Page 751 - Fundamentals of anatomy physiology

738 Unit 4 Fluids and Transport

Figure 20–19 A Simple Model of Stroke Volume. The stroke volume of the heart can be compared to the

amount of water ejected from a simple pump. The amount of water ejected varies with the amount of movement
of the pump handle.

Start When the pump handle is Filling
raised, pressure within the
cylinder decreases, and At the start of the pumping
water enters through a cycle, the amount of water in
one-way valve. This the cylinder corresponds to
corresponds to passive the amount of blood in a
filling during ventricular ventricle at the end of
diastole. ventricular diastole. This
amount is known as the
end-diastolic volume (EDV).

Ventricular
diastole

End-systolic End-diastolic
volume volume (EDV)
(ESV)
20 Pumping
Stroke
volume Ventricular As the pump handle is pushed
systole down, water is forced out of
When the handle is depressed as the cylinder. This corresponds
far as it will go, some water will to the period of ventricular
remain in the cylinder. That amount ejection.
corresponds to the end-systolic
volume (ESV) remaining in the
ventricle at the end of ventricular
systole. The amount of water
pumped out corresponds to the
stroke volume of the heart; the
stroke volume is the difference
between the EDV and the ESV.

individual factors involved. However, changes in cardiac output of other factors can influence cardiac output under abnormal
generally reflect changes in both heart rate and stroke volume. circumstances, and we consider several examples in the Clinical
Note on p. 734.
The heart rate can be adjusted by the activities of the
autonomic nervous system or by circulating hormones. The Factors Affecting the Heart Rate
stroke volume can be adjusted by changes in the end-diastolic
volume (how full a ventricle is when it starts to contract), the Under normal circumstances, autonomic activity and circulat-
end-systolic volume (how much blood remains in a ventricle ing hormones make homeostatic adjustments to the heart rate
after it contracts), or both. As we saw in Figure 20–19, stroke as cardiovascular demands change. These factors act by modify-
volume peaks when EDV is highest and ESV is lowest. A variety ing the natural rhythm of the heart. Even a heart removed for a