The digestive system consists of the set of organs and glands associated with the ingestion and digestion of food and the absorption of nutrients. It may be the system that is most taken for granted. Whether a person is eating foods of little value or a well-balanced diet, the task of the digestive system is the same—to nourish the cells of the body. The basic functions of the digestive system are to ingest, digest, and absorb the nutrients taken in, creating simple organic and inorganic molecules that can be transported to the body’s cells through blood and lymph. This system also eliminates solid, mostly indigestible, wastes from the body through the large intestine.
FIGURE. The digestive system
Often the digestive system is referred to as the upper gastrointestinal (GI) tract and the lower GI tract. The system is composed of the alimentary canal and the accessory organs of digestion. The canal is a tube that passes from the mouth to the anus. The parts of the alimentary canal are noted in Figure and include the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The colon extends from the small intestine to the anus.
Refer to the following information for a brief review of each digestive system organ in the alimentary canal:
- Mouth or oral cavity — In the mouth, the first part of the canal, the teeth, mechanically break down food that is mixed with saliva. The food is pushed about by the tongue; the tongue’s taste buds provide the pleasurable sensation of taste.
- Salivary glands — Three sets of salivary glands in the mouth secrete saliva (mostly water and amylase) in the mouth to moisten food for swallowing and begin digesting carbohydrates.
- Pharynx — The pharynx, or throat, is the portion of the alimentary canal that carries the swallowed food product to the esophagus.
- Esophagus — The food product is carried further through the canal by peristalsis into the stomach. No digestion takes place in either the pharynx or the esophagus. The lower esophageal sphincter (LES), or cardiac sphincter, relaxes to allow food into the stomach; it then contracts, preventing the backup of stomach contents.
- Stomach — The stomach sac holds the food for the digestion process to continue. A number of secretions occur in the stomach to form gastric juice. The secretions include mucus that coats and protects the stomach lining, hydrochloric acid and pepsin to digest proteins, intrinsic factor for the absorption vitamin B12, and the hormone gastrin to stimulate further production of gastric juice. When the food becomes a thick liquid called chyme, the pyloric sphincter relaxes to allow small amounts of chyme to pass into the first portion of the small intestine.
- Small intestine — Digestion is completed in the small intestine, a tube about 20 feet long. The first portion of the small intestine is called the duodenum. Bile secreted by the liver enters the duodenum to emulsify fats. The second portion, the jejunum, continues the digestion process, and absorption begins—of glucose and amino acids into blood capillaries and of fats into lymph capillaries (lacteals). The third portion, the ileum, is the final and longest segment of the small intestine. It is specifically responsible for the absorption of vitamin B12 and the reabsorption of bile salts.
- Large intestine (colon) — Refer to Figure for a diagram of the parts of the large intestine. Any undigested food and water pass into the first part of the large intestine, called the cecum. Peristalsis carries the waste to the ascending, transverse, and descending colon, the sigmoid colon, the rectum, and the anal canal. No digestion takes place in the colon. The colon absorbs water, minerals, and vitamins and eliminates the remaining waste products.
FIGURE. The large intestine shown in anterior view. The term flexure means a turn or bend. (From Scanlon, VC, and Sanders, T: Essentials of Anatomy and Physiology, ed 5. FA Davis, Philadelphia, 2007, p 385, with permission.)
The accessory glands include the pancreas, liver, and gallbladder. The pancreas is both an exocrine (produces digestive enzymes) and an endocrine (produces insulin and glucagon) gland. The liver, the largest organ in the abdominal cavity, produces bile and regulates the level of most chemicals in the blood. It plays a central role in cellular metabolism of the body. The gallbladder concentrates and stores bile that is received from the liver.
The two functions of the digestive system are digestion and absorption. In digestion, food is broken down into such simpler molecules as amino acids, fatty acids, and simple sugars through both mechanical and chemical processes. The mechanical process involves physically breaking down food as it is chewed and swallowed and moved to the stomach, where further chemical action takes place creating chyme. The chemical process occurs when the enzymes act on the digested food to create simpler chemical molecules. This prepares the food for absorption. In absorption, the simpler molecules as well as minerals, vitamins, and water move from the digestive tract across the digestive wall and into the blood to be carried into body tissue.
FIGURE. The liver and gallbladder with blood vessels and bile ducts
The various accessory organs secrete fluids into the digestive tube to help in the digestion and absorption of nutrients. The process of ingesting, digesting, and absorbing nutrients supplies the energy and chemical building blocks for growth and maintenance of the body.
FIGURE. (A) The pancreas, sectioned to show the pancreatic ducts. The main pancreatic duct joins the bile duct. (B) Microscopic section showing acini with their ducts and several islets of Langerhans. (From Scanlon, VC, and Sanders, T: Essentials of Anatomy and Physiology, ed 5. FA Davis, Philadelphia, 2007, p 382, with permission.)