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Lecture 7

lecture 7

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Biological Sciences
Ivana Stehlik

REGULATION OF ARTERIAL PRESSUREThe overall function of the cardiovascular system is to deliver blood to the tissues so that O and 2nutrients can be provided and waste products carried away Blood flow to the tissues is driven by the difference in pressure between the arterial and venous sides of the circulation Mean arterial pressure P is the driving force for blood flow and it must be maintained at a high constant alevel of approximately 100 mm Hg Because of the parallel arrangement of arteries off the aorta the pressure in the major artery serving each organ is equal to P The blood flow to each organ ais then independently regulated by changing the resistance of its arterioles through local control mechanismsThe mechanisms that help to maintain P at a constant value are discussed in this section The abasis for this regulation can be appreciated by examining the equation for P a PCardiac output x TPR a wherePMean arterial pressure mm Hg a Cardiac outputCardiac output mlmin TPRTotal peripheral resistance mm HgmlminInspection of this equation reveals that Pcan be changed by altering the cardiac output or any a of its parameters altering the TPR or any of its parameters or altering both cardiac output and TPRThe mechanisms responsible for maintaining a constant value for arterial pressure closely monitor P and compare it with the setpoint value of approximately 100 mm Hg If P increases aaabove the set point or decreases below the set point the cardiovascular system makes adjustments in cardiac output in TPR or in both attempting to return P to the setpoint valuea P is regulated by two major systems The first system is neurally mediated and known as the abaroreceptor reflex Figure 1 The baroreceptor reflex attempts to restore P to its setpoint avalue in a matter of seconds The second system is hormonally mediated and includes the reninangiotensinaldosterone system which regulates P more slowly primarily by its effect on ablood volumeBaroreceptor ReflexThe baroreceptor mechanisms are fast neurally mediated reflexes that attempt to keep arterial pressure constant via changes in the output of the sympathetic and parasympathetic nervous systems to the heart and blood vessels Pressure sensors the baroreceptors are located within the walls of the carotid sinus and the aortic arch Figure 1 and relay information about blood pressure to cardiovascular vasomotor centers in the brain stem The vasomotor centers in turn coordinate a change in output of the autonomic nervous system to effect the desired change in P aThus the reflex arc consists of sensors for blood pressure afferent neurons which carry the information to the brain stem brain stem centers which process the information and coordinate an appropriate response and efferent neurons which direct changes in the heart and blood vessels Figure2Figure 1BaroreceptorsThe baroreceptors are located in the walls of the carotid sinus where the common carotid artery bifurcates into the internal and external carotid arteries and in the aortic arch Figure 1 The carotid sinus baroreceptors are responsive to increases or decreases in arterial pressure whereas the aortic arch baroreceptors are primarily responsive to increases in arterial pressureThe baroreceptors are mechanoreceptors which are sensitive to pressure or stretch Thus changes in arterial pressure cause more or less stretch on the mechanoreceptors resulting in a change in their membrane potential Such a change in membrane potential is a receptor potential which increases or decreases the likelihood that action potentials will be fired in the afferent
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