What causes a reduction in the heart's stroke volume during illness?

The reduction in the heart's stroke volume during illness is primarily caused by decreased venous return. Stroke volume refers to the amount of blood the heart pumps with each contraction. Venous return is the volume of blood returning to the heart from the body's tissues. When a person is ill, several factors may contribute to reduced venous return, such as dehydration, blood loss, or changes in body posture, leading to a decrease in the volume of blood available to fill the heart.

As venous return declines, there is less blood available in the heart's chambers during diastole (the filling phase), which subsequently leads to a lower stroke volume. This can impact overall cardiac output, as stroke volume is a critical component of the volume of blood that the heart pumps per minute, along with heart rate.

Increased parasympathetic activity might slow heart rate but does not directly reduce stroke volume. Increased heart rate can sometimes compensate for a reduction in stroke volume but doesn't inherently decrease stroke volume itself. Increased contractility, which refers to the strength of the heart's contraction, typically increases stroke volume rather than reducing it. Therefore, decreased venous return stands out as the correct explanation for the reduced stroke volume during illness.