in the region. As with vasoconstriction, vasodilation is modulated by calcium ion concentration and myosin phosphorylation within vascular smooth muscle cells. Dephosphorylation by myosin light-chain phosphatase and induction of calcium symporters and antiporters that pump calcium ions out of the intracellular compartment both contribute to smooth muscle cell relaxation and therefore vasodilation. This is accomplished through reuptake of ions into the sarcoplasmic reticulum via exchangers and expulsion across the plasma membrane. Endogenous vasodilators include arginine and lactic acid. Long-Term Renal Regulation Consistent and long-term control of blood pressure is determined by the renin-angiotensin system. LEARNING OBJECTIVES 11/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology Describe the long-term renal regulation of blood pressure KEY TAKEAWAYS Key Points When blood volume is low, renin, excreted by the kidneys, stimulates production of angiotensin I, which is converted into angiotensin II. This substance has many e ects, including increase in blood pressure due to its vasoconstrictive properties. The cells that excrete renin are called juxtaglomerular cells. When blood volume is low, juxtaglomerular cells in the kidneys secrete renin directly into circulation. Plasma renin then carries out the conversion of angiotensinogen released by the liver to angiotensin I. Aldosterone secretion from the adrenal cortex is induced by angiotensin II and causes the tubules of the kidneys to increase the reabsorption of sodium and water into the blood, thereby increasing blood volume and blood pressure. Key Terms juxtaglomerular cells: The juxtaglomerular cells (JG cells, or granular cells) are cells in the kidney that synthesize, store, and secrete the enzyme renin. aldosterone: A mineralocorticoid hormone secreted by the adrenal cortex that regulates the balance of sodium and potassium in the body. adrenal cortex: The outer portion of the adrenal glands that produces hormones essential to homeostasis. Along with vessel morphology, blood viscosity is one of the key factors 12/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology in uencing resistance and hence blood pressure. A key modulator of blood viscosity is the renin-angiotensin system (RAS) or the reninangiotensin-aldosterone system (RAAS), a hormone system that regulates blood pressure and water balance. When blood volume is low, juxtaglomerular cells in the kidneys secrete renin directly into circulation. Plasma renin then carries out the conversion of angiotensinogen released by the liver to angiotensin I. Angiotensin I is subsequently converted to angiotensin II by the enzyme angiotensin converting enzyme found in the lungs. Angiotensin II is a potent vasoactive peptide that causes blood vessels to constrict, resulting in increased blood pressure. Angiotensin II also stimulates the secretion of the hormone aldosterone from the adrenal cortex. Aldosterone causes the tubules of the kidneys to increase the reabsorption of sodium and water into the blood. This increases the volume of uid in the body, which also increases blood pressure. If the renin-angiotensin-aldosterone system is too active, blood pressure will be too high. Many drugs interrupt di erent steps in this system to lower blood pressure. These drugs are one of the main ways to control high blood pressure (hypertension), heart failure, kidney failure, and harmful e ects of diabetes. It is believed that angiotensin I may have some minor activity, but angiotensin II is the major bioactive product. Angiotensin II has a variety of e ects on the body: throughout the body, it is a potent vasoconstrictor of arterioles. 13/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology The renin-angiotensin pathway: The gures outlines the origination of the renin-angiotensin pathway molecules, as well as e ects on target organs and systems. Checking Circulation Checking circulation involves measurement of blood pressure and pulse through a variety of invasive and noninvasive methods. LEARNING OBJECTIVES Choose the appropriate method for checking circulation KEY TAKEAWAYS Key Points Pulse rate is most commonly measured manually at the wrist by a trained medical professional. Arterial catheters and pulse oximetry allow for more accurate and long-term measurement of pulse rate. 14/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology Heart rate can be measured directly by listening to the heart through the chest. Electrocardiography, which detects the electrical pattern of the heart muscle through the skin, can be used for more accurate or long-term measurements. Arterial pressure is most commonly measured via a sphygmomanometer. Blood pressure values are generally reported in millimeters of mercury (mmHg), though aneroid and electronic devices do not use mercury. The auscultatory method for determining blood pressure uses a stethoscope and a sphygmomanometer. Key Terms electrocardiography: A measure of the electrical output of the heart detected through the skin. sphygmomanometer: A device used to measure blood pressure. Circulatory health can be measured in a variety of ways as follows. Pulse While a simple pulse rate measurement can be achieved by anyone, trained medical sta are capable of much more accurate measurements. Radial pulse is commonly measured using three ngers: the nger closest to the heart is used to occlude the pulse pressure, the middle nger is used to get a crude estimate of blood pressure, and the nger most distal to the heart is used to nullify the e ect of the ulnar pulse as the two arteries are connected via the palmar arches. Where more accurate or long-term measurements are required, pulse rate, pulse de cits, and much more physiologic data are readily visualized by the use of one or more arterial catheters connected to a 15/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology transducer and oscilloscope. This invasive technique has been commonly used in intensive care since the 1970’s. The rate of the pulse is observed and measured by tactile or visual means on the outside of an artery and recorded as beats per minute (BPM). The pulse may be further indirectly observed under light absorbencies of varying wavelengths with assigned and inexpensively-reproduced mathematical ratios. Applied capture of variances of light signal from the blood component hemoglobin under oxygenated vs. deoxygenated conditions allows the technology of pulse oximetry. Heart Rate Heart rate can be measured by listening to the heart directly though the chest, traditionally using a stethoscope. For more accurate or long-term measurements, electrocardiography may be used. During each heartbeat, a healthy heart has an orderly progression of depolarization that starts with pacemaker cells in the sinoatrial ECG Graph: ECG graph of a normal heartbeat. (SA) node, spreads out through the atrium, passes through the atrioventricular node down into the bundle of His and into the Purkinje bers, and down and to the left throughout the ventricles. This organized pattern of depolarization can be detected through electrodes placed on the skin and recorded as the commonly seen ECG tracing. ECG provides a very accurate means to measure heart rate, rhythm, and other factors such as chamber sizing, as well as identifying possible regions of damage. Blood Pressure Arterial pressure is most commonly measured via a sphygmomanometer, which historically...
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