Why does the heart automatically adjust the flow of blood to match activity levels? a. extreme activity damages blood cells and the body needs replacements. b. the heart requires more oxygen when exercising. c. it builds up a higher level of oxygen in the blood. d. organs and muscles require more blood under stress.

Option C. It builds up a higher level of oxygen in the blood.

At rest, the heart pumps out about 5,000mL/min of blood. This is called the cardiac output. One can say that, at rest, the total systemic blood flow throughout the body is about 5L/min. At the extreme level of activity the heart pumps much harder and faster and is usually able to quintuple its output to about 25L/min of blood flow. This amount applies to the right side of the heart, which pumps blood to the lungs so it can pick up oxygen and drop off Carbon dioxide, and the left side of the heart that pumps it to the rest of the organs and tissues of the body.