Question

Why does the heart automatically adjust the flow of blood to match activity levels?

Answer 1

D. Organs and muscles require more blood under stress.

A is incorrect because blood cells don't get damaged due to activity.
B is incorrect because it is not the heart that needs oxygen; it's the body cells that are being worked, such as muscle cells in you're arm when you're lifting weights.
C is incorrect because the flow of blood doesn't change the level of oxygen in the blood; breathing does.

Answer 2

D. Organs and muscles require more blood under stretch.
Just think about it, when you go on a run or do something physical your blood levels increase. Exercising is all about getting your pulse up, because that's how you measure intensity.