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 answer is B, the heart requires more oxygen when exercising. This is because of the off chance that it didn't you'd be extremely wasteful. Either yield would be lower than required and you'd tumble to work or higher, which squanders vitality. The volume of blood that the heart pumps out is kept up so that body organs get the important measure of oxygen and nourishment they require.

During exercise, heart beats faser, and harder. Therefore, it requires more oxygen to keep to the pace. More energy is required during exrercise, and thus for production of energy, extra load is put on overall process, leading to high respiratory rates.