Digestive System

Humans need to eat food to supply their bodies with energy and nutrients. The digestive system is responsible for processing this food and is comprised of the mouth, oesophagus, stomach and intestines.

You start digesting food the moment you bite into it. As your teeth mush the food up, enzymes in your saliva begin to break it down chemically to make it easier to chew and swallow. By the time it reaches your stomach, your meal is unrecognisable. 

The tongue is a very strong and flexible bundle of muscle that pushes food against your teeth as you chew. 

Swallowing is a reflex action, which means it happens automatically without you having to control it. As the tongue pushes food to the back of the mouth, the swallowing reflex begins.

Swallowed food gets pushed down a muscular tube called the oesophagus. The muscles even work when you are upside down or in outer space, where gravity would not help push the food toward the stomach.

The stomach has a stretchy lining wall so that it can expand to hold larger meals. Glands in the wall excrete acid and enzymes that start digesting protein in meat. Muscles here churn food about and squeeze it out of the stomach when it is ready.

When food leaves the stomach, it enters a long, tangled tube divided into two parts.
The first is called the small intestine, which is long and narrow. Digested food soaks through the walls of the small intestine and enters the blood to be carried away to provide cells and organs with nutrients and energy.
The second is called the large intestine, which is shorter but fatter. Here water and some vitamins are absorbed from undigested leftovers. The rest passes out of the body as poo.
The intestines use a special kind of muscle action to move food along. Rings of muscles squeeze behind the food, pushing it along, like a hand pushing a ball along a stocking.

The body gets rid of waste chemicals and excess water by making urine. Urine comes from two organs called kidneys. The kidneys filter and clean blood, removing chemicals that your body doesn't need.
If you put your hands on your hips and thumbs on your back, your kidneys are next to your thumb tips.

Further Reading