Your heartbeat is created when the muscular walls of the heart
vibrate
O expand
O contract
O None of the above

contract

Explanation:

in order to pump blood it has to be pumped

fluid balance is an aspect of the homeostasis of organisms in which the amount of water in the organism needs to be controlled, via osmoregulation and behavior, such that the concentrations of electrolytes (salts in solution) in the various body fluids are kept within healthy ranges. the core principle of fluid balance is that the amount of water lost from the body must equal the amount of water taken in.