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Stroke volume is the amount of blood that is sent out of the heart during the systole stroke of the heartbeat. This is the combined output of both the left and the right ventricles. For a healthy male, this volume is somewhere around 5.6 liters per hour, for a healthy female, 4.9 liters per hour. The factors that could affect stroke volume would be heart disease, any of which narrow the ventricles chambers. Some of these diseases weaken the muscle of the ventricles, causing a decreased heart rate. Atherosclerosis, or hardening of the arteries, can have a dibilitating effect on cardiac output as well. The interaction of the three items mentioned are this: cardiac output is equal to the heart rate multiplied by the stroke volume. The amount of blood being pumped by the heart times the number of times the heart beats within a given time frame represents the total amount of blood the heart puts out, or the cardiac output.
The relationship among cardiac output, stroke volume, and heart rate is quite direct.
Cardiac Output = Stroke Volume X Heart Rate
Stroke Volume = End Diastolic Volume - End Systolic Volume
Although the concept applies to either ventricle, usually stroke volume measurements are made on the left ventricle, which has the larger workload.
The Frank-Starling Law states that the larger the preload, or volume of blood entering the heart during diastole (relaxation) the greater the volume ejected during the subsequent systole, or contraction. This happens because a greater volume of blood entering the heart causes stretching of the cardiac muscle, which in turn causes a larger number of actin/myosin cross bridges to form, increasing the power of the heart beat that follows.
Elevated afterload, which is resistance in the aorta, causes reductions in stroke volume. This is uncommon in healthy hearts, but can be seen in individuals with hypertension and/or aortic blockages.
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