You have been advised to check your pulse during exercise in order to monitor your heart rate. what is happening in your heart as your pulse is produced

its pumping blood into your heart and through out the rest of your body to keep the blood flow steady

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.