Define preload and describe its effect on stroke volume via the Frank-Starling mechanism.

Define preload and describe its effect on stroke volume via the Frank-Starling mechanism.

• A. Preload is the degree of stretch of the ventricles at end-diastole; greater preload increases stroke volume through the Frank-Starling mechanism, up to physiological limits.
• B. Preload is the volume of blood in the aorta; greater preload reduces stroke volume via afterload.
• C. Preload is the heart rate at rest; higher preload slows heart rate during exercise.
• D. Preload is the pressure in the pulmonary artery; greater preload has no effect on stroke volume.

Answer: (A)

Preload is the degree to which the ventricular walls are stretched at the end of filling (end-diastolic volume). When preload increases, the heart muscle fibers start the next heartbeat more stretched. In the Frank-Starling mechanism, this stretch moves the sarcomeres toward an optimal length for actin–myosin cross-bridge formation, so the contraction is stronger and a greater volume of blood is ejected with each beat. This increases stroke volume, up to the limits of the heart’s filling and contractile capacity. Preload is not the volume in the aorta or pulmonary artery and it’s not heart rate; those relate to afterload or other factors.

Preload is the degree to which the ventricular walls are stretched at the end of filling (end-diastolic volume). When preload increases, the heart muscle fibers start the next heartbeat more stretched. In the Frank-Starling mechanism, this stretch moves the sarcomeres toward an optimal length for actin–myosin cross-bridge formation, so the contraction is stronger and a greater volume of blood is ejected with each beat. This increases stroke volume, up to the limits of the heart’s filling and contractile capacity. Preload is not the volume in the aorta or pulmonary artery and it’s not heart rate; those relate to afterload or other factors.