Key Points

• Venous pressure is a major determinant of cardiac output because it is the filling pressure for the heart. Venous pressure at any cardiac output can be predicted by the vascular function curve.

• Venous pressure tends to be reciprocally related to arterial pressure. Because venous capacitance is much larger than arterial capacitance, large changes in arterial pressure cause much smaller changes in venous pressure as blood shifts from the arteries to the veins.

• The periphery interacts with the heart through changes in venous pressure because venous pressure falls as cardiac output increases.

• The crossing point of the cardiac and the venous function curves determines cardiac output.

• Increasing contractility increases cardiac output and reduces venous pressure.

• Increasing blood volume increases cardiac output and venous pressure.

• Increasing venous tone increases cardiac output and venous pressure.

• Increasing arteriolar tone decreases cardiac output but has an unpredictable effect on venous pressure.

• Heart rate affects cardiac output in a triphasic manner. Very low heart rates (bradycardia) limit cardiac output. In the normal range of heart rates (50-150 beats/min), stroke volume changes reciprocally with heart rate so that little effect is seen on cardiac output. High heart rates (tachycardia) limit cardiac output because of inadequate time for ventricular filling.

Essential Medical Physiology, Third Edition

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