Question 20

Describe the physiological control of systemic vascular resistance (SVR)

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College Answer

A definition or description of SVR that recognised the importance of radius in small arteries/arterioles as the major determinant attracted marks. Resistance is ΔP/flow; where ΔP is not only MAP and flow is volume/time. The systemic vascular resistance is the resistance of several circuits in parallel, which have both common and independent factors in their regulation. As they are in parallel the sum of reciprocals is used, 1/SVR = 1/R1 + 1/R2 to determine the overall value. A detailed explanation of the Hagen-Pouiselle law was not required, attracted few marks and wasted writing time. The remainder of the answer focus was on the factors that control the radius of these vessels. As a question regarding control, an approach that included sensors, integrators and effectors tended to yield a more comprehensive answer with resultant higher marks. Other useful structures included divisions into intrinsic/local factors (including endothelial input and autoregulation), neural control (reflexes and central controller) and hormonal control. As the question was regarding physiological control, no marks were awarded to pharmacological manipulation of SVR. Given the potential scope of the question, detailed descriptions of how noradrenaline exerts its effect were not required beyond receptor level although stating 'sympathetic nervous system activation results in vasoconstriction' were too simplistic to attract full marks.