Stroke volume is influenced by preload, afterload, and contractility.

Stroke volume is influenced by preload, afterload, and contractility.

• A. Heart rate, respiratory rate, and temperature
• B. Oxygen saturation, pH, and pressure
• C. Venous pooling, diffusion distance, and viscosity
• D. Preload, afterload, and contractility

Answer: (D)

Stroke volume is the amount of blood the ventricle ejects with each beat. It is controlled by three main factors: preload, afterload, and contractility. Preload reflects how much blood fills the ventricle before it contracts (end-diastolic volume). When venous return increases, the ventricle stretches more, and the force of contraction rises (the Frank-Starling mechanism), increasing stroke volume—within physiological limits. Afterload is the pressure the ventricle must overcome to open the aortic or pulmonary valve. Higher afterload makes it harder to eject blood, so stroke volume tends to decrease as the ventricle must work against greater resistance. Contractility is the intrinsic strength of the cardiac muscle during systole; increased contractility means the heart can eject more blood for the same preload, raising stroke volume, while reduced contractility lowers it. Other listed factors—such as heart rate, respiratory rate, and temperature—affect overall cardiovascular output and metabolic demand but do not directly set the amount of blood pumped per beat. Parameters like oxygen saturation, pH, and pressure relate to gas exchange and systemic conditions rather than the direct mechanical determinants of stroke volume. Venous pooling, diffusion distance, and viscosity influence return and flow indirectly, but the direct determinants of stroke volume are the three mentioned: preload, afterload, and contractility.

Stroke volume is the amount of blood the ventricle ejects with each beat. It is controlled by three main factors: preload, afterload, and contractility. Preload reflects how much blood fills the ventricle before it contracts (end-diastolic volume). When venous return increases, the ventricle stretches more, and the force of contraction rises (the Frank-Starling mechanism), increasing stroke volume—within physiological limits. Afterload is the pressure the ventricle must overcome to open the aortic or pulmonary valve. Higher afterload makes it harder to eject blood, so stroke volume tends to decrease as the ventricle must work against greater resistance. Contractility is the intrinsic strength of the cardiac muscle during systole; increased contractility means the heart can eject more blood for the same preload, raising stroke volume, while reduced contractility lowers it.

Other listed factors—such as heart rate, respiratory rate, and temperature—affect overall cardiovascular output and metabolic demand but do not directly set the amount of blood pumped per beat. Parameters like oxygen saturation, pH, and pressure relate to gas exchange and systemic conditions rather than the direct mechanical determinants of stroke volume. Venous pooling, diffusion distance, and viscosity influence return and flow indirectly, but the direct determinants of stroke volume are the three mentioned: preload, afterload, and contractility.