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.

Explanation:

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explanation:

since you have not provided the options here i   am going to give you general overview about hormone.hormone are actually the part of endocrine system in our body and produced by the endocrine glands. the hormones act like a chemical messenger   within human body. once produce by the glands they   travel through the blood stream and reach at specific or target tissue and organs   which result in control of our body function. the hormones are specific in their work and work on only the targeted tissue or organ   . with the of hormone endocrine system regulate the heart rate , metabolism, our mood and   growth as well as the development and even the sleeping cycle etc. they actually are the messenger for body cell and organs .