Discuss the regulation of cardiac output. Illustrate your answer by using a graph to describe the important physiological relationships.

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College Answer

Most candidates approached this question by defining cardiac output as stroke volume × heart 
rate and then discussing the determinants of cardiac output - preload, contractility, afterload and 
heart rate rather than focusing on the regulation of cardiac output. Under preload a brief 
description of the Frank Starling mechanism was required. Important was the concept that at rest 
cardiac output is controlled almost entirely by peripheral factors that determine venous return. 
These concepts were best illustrated by graphing vascular function (venous return vs right atrial 
pressure) and cardiac function (cardiac output vs right atrial pressure) curves. Then 
demonstrating on these curves the factors that affected preload, contractility and afterload such 
as changes in blood volume, sympathetic and parasympathetic stimulation and exercise as 
examples. Also important to demonstrate on these curves was the fact that venous return and 
cardiac output are equal at steady state. Most candidates tried to illustrate these cardiovascular 
concepts with a series of left ventricular pressure volume loops rather than use the vascular and 
cardiac function curves. They then went on to demonstrate via these pressure volume loops the 
effects of changes in preload, contractility and afterload on stroke volume. Candidates who took 
this approach were not penalised, if there were clear, correct diagrams with explanations 
indicating comprehension of these concepts. On the whole graphs were poorly drawn and were 
not well integrated into the answer. Some candidates also wasted time by unnecessarily 
describing excitation-contraction coupling and sympathetic nerve reflex pathways

Recommended sources: Guyton Textbook of Medical Physiology 11th edition pgs 241-243