10–8. a. SV = CO/HR
SV = 6000/70 = 86 mL/beat at rest
SV = 18,000/160 = 113 mL/beat during exercise
[You may recall that, in the absence of other information, changes in SV can be estimated from changes in arterial pulse pressure, Pp. The information in Figure 10–4 indicates that Pp increased 1.75 times (from 40 to 70 mm Hg) as a result of exercise, whereas SV actually increased only 1.32 times (from 86 to 113 mL), as calculated earlier. This discrepancy emphasizes that although SV is a major determinant of Pp, changes in other factors, such as the compliance of arteries (CA), can influence Pp as well (see Appendix C). Part of the increase in Pp that accompanies exercise is due to a decrease in effective arterial compliance. The latter is due to (1) an increase in mean arterial pressure with exercise and (2) the nonlinear nature of the arterial volume–pressure relationship (see Figures 6–8 and 6–10).]
b. Ejection fraction = SV/EDV, therefore EDV = SV/ejection fraction
EDV = 86/0.60 = 143 mL at rest
EDV = 113/0.80 = 141 mL during exercise
[Recall that central venous pressure, PCV, is the cardiac filling pressure or preload and is therefore the primary determinant of EDV. The EDV changed little with exercise because exercise caused little or no change in PCV.]
c. SV = EDV − ESV, therefore ESV = EDV − SV
ESV = 143 − 86 = 57 mL at rest
ESV = 141 − 113 = 28 mL during exercise
[Recall that the primary determinants of ESV are cardiac afterload (mean arterial pressure) and myocardial contractility (see Appendix C). Cardiac afterload increases during exercise and thus goes in the wrong direction to account for a decrease in ESV. Therefore, an increased myocardial contractility, secondary to increased cardiac sympathetic nerve activity, must be primarily responsible for the decrease in ESV that accompanies exercise.]
1. End-diastolic volume during both rest and exercise is approximately 140 mL.
2. Ventricular ejection (decreasing ventricular volume) begins when intraventricular pressure reaches the diastolic aortic pressure and the aortic valve opens. Figure 10–4 indicates an arterial diastolic pressure of 80 mm Hg both at rest and during exercise. Thus, ventricular ejection will begin at an intraventricular pressure of 80 mm Hg in both situations.
3 and 4. Peak intraventricular pressure normally equals peak (systolic) arterial pressure. Hence, the systolic arterial pressure values in Figure 10–4 indicate peak intraventricular pressures of 120 and 150 mm Hg during rest and exercise, respectively.
5 and 6. As calculated in c earlier, end-systolic volume is 57 mL at rest and decreases to 28 mL during exercise. The reduction in end-systolic volume accounts for the increase in stroke volume during exercise.