6th Lecture - Cadiovascular System for 200L OPTOMETRY AND PHARMACY 2023-2024.pdf

GOOD MORNING CLASS 7/22/2024 Jesus is Lord 1  SIXTH LECTURE 7/22/2024 Jesus is Lord 2 CARDIOVASCULAR SYSTEM (PHS213 & OPT218) Dr. Eghe Osawaru AIHIE Department of Physiology, School of Basic Medical Sciences, College of Medical Sciences, U...

GOOD MORNING CLASS 7/22/2024 Jesus is Lord 1  SIXTH LECTURE 7/22/2024 Jesus is Lord 2 CARDIOVASCULAR SYSTEM (PHS213 & OPT218) Dr. Eghe Osawaru AIHIE Department of Physiology, School of Basic Medical Sciences, College of Medical Sciences, University of Benin. Benin City, Nigeria. COURSE OUTLINE: CARDIOVASCULAR SYSTEM Definition and functions of the cardiovascular system Cardiac muscle Cardiac myoelectrophysiology Cardiac cycle Circulation of blood: cardiac output and regulation Blood pressure Haemodynamics and microcirculation Pulmonary, Coronary, Splanchnic and muscle circulation. Shock and cardiovascular changes in exercise. 7/22/2024 Jesus is Lord 4 Figure 1: General components of a reflex arc that functions as a negative feedback control system 7/22/2024 Jesus is Lord 5  BLOOD PRESSURE CONTD. 7/22/2024 Jesus is Lord 6  REGULATION OF ARTERIAL BLOOD PRESSURE 7/22/2024 Jesus is Lord 7  INTRODUCTION The mean arterial pressure is the major cardiovascular variable that is regulated in the systemic circulation. This is because pressure is the driving force for blood flow through all the organs except the lungs. Maintaining it is therefore a prerequisite for ensuring adequate blood flow to these organs. 7/22/2024 Jesus is Lord 8  Introduction contd. The importance of maintaining blood pressure within a normal range demonstrates the general principle of physiology that homeostasis is essential for health and survival. Without a homeostatic control system operating to maintain blood pressure, the tissues of the body would quickly die if pressure were to decrease significantly. 7/22/2024 Jesus is Lord 9  Blood Pressure Equation In simple physiologic terms, Blood Pressure (BP) is represented mathematically as: BP = CO x TPR (BP = arterial blood pressure; CO = cardiac output; TPR = total peripheral resistance). 7/22/2024 Jesus is Lord 10  Blood Pressure Equation contd. Cardiac output and total peripheral resistance set the mean systemic arterial pressure because they determine the average volume of blood in the systemic arteries over time; it is this blood volume that causes the pressure. All changes in mean arterial pressure must be the result of changes in cardiac output and/or total peripheral resistance. 7/22/2024 Jesus is Lord 11  Blood Pressure Equation contd. Inspection of the equation reveals that arterial blood pressure can be changed by altering the cardiac output (or any of its parameters), altering the total peripheral resistance (or any of its parameters), or altering both cardiac output and total peripheral resistance. 7/22/2024 Jesus is Lord 12  Blood Pressure Equation contd. This equation is however not as simple as it appears Cardiac output and total peripheral resistance are not independent variables. Changes in total peripheral resistance can alter cardiac output and Changes in cardiac output can alter total peripheral resistance. 7/22/2024 Jesus is Lord 13  Blood Pressure Equation contd. Therefore, it cannot be stated that if total peripheral resistance doubles, arterial blood pressure also doubles. In fact, when total peripheral resistance doubles, cardiac output simultaneously is almost halved and arterial blood pressure will increase only modestly. 7/22/2024 Jesus is Lord 14  Blood Pressure Equation contd. Likewise, if cardiac output is halved, there is a compensatory increase in total peripheral resistance and arterial blood pressure will decrease but not be halved. This happens because there are compensatory mechanisms that help to regulate the arterial blood pressure to ensure that it is constant at a set point. 7/22/2024 Jesus is Lord 15  Blood Pressure Equation contd. Arterial blood pressure could either increase above the set point or decrease below the set point. The cardiovascular system makes adjustments in cardiac output, in total peripheral resistance, or in both, attempting to return it to the set-point value. 7/22/2024 Jesus is Lord 16 Factors that Influence Mean Arterial Pressure 7/22/2024 Jesus is Lord 17  Regulation of Arterial Blood Pressure Arterial pressure is regulated not by a single pressure controlling system but instead by several inter-related systems, each of which performs a specific function. 7/22/2024 Jesus is Lord 18  Regulation of Arterial Blood Pressure contd. For example, when a person bleeds so severely that the pressure falls suddenly, two problems confront the pressure control system. The first is survival; the arterial pressure must be rapidly returned to a high enough level that the person can live through the acute episode. The second is to return the blood volume and arterial pressure eventually to their normal levels so that the circulatory system can reestablish full normality. 7/22/2024 Jesus is Lord 19  Regulation of Arterial Blood Pressure contd. These mechanisms for regulation of arterial blood pressure can be divided broadly into three groups: 1. those that react rapidly, within seconds or minutes 2. those that respond over an intermediate time period, that is, minutes or hours; and 3. those that provide long-term arterial pressure regulation for days, months, and years. 7/22/2024 Jesus is Lord 20  Figure 2: Approximate potency of various arterial pressure control mechanisms at different time intervals after the onset of a disturbance to the arterial pressure. Note especially the near-infinite gain (∞) of the renal body fluid pressure control mechanism that occurs after a few weeks’ time. CNS, Central nervous system. (Modified from Guyton AC: Arterial Pressure and Hypertension. Philadelphia: WB 7/22/2024 Jesus is Lord Saunders, 1980.)21  1. Rapidly Acting Pressure Control Mechanisms The rapidly acting pressure control mechanisms are almost entirely acute nervous reflexes or other nervous responses that begin to react within seconds and are powerful. The three mechanisms that show responses within seconds are: 1. The baroreceptor feedback mechanism 2. The central nervous system ischemic mechanism 3. The chemoreceptor mechanism. 7/22/2024 Jesus is Lord 22  2. Those that respond over an intermediate time period Several pressure control mechanisms exhibit significant responses only after a few minutes following acute arterial pressure change. Three of these mechanisms are 1. The renin-angiotensin vasoconstrictor mechanism 2. The stress-relaxation mechanism of the vasculature 3. The capillary fluid shift mechanism These three intermediate mechanisms become mostly activated within 30 minutes to several hours. 7/22/2024 Jesus is Lord 23  3. Long-Term Mechanisms for Arterial Pressure Regulation It takes few hours for this mechanism to begin to show a significant response and spans to days, months, and years. It involves the renal–blood volume pressure control mechanism also known as the renal–body fluid pressure control mechanism. This mechanism can eventually return the arterial pressure nearly all the way back, not merely partway back, to the pressure level that provides normal output of salt and water by the kidneys. 7/22/2024 Jesus is Lord 24  Figure 2: Approximate potency of various arterial pressure control mechanisms at different time intervals after the onset of a disturbance to the arterial pressure. Note especially the near-infinite gain (∞) of the renal body fluid pressure control mechanism that occurs after a few weeks’ time. CNS, Central nervous system. (Modified from Guyton AC: Arterial Pressure and Hypertension. Philadelphia: WB 7/22/2024 Jesus is Lord Saunders, 1980.)25  BARORECEPTOR ARTERIAL PRESSURE CONTROL SYSTEM — BARORECEPTOR REFLEXES 7/22/2024 Jesus is Lord 26 Figure 1: General components of a reflex arc that functions as a negative feedback control system 7/22/2024 Jesus is Lord 27  The Baroreceptor Reflex The primary reflex pathway for homeostatic control of mean arterial blood pressure is the baroreceptor reflex. It is by far the best known of the nervous mechanisms for arterial pressure control. The baroreceptor mechanisms are fast, neurally mediated reflexes that attempt to keep arterial pressure constant through changes in the output of the sympathetic and parasympathetic nervous systems to the heart and blood vessels. 7/22/2024 Jesus is Lord 28  Figure 3: Sympathetic nervous system. 7/22/2024 Jesus is Lord 29  Figure 4: Parasympathetic nervous system. 7/22/2024 Jesus is Lord 30  Figure 5: Anatomy of sympathetic nervous control of the circulation. Also shown by the dashed red line, is a vagus nerve that carries parasympathetic signals to the heart. 7/22/2024 Jesus is Lord 31 Figure 6: 7/22/2024 Sympathetic innervation of the Jesus is Lord systemic circulation 32.  Cardiovascular Brain Stem Centers The cardiovascular brain stem centers are located bilaterally mainly in the reticular substance of the medulla and lower third of the pons is an area called the vasomotor center. This center transmits i. parasympathetic impulses through the vagus nerves to the heart and ii. sympathetic impulses through the spinal cord and peripheral sympathetic nerves to virtually all arteries, arterioles, and veins of the body. 7/22/2024 Jesus is Lord 33  Cardiovascular Brain Stem Centers contd. These centers function in a coordinated fashion, receiving information about blood pressure from the baroreceptors and then directing changes in output of the sympathetic and parasympathetic nervous systems to correct the blood pressure as needed. 7/22/2024 Jesus is Lord 34  Figure 7: Areas of the brain that play important roles in the nervous regulation of the circulation. The dashed lines represent inhibitory pathways. 7/22/2024 Jesus is Lord 35  Cardiovascular Brain Stem Centers contd. 1. The parasympathetic outflow is the effect of the vagus nerve on the sinoatrial node to decrease the heart rate. 2. The sympathetic outflow has four components: i. an effect on the sinoatrial node to increase heart rate, ii. an effect on cardiac muscle to increase contractility and stroke volume, 7/22/2024 Jesus is Lord 36  Cardiovascular Brain Stem Centers contd. iii. an effect on the arterioles to produce vasoconstriction and increase total peripheral resistance, and iv. an effect on veins to produce venoconstriction and decrease unstressed volume. 7/22/2024 Jesus is Lord 37  Cardiovascular Brain Stem Centers contd. The cardiovascular brain stem centers are as follows: 1. The vasoconstrictor center: is located in the upper medulla and the lower pons. Efferent neurons from this vasomotor center are part of the sympathetic nervous system and synapse in the spinal cord, then in sympathetic ganglia, and finally on the target organs, producing vasoconstriction in the arterioles and venules. 7/22/2024 Jesus is Lord 38  Cardiovascular Brain Stem Centers contd. 2. The cardiac accelerator center: Efferent neurons from the cardiac accelerator center are also part of the sympathetic nervous system and synapse in the spinal cord, in sympathetic ganglia, and finally in the heart. In the heart, the effects of this activity are a. an increased firing rate of the sinoatrial node (to increase heart rate) - Chronotropic effects b. increased conduction velocity through the atrioventricular node - dromotropic effects c. increased contractility – inotropic effects 7/22/2024 Jesus is Lord 39  Cardiovascular Brain Stem Centers contd. 3. The cardiac decelerator center Efferent fibers from the cardiac decelerator center are part of the parasympathetic nervous system. They travel in the vagus nerve and synapse on the sinoatrial node to decrease heart rate. 7/22/2024 Jesus is Lord 40  The Baroreceptor Reflex contd. Basically, this reflex is initiated by stretch receptors, called baroreceptors or pressoreceptors or mechanoreceptors. These receptors are located at specific points in the walls of several large systemic arteries. A rise in arterial pressure stretches the baroreceptors and causes them to transmit signals into the central nervous system (CNS). 7/22/2024 Jesus is Lord 41  The Baroreceptor Reflex contd. “Feedback” signals are then sent back through the autonomic nervous system to the circulation to reduce arterial pressure downward toward the normal level. 7/22/2024 Jesus is Lord 42 Figure 8: 7/22/2024 CNS control of the heart and blood Jesus is Lord vessels. 43  The Baroreceptor Reflex contd. “Feedback” signals are then sent back through the autonomic nervous system to the circulation to reduce arterial pressure downward toward the normal level. 7/22/2024 Jesus is Lord 44  Physiologic Anatomy of the Baroreceptors Baroreceptors are spray-type nerve endings that lie in the walls of the arteries and are stimulated when stretched. A few baroreceptors are located in the wall of almost every large artery of the thoracic and neck regions But baroreceptors are extremely abundant in: 1. The walls of the carotid sinus 2. The wall of the aortic arch. 7/22/2024 Jesus is Lord 45 Figure 9: Baroreceptor system for controlling arterial pressure. 7/22/2024 Jesus is Lord 46  Physiologic Anatomy of the Baroreceptors Figure 9 shows that Signals from the carotid baroreceptors are transmitted through small Hering’s nerves to the glossopharyngeal nerves in the high neck and then to the nucleus tractus solitarius in the medulla. Signals from the aortic baroreceptors in the arch of the aorta are transmitted through the vagus nerves to the same nucleus tractus solitarius in the medulla. 7/22/2024 Jesus is Lord 47  Physiologic Anatomy of the Baroreceptors Based on their location, they can monitor the pressure of blood flowing to the brain (carotid baroreceptors) and to the body (aortic baroreceptors) The carotid and aortic baroreceptors are tonically active stretch receptors that fire action potentials continuously at normal blood pressures. 7/22/2024 Jesus is Lord 48  Response of the Baroreceptors to Changes in Arterial Pressure The baroreceptors are sensitive to pressure or stretch. Changes in arterial pressure cause more or less stretch on the baroreceptors, resulting in a change in their membrane potential (a receptor potential). A receptor potential increases or decreases the likelihood that action potentials will be fired in the afferent nerves that travel from the baroreceptors to the brain stem. 7/22/2024 Jesus is Lord 49 Response of the Baroreceptors to Changes in Arterial Pressure contd. Increases in arterial pressure cause increased stretch on the baroreceptors and increased firing rate in the afferent nerves. 7/22/2024 Jesus is Lord 50 Response of the Baroreceptors to Changes in Arterial Pressure contd. Decreases in arterial pressure cause decreased stretch on the baroreceptors and decreased firing rate in the afferent nerves. 7/22/2024 Jesus is Lord 51  Response of the Baroreceptors to Changes in Arterial Pressure contd. Although the baroreceptors are sensitive to the absolute level of pressure, they are even more sensitive to: 1. changes in pressure 2. the rate of change of pressure. The strongest stimulus for the baroreceptors is a rapid change in arterial pressure. 7/22/2024 Jesus is Lord 52  Integrated Function of the Baroreceptor Reflex 7/22/2024 Jesus is Lord 53  Introduction The function of the baroreceptor reflex can be illustrated by examining its response to an increase in arterial pressure 7/22/2024 Jesus is Lord 54  Response of the Baroreceptor Reflex to an increase in Arterial Pressure When there is an increase in arterial pressure The increase is detected by baroreceptors in the carotid sinus and in the aortic arch. This increase in pressure results in increased firing rate of the Hering’s carotid sinus nerve to the glossopharyngeal nerves (CN IX) and in afferent fibers in the vagus nerve (CN X). 7/22/2024 Jesus is Lord 55  Response of the Baroreceptor Reflex to an increase in Arterial Pressure contd. Afferent information about the increase in blood pressure is then sent through the afferent nerves and is integrated in the nucleus tractus solitarius of the medulla. The nucleus tractus solitarius then directs a series of coordinated responses, using the medullary cardiovascular centers, to reduce pressure back to normal. 7/22/2024 Jesus is Lord 56  Response of the Baroreceptor Reflex to an increase in Arterial Pressure contd. These coordinated responses to an increase in blood pressure include; 1. an increase in parasympathetic outflow to the heart 2. a decrease in sympathetic outflow to the heart and blood vessels. The increase in parasympathetic activity to the sinoatrial node results in a decrease in heart rate (negative chronotropic effects). 7/22/2024 Jesus is Lord 57  Response of the Baroreceptor Reflex to an increase in Arterial Pressure contd. The decrease in sympathetic activity to the sinoatrial node complements the increase in parasympathetic activity and also decreases heart rate (negative chronotropic effects). Decreased sympathetic activity also decreases cardiac contractility (negative inotropic effect). Together, the decreased heart rate and decreased cardiac contractility produce a decrease in cardiac output, which tends to reduce pressure back to normal. 7/22/2024 Jesus is Lord 58  Response of the Baroreceptor Reflex to an increase in Arterial Pressure contd. The decrease in sympathetic activity also affects the tone of the blood vessels. 1. There is decreased constriction of arterioles, or arteriolar vasodilation, which decreases total peripheral resistance and reduces pressure. 2. There is decreased constriction of veins, which increases the compliance of the veins, thereby increasing the unstressed volume. 7/22/2024 Jesus is Lord 59  Response of the Baroreceptor Reflex to an increase in Arterial Pressure contd. When unstressed volume increases, stressed volume decreases, which further contributes to a reduction in pressure. Once these coordinated reflexes reduce arterial blood pressure back to the set-point pressure, then activity of the baroreceptors and the cardiovascular brain stem centers will return to the tonic (baseline) level. 7/22/2024 Jesus is Lord 60 Figure 10: Response of baroreceptor reflex to increased arterial pressure. The + symbol shows increases in activity; the − symbol shows decreases in activity; the dashed lines show inhibitory pathways. CN, Cranial nerve. 7/22/2024 Jesus is Lord 61 Figure 11: This map shows the reflex response to an increase in mean arterial pressure. 7/22/2024 Jesus is Lord 62  Response of the Baroreceptor Reflex to Hemorrhage Another example of the operation of the baroreceptor reflex is the response to loss of blood volume or hemorrhage. Hemorrhage produces a decrease in arterial blood pressure because, as blood volume decreases, stressed volume also decreases. In response to an acute reduction in arterial blood pressure, the baroreceptor reflex is activated and attempts to restore blood pressure back toward normal. 7/22/2024 Jesus is Lord 63  Response of the Baroreceptor Reflex to Hemorrhage contd. The responses of the baroreceptor reflex to a decrease in blood pressure are the exact opposite of those described for the response to an increase in blood pressure. Decreases in blood pressure produce decreased stretch on the baroreceptors and decreased firing rate of the carotid sinus nerve (Herings nerve). This information is received in the nucleus tractus solitarius of the medulla, which produces coordinated responses. 7/22/2024 Jesus is Lord 64  Response of the Baroreceptor Reflex to Hemorrhage contd. These coordinated responses to a decrease in blood pressure include; 1. an decrease in parasympathetic outflow to the heart 2. a increase in sympathetic outflow to the heart and blood vessels. Decrease in parasympathetic activity to the sinoatrial node results in a increase in heart rate (positive chronotropic effects). 7/22/2024 Jesus is Lord 65  Response of the Baroreceptor Reflex to Hemorrhage contd. The increase in sympathetic activity to the sinoatrial node results in increases in the heart rate (positive chronotropic effects). Increased sympathetic activity increases cardiac contractility (positive inotropic effect). Together, the increased heart rate and increased cardiac contractility produce a increase in cardiac output, which tends to increase pressure back to normal. 7/22/2024 Jesus is Lord 66  Response of the Baroreceptor Reflex to Hemorrhage contd. The increase in sympathetic activity also affects the tone of the blood vessels. 1. There is increased constriction of arterioles which increases total peripheral resistance and increases pressure. 2. There is increased constriction of veins, which decreases the compliance of the veins, thereby decreasing the unstressed volume. 7/22/2024 Jesus is Lord 67  Response of the Baroreceptor Reflex to Hemorrhage contd. When unstressed volume decreases, stressed volume increases, which further contributes to an increase in pressure. Once these coordinated reflexes increase arterial blood pressure back to the set-point pressure, then activity of the baroreceptors and the cardiovascular brain stem centers will return to the tonic (baseline) level. 7/22/2024 Jesus is Lord 68  Figure 12: Response of the baroreceptor reflex to acute hemorrhage. The reflex is initiated by a decrease in mean arterial pressure (Pa). The compensatory responses attempt to increase Pa back to normal. TPR, Total peripheral resistance. 7/22/2024 Jesus is Lord 69  Figure 13: Arterial baroreceptor reflex compensation for hemorrhage. The compensatory mechanisms do not restore arterial pressure completely to normal. The increases designated “toward normal” are relative to prehemorrhage values; for example, the stroke volume is increased reflexively “toward normal” relative to the low point caused by the hemorrhage (i.e., before the reflex occurs), but it does not reach the level it had prior to the hemorrhage. For simplicity, the fact that plasma angiotensin II and vasopressin are also reflexively increased and help 7/22/2024 Jesus is Lord constrict arterioles is not shown. 70  Clinical Relevance The sensitivity of the baroreceptors can be altered by disease. For example, in chronic hypertension (elevated blood pressure), the baroreceptors do not "see" the elevated blood pressure as abnormal. In such cases, the hypertension will be maintained, rather than corrected, by the baroreceptor reflex. 7/22/2024 Jesus is Lord 71  Clinical Relevance contd. The mechanism of this defect is either decreased sensitivity of the baroreceptors to increases in arterial pressure or an increase in the blood pressure set point of the brain stem centers. 7/22/2024 Jesus is Lord 72  Other short term mechanisms involved in the regulation of Arterial Blood Pressure 1. Chemoreceptor reflex 2. CNS ischemic response. 3. Atrial and Pulmonary Artery Reflexes 7/22/2024 Jesus is Lord 73  1. Chemoreceptor reflex The chemoreceptor reflex operates in much the same way as the baroreceptor reflex except that chemoreceptors, instead of stretch receptors initiate the response. The chemoreceptors are chemosensitive cells sensitive to i. low oxygen, ii. carbon dioxide excess, and iii. hydrogen ion excess. 7/22/2024 Jesus is Lord 74  1. Chemoreceptor reflex contd. Peripheral chemoreceptors for O2 are located in the carotid bodies near the bifurcation of the common carotid arteries and in the aortic bodies along the aortic arch. The chemoreceptors excite nerve fibers that, along with the baroreceptor fibers, pass through the Hering’s nerves and the vagus nerves into the vasomotor center of the brain stem. 7/22/2024 Jesus is Lord 75 Figure 14: peripheral chemoreceptors in the carotid and aortic bodies. 7/22/2024 Jesus is Lord 76  1. Chemoreceptor reflex contd. They have very high rates of O2 consumption and are very sensitive to decreases in the partial pressure of oxygen (PO2). They also are sensitive to increases in the partial pressure of CO2 (PCO2) and decreases in pH, particularly when PO2 is simultaneously decreased. The response of the peripheral chemoreceptors to decreased arterial PO2 is greater when the PCO2 is increased or the pH is decreased. 7/22/2024 Jesus is Lord 77  1. Chemoreceptor reflex contd. When arterial PO2 decreases, there is an increased firing rate of afferent nerves from the carotid and aortic bodies that activates sympathetic vasoconstrictor centers. The resultant effects are vasoconstriction, an increase in TPR, and an increase in arterial pressure. 7/22/2024 Jesus is Lord 78  1. Chemoreceptor reflex contd. The chemoreceptor reflex is not a powerful arterial pressure controller until the arterial pressure falls below 80 mm Hg. Therefore, it is at the lower pressures that this reflex becomes important to help prevent further decreases in arterial pressure. They play a far more important role in relation to respiratory control than in blood pressure control. 7/22/2024 Jesus is Lord 79  2. CNS ischemic response Central chemoreceptors are located in the medulla itself. The brain is intolerant of decreases in blood flow. These chemoreceptors are most sensitive to CO2 and pH and less sensitive to O2. Changes in PCO2 or pH stimulate the medullary chemoreceptors, which then direct changes in outflow of the medullary cardiovascular centers. 7/22/2024 Jesus is Lord 80  2. CNS ischemic response contd. When blood flow to the vasomotor center in the brain stem becomes decreased severely enough to cause nutritional deficiency (cerebral ischemia) The vasoconstrictor and cardioaccelerator neurons in the vasomotor center respond directly to the ischemia and become strongly excited. When this excitation occurs, the systemic arterial pressure often rises to a level as high as the heart can possibly pump. 7/22/2024 Jesus is Lord 81  2. CNS ischemic response contd. This effect is believed to be caused by failure of the slowly flowing blood to carry carbon dioxide away from the brain stem vasomotor center. This arterial pressure elevation in response to cerebral ischemia is known as the CNS ischemic response. The ischemic effect on vasomotor activity can elevate the mean arterial pressure dramatically, sometimes to as high as 250 mm Hg for as long as 10 minutes. 7/22/2024 Jesus is Lord 82  2. CNS ischemic response contd. The degree of sympathetic vasoconstriction caused by intense cerebral ischemia is often so great that some of the peripheral vessels become totally or almost totally occluded. The kidneys, for instance, often entirely cease their production of urine because of renal arteriolar constriction in response to the sympathetic discharge. 7/22/2024 Jesus is Lord 83  2. CNS ischemic response contd. The CNS ischemic response is one of the most powerful of all the activators of the sympathetic vasoconstrictor system. Despite the powerful nature of the CNS ischemic response, it does not become significant until the arterial pressure falls far below normal, down to 60 mm Hg and below, reaching its greatest degree of stimulation at a pressure of 15 to 20 mm Hg. 7/22/2024 Jesus is Lord 84  2. CNS ischemic response contd. Therefore, the CNS ischemic response is not one of the normal mechanisms for regulating arterial pressure. Instead, it operates principally as an emergency pressure control system that acts rapidly and powerfully to prevent further decrease in arterial pressure whenever blood flow to the brain decreases dangerously close to the lethal level. It is sometimes called the “last-ditch stand” pressure control mechanism. 7/22/2024 Jesus is Lord 85  3. Atrial and Pulmonary Artery Reflexes Both the atria and the pulmonary arteries have in their walls stretch receptors called low pressure receptors. Low-pressure receptors are similar to the baroreceptor stretch receptors of the large systemic arteries. These low-pressure receptors play an important role, especially in minimizing arterial pressure changes in response to changes in blood volume. 7/22/2024 Jesus is Lord 86  4. Stress Relaxation Mechanism The stress relaxation mechanism is demonstrated by the following example. When the pressure in the blood vessels becomes too high, they become stretched more and more for minutes or hours; as a result, the pressure in the vessels falls toward normal. This continuing stretch of the vessels, called stress relaxation, can serve as an intermediate-term pressure “buffer.” 7/22/2024 Jesus is Lord 87  5. Capillary Fluid Shift Mechanism The capillary fluid shift mechanism means simply that whenever capillary pressure falls too low, fluid is absorbed from the tissues through the capillary membranes and into the circulation, thus building up the blood volume and increasing the pressure in the circulation. 7/22/2024 Jesus is Lord 88  5. Capillary Fluid Shift Mechanism contd. Conversely, when the capillary pressure rises too high, fluid is lost out of the circulation into the tissues, thus reducing the blood volume, as well as virtually all the pressures throughout the circulation. 7/22/2024 Jesus is Lord 89  ANY QUESTIONS PLEASE 7/22/2024 Jesus is Lord 90  A REMINDER: The best time to start reading was yesterday but the next best time to start reading is “TODAY” 05/03/2018 Jesus is Lord 91  MAKE GOOD USE OF THE REST OF TODAY AND THE WEEK 05/03/2018 Jesus is Lord 92  THANK YOU FOR LISTENING 7/22/2024 Jesus is Lord 93  THE END 7/22/2024 Jesus is Lord 94