The Digestive and Excretory System

Digestion is the breaking down of food in the body, into a form that can be absorbed and used or excreted. It is also the process by which the body breaks down food into smaller components that can be absorbed by the blood stream. In mammals, preparation for digestion begins with the cephalic phase in which saliva is produced in the mouth and digestive enzymes are produced in the stomach. Mechanical and chemical digestion begin in the mouth where food is chewed, and mixed with saliva to break down starches. The stomach continues to break food down mechanically and chemically through the churning of the stomach and mixing with enzymes. Absorption occurs in the stomach and gastrointestinal tract, and the process finishes with excretion

The excretory system is an organ system that performs the function of excretion, the bodily process of discharging nitrogeneous wastes. It is responsible for the elimination of the nitrogeneous waste products of metabolism as well as other non-useful nitrogeneous materials. The main components of the excretory system are your two kidneys, two tubes that carry urine called ureters, the bladder, and the urethra.

The energy required for maintenance and proper functioning of the human body is supplied by food. After it is broken into fragments by chewing (see Teeth) and mixed with saliva, digestion begins. The food passes down the gullet into the stomach, where the process is continued by the gastric and intestinal juices. Thereafter, the mixture of food and secretions, called chyme, is pushed down the alimentary canal by peristalsis, rhythmic contractions of the smooth muscle of the gastrointestinal system.

The contractions are initiated by the parasympathetic nervous system; such muscular activity can be inhibited by the sympathetic nervous system. Absorption of nutrients from chyme occurs mainly in the small intestine; unabsorbed food and secretions and waste substances from the liver pass to the large intestines and are expelled as feces. Water and water-soluble substances travel via the bloodstream from the intestines to the kidneys, which absorb all the constituents of the blood plasma except its proteins. The kidneys return most of the water and salts to the body, while excreting other salts and waste products, along with excess water, as urine.
source: wiki/encarta encyclopedia