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 If venous return increases and there is no change in sympathetic tone, venous pressure rises, diastolic inflow is greater, ventricular end-diastolic pressure increases, and the heart muscle contracts more forcefully. During muscular exercise, venous return is increased by the pumping action of the muscles and the increase in respiration. In addition, because of vasodilation in the contracting muscles, peripheral resistance and, consequently, afterload are decreased. The end result in both normal and transplanted hearts is thus a prompt and marked.

During muscular exercise, there is increased sympathetic discharge, so that myocardial contractility is increased and the heart rate rises. The increase in heart rate is particularly prominent in normal individuals, and there is only a modest increase in stroke volume. However, patients with transplanted hearts are able to increase their cardiac output during exercise in the absence of cardiac innervation through the operation of the Frank–Starling mechanism. Circulating catecholamines also contribute. If venous return increases and there is no change in sympathetic tone, venous pressure rises, diastolic inflow is greater, ventricular end-diastolic pressure increases, and the heart muscle contracts more forcefully. During muscular exercise, venous return is increased by the pumping action of the muscles and the increase in respiration. In addition, because of vasodilation in the contracting muscles, peripheral resistance and, consequently, afterload are decreased. The end result in both normal and transplanted hearts is thus a prompt and marked increase in cardiac output.
 
One of the differences between untrained individuals and trained athletes is that the athletes have lower heart rates, greater end-systolic ventricular volumes, and greater stroke volumes at rest. Therefore, they can potentially achieve a given increase in cardiac output by further increases in stroke volume without increasing their heart rate to as great a degree as an untrained individual.
 
The basal O2 consumption by the myocardium is about 2 mL/100 g/min. This value is considerably higher than that of resting skeletal muscle. O2 consumption by the beating heart is about 9 mL/100 g/min at rest. Increases occur during exercise and in a number of different states. Cardiac venous O2 tension is low, and little additional O2 can be extracted from the blood in the coronaries, so increases in O2 consumption require increases in coronary blood flow.O2 consumption by the heart is determined primarily by the intramyocardial tension, the contractile state of the myocardium, and the heart rate. Ventricular work per beat correlates with O2 consumption. The work is the product of stroke volume and mean arterial pressure in the pulmonary artery or the aorta (for the right and left ventricle, respectively). Because aortic pressure is 7 times greater than pulmonary artery pressure, the stroke work of the left ventricle is approximately
 
7 times the stroke work of the right. In theory, a 25% increase in stroke volume without a change in arterial pressure should produce the same increase in O2 consumption as a 25% increase in arterial pressure without a change in stroke volume. However, for reasons that are incompletely understood, pressure work produces a greater increase in O2 consumption than volume work. In other words, an increase in afterload causes a greater increase in cardiac O2 consumption than does an increase in preload. This is why angina pectoris due to deficient delivery of O2 to the myocardium is more common in aortic stenosis than in aortic insufficiency. In aortic stenosis, intraventricular pressure must be increased to force blood through the stenotic valve, whereas in aortic insufficiency, regurgitation of blood produces an increase in stroke volume with little change in aortic impedance.
 
It is worth noting that the increase in O2 consumption produced by increased stroke volume when the myocardial fibers are stretched is an example of the operation of the law of
Laplace. This law states that the tension developed in the wall of a hollow viscus is proportionate to the radius of the viscus, and the radius of a dilated heart is increased. O2 consumption per unit time increases when the heart rate is increased by sympathetic stimulation because of the increased number of beats and the increased velocity and strength of each contraction. However, this is somewhat offset by the decrease in end-systolic volume and hence in the radius of the heart.