Body size, age, and fitness level are factors in heart rate. why is this?

i know the answer but i have to give it in "detail"

"Heart rate is like a variable, if someone is bigger blood has to flow farther, and takes more pressure to deliver the blood. As you age, cholesterol accumulates in arteries and makes each pump of the heart less efficient, and someone with good fitness needs less beats per minute to deliver all the blood/oxygen they need. Michael Phelps, for example, probably has a heart rate of less than 40 BPM; Whereas as person of the same age that is less involved in an exercise regiment may have a heart rate around 80 BPM."

Body size= How far the blood has to travel from the heart.
Age = How fast the blood can be pumped along with how far.
Fitness level = how strong the heart is.

Predator instincts is what causes it

C- a hypothesis or d- an observation