Excessively high hear rate `( gt 180 )` can reduce cardiac output because

Options

A. blood is moving too fast through the lungs to pick up enough oxygen
B. it tires out the heart muscles and so they pump slower
C. it reduces the time for ventricular filling which reduces storke volume
D. the PR-interval increases which leads to longer ventricular diastole and shorter ventricular systole

Answer

C

Explanation

The cardiac output is determined by multiplying the heart rate (HR) - the number of beats per minute, and the stroke volume (SV) - the blood volume ejected by each ventricle during systole. During diastole, normal filling of the ventricles occurs. In the first third period of diastole, there is rapid filling of ventricles. The middle period of diastole accounts only for a small amount of blood that continues to empty into the atria from the veins and passes through the atria directly into the ventricles. During the last third period of diastole, the atria contract and give an additional thrust to the inflow of blood into the ventricles. The fact that most of ventricular filling is completed during early diastole is of great importance. It ensures that filling is not seriously impaired during period when the heart is beating very rapidly and the duration of diastole and therefore total filling times are reduced. Furthermore, this leads to a decrease in the blood volume ejected at the time of ventricular systole. Hence, the stroke volume reduces.