how is blood flow related to lung function during exercise? a. blood vessels are dilated to increase the amount of oxygen carried in the blood, and the lungs work harder to meet the body's oxygen demands. b. blood volume increases during exercise, making the body more efficient, so it is not necessary for the lungs to work as hard. c. the lungs determine the speed at which the heart pumps, so blood flows at a level that accommodates oxygen absorption in the muscles. d. the lungs expand and contract, which increases the rate of blood flow so the muscles can convert stored energy.