he gastrointestinal (GI) tract consists of the alimentary tract from the mouth to the anus and includes the associated glandular organs that empty their contents into the tract. The overall function of the GI tract is to absorb nutrients and water into the circulation and eliminate waste products. The major physiological processes that occur in the GI tract are motility, secretion, digestion, and absorption. Most of the nutrients in the diet of mammals are taken in as solids and as macromolecules that are not readily transported across cell membranes to enter the circulation. Thus digestion consists of physical and chemical modification of food such that absorption can occur across intestinal epithelial cells. Digestion and absorption require motility of the muscular wall of the GI tract to move the contents along the tract and to mix the food with secretions. Secretions from the GI tract and associated organs consist of enzymes, biological detergents, and ions that provide an intraluminal environment optimized for digestion and absorption. These physiological processes are highly regulated to maximize digestion and absorption, and the GI tract is endowed with complex regulatory systems to ensure this occurs. In addition the GI tract absorbs drugs administered by the oral or rectal routes.
The GI tract also serves as an important organ for excretion of substances. It stores and excretes waste substances from ingested food materials and excretes products from the liver such as cholesterol, steroids, and drug metabolites (all sharing the common property of being lipid-soluble molecules).
When considering the physiology of the GI tract, it is important to remember that it is a long tube that is in contact with the body’s external environment. As such, it is vulnerable to infectious microorganisms that can enter along with food and water. To protect itself the GI tract possesses a complex system of defenses consisting of immune cells and other nonspecific defense mechanisms. In fact the GI tract represents the largest immune organ of the body. This chapter provides an overview of the functional anatomy and general principles of regulation in the GI system.
The structure of the GI tract varies greatly from region to region, but there are common features in the overall organization of the tissue. Essentially the GI tract is a hollow tube divided into major functional segments; the major structures along the tube are the mouth, pharynx, esophagus, stomach, duodenum, jejunum, ileum, colon, rectum, and anus (
Fig. 27.1 ). Together the duodenum, jejunum, and ileum make up the small intestine, and the colon is sometimes referred to as the large intestine. Associated with the tube are blind-ending glandular structures that are invaginations of the lining of the tube; these glands empty their secretions into the gut lumen (e.g., Brunner’s glands in the duodenum, which secrete copious amounts of HCO3 −). Additionally there are glandular organs attached to the tube via ducts through which secretions empty into the gut lumen—for example, the salivary glands and pancreas.
The major structures along the GI tract have many functions. One important function is storage; the stomach and colon are important storage organs for processed food (also referred to as chyme) and exhibit specialization in terms of both their functional anatomy (e.g., shape and size) and control mechanisms (characteristics of smooth muscle to produce tonic contractions) that enable them to perform this function efficiently. The predominant function of the small intestine is digestion and absorption; the major specialization of this region of the GI tract is a large surface area over which absorption can occur. The colon reabsorbs water and ions to ensure they do not get eliminated from the body. Ingested food is moved along the GI tract by the action of muscle in its walls. Separating the regions  of the GI tract are also specialized muscle structures called sphincters. These function to isolate one region from the next and provide selective retention of contents or prevent backflow, or both.