Which statement best describes the relationship between blood flow through the vascular system and cardiac output?

• A. Blood flow through the entire vascular system is proportional to cardiac output.
• B. Blood flow is independent of cardiac output.
• C. Blood flow is inversely proportional to cardiac output.
• D. Blood flow depends only on heart rate.

Answer: (A)

The main idea is that the circulatory system is a closed loop and the heart’s output sets the total amount of blood flowing per minute. Cardiac output is the volume the heart ejects each minute (heart rate × stroke volume). In a steady state, the total blood flow through the entire vascular system equals this cardiac output, because what leaves the heart must circulate through the body and return to the heart. The distribution of that flow among different tissues depends on regional vascular resistances, but the sum of flows to all tissues matches the cardiac output. So, increasing or decreasing the heart’s pump changes the overall flow correspondingly, rather than flow being independent, inversely related, or dictated only by heart rate.