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.

The correct answer for above statement is:

The heart is that organ of the body that pumps blood throughout the body to all the tissues. The heart automatically regulates the flow of blood to meet a person’s activity level because the heart needs extra oxygen when a person is doing exercise. When you exercise your heart has to work harder and beats faster to pump blood throughout your blood vessels. Blood movement retains the body and its organs operating accurately.

Organs and muscles require more blood under stress.