Cardiovascular Physiology Regulation of Arterial Pressure PDF

Week 5: Cardiovascular Physiology Regulation of Arterial Pressure 1 Relationships between cardiac output and venous return It should be clear from the previous discussion that one of the most important factors determining cardiac output is left ventricular end-diast...

Week 5: Cardiovascular Physiology Regulation of Arterial Pressure 1 Relationships between cardiac output and venous return It should be clear from the previous discussion that one of the most important factors determining cardiac output is left ventricular end-diastolic volume. In turn, left ventricular end-diastolic volume depends on venous return, which also determines right atrial pressure. Thus, it follows that there is not only a relationship between cardiac output and end-diastolic volume but also a relationship between cardiac output and right atrial pressure. Cardiac output and venous return each can be examined separately as a function of right atrial pressure. These separate relationships also can be combined in a single graph to visualize the normal interrelationship between cardiac output and venous return (see Fig. 4.25). The combined graphs can be used to predict the effects of changes in various cardiovascular parameters on cardiac output, venous return, and right atrial pressure. Cardiac function curve The cardiac function curve or cardiac output curve, shown in Figure 4.26, is based on the Frank-Starling relationship for the left ventricle. The cardiac function curve is a plot of the relationship between cardiac output of the left ventricle and right atrial pressure. Again, recall that right atrial pressure is related to venous return, end-diastolic volume, and end- diastolic fiber length: As venous return increases, right atrial pressure increases, and end- diastolic volume and end-diastolic fiber length increase. Increases in end-diastolic fiber length produce increases in stroke volume and cardiac output. Thus, in the steady state, the volume of blood the left ventricle ejects as cardiac output equals or matches the volume it receives in venous return. Increases in end-diastolic volume (i.e., right atrial pressure) produce increases in cardiac output by the Frank-Starling mechanism. However, this “matching” occurs only up to a point: When right atrial pressure reaches a value of approximately 4 mm Hg, cardiac output can no longer keep up with venous return, and the cardiac function curve levels off. This maximum level of cardiac output is approximately 9 L/min.