The Digestive and urinary systems are the body's power plant and its waste-disposal mechanism. The digestive system (also called the bowel) begins at the mouth and ends at the anus. At the top, food enters the mouth where it is chewed and combined with saliva, which has enzymes to begin the breakdown of the food into useful components. It passes into the digestive tract, where more juices break the food into proteins, fats, and carbohydrates and pass it into the stomach.

Some substances (water, salts, glucose, alcohol, and certain drugs) are absorbed directly into the system from the stomach. Most food, however, is transformed by the acidic gastric juices of the stomach into a substance called chyme. Chyme passes from the stomach via the duodenum into the small intestine, where it is bathed in bile from the gall bladder and juices from the pancreas. Powerful muscular contractions (peristalsis) push the chyme along the small intestine, and as it proceeds more digestive juices and enzymes break it down into fats, carbohydrate, and proteins. These are absorbed into the bloodstream through the walls of the small intestine. From the bloodstream they pass to the liver, which is very important in the digestive process, storing sugar, fats, and proteins, and creating bile. The liver is also responsible for neutralizing toxins such as alcohol and drugs.

By the time the chyme reaches the colon (large intestine), all that is left of it is indigestible roughage and water. The water is absorbed back into the bloodstream through the intestinal walls. The main function of the large intestine is to reclaim water from food. The bulky remains are passed into the rectum as feces, and from here they are expelled as a bowel movement.

The function of the urinary system, which composed of the kidneys and bladder together with their connecting tubes, is to maintain the body's internal balance of water and salts. It is also responsible, for filtering the blood and expelling excess waste and waste products. The kidneys also playa vital role in removing toxins and secreting important hormones. The kidneys are made up of nephrons, which are responsible for filtering out the smaller molecules from the blood, including water, glucose, and waste products. The kidneys have an impressive blood supply - more than 300pt/150l of blood passes through them each day to form the 2pt/1l of urine that the body produces daily. Urine formed in the kidneys passes down the ureters and is stored in the bladder until it is convenient to empty it. The bladder drains through the urethra. In the female, the urethra is about 1 1/2in/4cm long but it is 10in/25cm long in males. The bladder is a fibrous organ that stretches as it fills with urine. When it reaches its capacity it sends a message to the brain, which acknowledges that water should be passed.