Which factor is primarily responsible for increasing cardiac output?

Increasing cardiac output is primarily influenced by stroke volume, which is defined as the amount of blood pumped by the heart in one contraction. Cardiac output is calculated using the formula:

Cardiac Output (CO) = Stroke Volume (SV) × Heart Rate (HR).

When stroke volume increases, assuming the heart rate remains constant, there is a direct, proportional increase in cardiac output. This increase in stroke volume can be due to a variety of factors, such as enhanced venous return, increased contractility of the heart muscle, or improved filling of the ventricles.

In contrast, higher blood viscosity can lead to increased resistance in the circulatory system, which generally decreases cardiac output rather than increasing it. A decreased heart rate would also result in a lower cardiac output because it would reduce the number of times the heart pumps in a given time frame. Additionally, decreased end diastolic volume typically means that there is less blood available to be pumped out of the heart, which would lower stroke volume and thereby decrease cardiac output.

Thus, the primary factor responsible for increasing cardiac output is indeed an increase in stroke volume.