In this topic you will learn about how your body prepares food for your cells and eliminates the waste products that are left over.

Every day people eat food. Some of the food becomes waste in the body. Before getting rid of the wastes, the body separates out the useful nutrients. Digestion is the process of breaking down food into simple substances your body can use.

Digestion begins in the mouth when you chew your food. Chewing breaks the large pieces into smaller pieces. Chewing also moistens food with saliva, which contains digestive enzymes. An enzyme is a chemical that breaks down food into simple pieces that cells can use.

When you swallow, the food travels down a tube called the esophagus to your stomach. In the stomach, food is mixed with gastric juice. Gastric juice contains acids and more enzymes to break down the proteins in your food, turning the food into a thick liquid. The food then enters the small intestine. In the small intestine, juices from the pancreas and liver work on the food.

The liver is the largest organ in the body. Bile is a greenish-yellow fluid produced by the liver to digest animal fats. Bile, pancreatic juices, and other intestinal juices break down most of the remaining foods. Now the nutrients must be absorbed. The walls of the small intestine are lined with villi. Villi are tiny fingerlike structures in the small intestine that absorb digested food. The villi are lined with capillaries. Digested food passes through the villi and capillaries to enter the bloodstream. Not all food is changed into nutrients. Undigested food and water goes to the large intestine. At the end of the large intestine, wastes exit the body through the anus.

Your liver purifies your blood. It filters out waste, bacteria, drugs, and certain chemicals. Your liver converts these waste products to urea. Urea is a substance formed from waste material in the liver and carried by blood to the kidneys for excretion in urine. Excretion is the process of removing waste products from the body. Kidneys filter and clean the blood of urea and other wastes. Each kidney contains more than one million nephrons. A nephron is a structure in the kidney that filters blood. The cleaned blood leaves through veins. The wastes are collected in your kidneys as liquid urine. The urine flows down long tubes called ureters to a muscular bag called the bladder.

Your skin also takes part in excretion when you sweat. Glands in the skin produce sweat. Sweat, which is mostly water, contains mineral salts that the body doesn't need. There is also a tiny amount of urea in sweat.

Your body is about two-thirds water. There is water inside and outside of your cells, in your blood, and in your lymph. Without water you could not cool down by sweating or remove wastes from your body.