The digestive system is responsible for processing food, breaking it down into usable proteins, carbohydrates, minerals, fats, and other substances, and introducing these into the bloodstream so that they can be used by the body. The digestive, or alimentary, tract begins at the mouth, where the teeth and tongue begin the breakdown of food, aided by saliva secreted by the salivary glands.
The chewed food, combined with saliva, is swallowed, carrying it in peristaltic (contractile) waves down the esophagus to the stomach. In the stomach, the food combines with hydrochloric acid which further assists in breaking it down. When the food is thoroughly digested, the fluid remaining, called chyme, is passed through the pylorus sphincter to the small intestine and large intestines.
Within the long, convoluted intestinal canals, the nutrients are absorbed from the chyme into the bloodstream, leaving the unusable residue. This residue passes through the colon (where most of the water is absorbed into the bloodstream) and into the rectum where it is stored prior to excretion. This solid waste, called feces, is compacted together and, upon excretion, passes through the anal canal and the anus. Along the way through the digestive tract, the pancreas, spleen, liver, and gallbladder secrete enzymes which aid in the digestive process.