The digestive system begins at the mouth. Food is chewed to facilitate swallowing. Saliva lubricates the food and the enzymes in the saliva initiates digestion of carbohydrates. Chewing also allows for the appreciation of taste, hopefully a pleasant one. Swallowing involves a complex and coordinated series of muscular contraction and relaxation by the muscles of the tongue, pharynx, larynx and palate whereby food and drink is prevented from entering the airway. Once it passes into the pharynx, it becomes an involuntary process and the food or liquid bolus is propelled into the esophagus. Strokes and diseases of the larynx or pharynx may impair swallowing leading to choking and aspiration.

This is a muscular and straight tube leading into the stomach, running inside the chest. Muscular rings, called sphincters, act as valves in the upper and lower esophagus. Inflammation, ulcers and spasms will cause pain or heartburn and these may often be confused with symptoms from heart disease. If there is a narrowing, food and even liquids can get stuck causing pain and vomiting. If the lower sphincter is weak, then acid-reflux will cause inflammation.

The initial role of the stomach on receiving food is that of storage, and then preparing it for the digestive process later. The high acid levels found in the gastric secretion in an empty stomach serve to keep a sterile environment and to pave the way for activation of pepsin, an enzyme for protein digestion. Acid is produced in the upper two thirds of the stomach. The lower third of the stomach acts as a grinder, further pulverizing the food for digestion in the small intestines. The pylorus is the opening leading out of the stomach into the duodenum (start of the small intestine). It is able to regulate the amount and speed of food leaving the stomach depending on the type of the food (solids slower than liquids) and its composition (fat slower than protein or carbohydrates).

There are almost 20 feet of small intestines on average and it is divided into 3 sections - duodenum, jejunum, and ileum. The important functions of the small intestine are those of digestion and absorption of food and liquids. Enzymes produced in the intestinal lining, together with those by the pancreas, mix with the food from the stomach and digestion takes place. During digestion, carbohydrates, protein and fats are "broken down" into their simpler constituents, which are then absorbed through the intestinal lining and into the blood stream and lymphatic system. Water, electrolytes (e.g. sodium, potassium, chloride, bicarbonate etc.), vitamins and minerals are also absorbed. A rich blood circulation is essential for its proper function. Without an adequate length of small intestine, a person will not be able to survive. It continues into the colon.

This begins as the cecum, which is located in the lower right part of the abdomen, and where the appendix can be found attached to its base. The total length of the colon is approximately 3 ~ 5 feet. The colon is named according to its position e.g. ascending, transverse, and descending colon, and then sigmoid colon before it finally ends in the rectum and anus. The main function of the colon is to give form to the stools, and to regulate the absorbtion of water and electrolytes. It is natural to find bacteria populating the colon and, this is largely responsible for the intestinal gas a person produces. When stools reach the sigmoid colon and rectum, the urge to defecate becomes established causing one to open the bowels. Receptors in the rectum can distinguish between solid, liquid and gas, and the appropriate response for the individual can thus be made. Anal sphincters prevent accidents e.g. soiling or incontinence.

The liver is part of the digestive system having had similar origins from the embryonic tissue that formed the digestive tract. It is almost pyramidal or wedge shaped and lies in the upper right side of the abdomen, protected by the lower rib cage. It has a smooth capsule on the surface and is normally soft in consistency. The liver is responsible for all the major metabolic and homeostatic processes that sustain life. It processes all the nutrients derived from digestion, stores glycogen as an energy source, and, degrades substances that are to be deactivated or detoxified and removed from the body. Its ability to remove bilirubin and manufacture proteins and clotting factors are commonly used as a measure of its functional level. Although it has great regenerative abilities, chronic damage may result in scar tissue and hardening of the liver. This is called cirrhosis, and when this is severe enough the liver loses its function.

An integral part of the liver is the biliary system, a system for draining the complex liquid called bile that is actually manufactured by the liver cells. The bile ducts(biliary tract) and gallbladder make up this system, and it drains into the duodenum. Bile carries a large number of metabolized substances, cholesterol, bilirubin, and bile salts. Bile is necessary for "packaging" digested fat so that it can be absorbed. When crystals form in bile, they may coalesce to form stones. This happens most often in the gallbladder.

This organ occupies the C-shaped space made by the duodenum centrally in the upper abdomen, continuing behind the stomach and to the spleen on the left. It is situated deep inside and lies across the spine. Its shape resembles a tongue. Digestive enzymes for fat, protein and carbohydrates are produced in the pancreas, together with bicarbonate. These are secreted along the pancreatic duct into the duodenum for digestion. The bile duct runs through the head of the pancreas and joins the pancreatic duct just before opening into the duodenum. The pancreas also produces hormones such as insulin and glucagon. Malabsorption, diarrhoea and malnutrition, and later diabetes characterize pancreatic insufficiency.