Preload is most directly influenced by which factor?

Preload is most directly influenced by which factor?

• A. Venous return to the heart
• B. Afterload
• C. Heart rate
• D. Calcium availability

Answer: (A)

Preload is the degree of stretch of the ventricular muscle fibers at the end of diastole, which reflects how much blood has filled the ventricle. It is determined most directly by venous return—the amount of blood returning to the heart. When venous return increases, more blood fills the ventricles, end-diastolic volume rises, and the heart muscle lengthens more before contraction, producing a stronger pump through the Frank-Starling mechanism. Afterload matters for how much blood is expelled during systole, not for how much blood fills the ventricle. Heart rate can influence how long the ventricle has to fill, so there is some indirect effect on preload, but the direct determinant is venous return. Calcium availability mainly affects contractility itself, not the filling of the ventricle.

Preload is the degree of stretch of the ventricular muscle fibers at the end of diastole, which reflects how much blood has filled the ventricle. It is determined most directly by venous return—the amount of blood returning to the heart. When venous return increases, more blood fills the ventricles, end-diastolic volume rises, and the heart muscle lengthens more before contraction, producing a stronger pump through the Frank-Starling mechanism.

Afterload matters for how much blood is expelled during systole, not for how much blood fills the ventricle. Heart rate can influence how long the ventricle has to fill, so there is some indirect effect on preload, but the direct determinant is venous return. Calcium availability mainly affects contractility itself, not the filling of the ventricle.