When the body responds to the need of food in is called?
Digestion breaks down food into simple forms that the body can absorb and use. There are something collectively called essential nutrients, meaning they are nutrients the body need in order to function properly and ultimately survive, but that the body can't produce itself. Through digestion of food that does contain these nutrients, the body can obtain them.
The human body has a system of getting rid of (eliminating) what it does not need in two ways. After food is eaten and digested, it moves through the body and what is not needed comes out when you go to the bathroom. Liquid waste is called urine ("pee" in slang) and solid waste is called feces ("poop" or "poo" in slang).