The gastrointestinal (GI) tract is composed of the alimentary tract, which extends from the mouth to the anus, and includes the associated glandular organs that release their contents into the tract. Its primary function is to absorb nutrients and water into the circulation and eliminate waste products. Motility, secretion, digestion, and absorption are the major physiological processes that occur in the GI tract. Digestion involves the physical and chemical modification of food to enable absorption across intestinal epithelial cells. The movement of the muscular wall of the GI tract is essential for digestion and absorption, as it facilitates the transportation of contents along the tract and the mixing of food with secretions. The secretions from the GI tract and associated organs create an optimal intraluminal environment for digestion and absorption, containing enzymes, biological detergents, and ions. These physiological processes are tightly regulated to maximize digestion and absorption, and the GI tract is equipped with complex regulatory systems to ensure their efficiency. Additionally, the GI tract is responsible for the absorption of orally or rectally administered drugs.

The GI tract also serves as an important organ for excretion, storing and eliminating waste substances from ingested food materials, as well as excreting lipid-soluble molecules such as cholesterol, steroids, and drug metabolites produced by the liver. As the GI tract is in contact with the external environment, it is susceptible to infectious microorganisms that can enter through food and water. To protect itself, the GI tract possesses a complex defense system consisting of immune cells and other nonspecific defense mechanisms, making it the largest immune organ in 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 in different regions but shares common features in its overall organization. It is a hollow tube divided into major functional segments, including the mouth, pharynx, esophagus, stomach, duodenum, jejunum, ileum, colon, rectum, and anus (Fig. 27.1). The small intestine is composed of the duodenum, jejunum, and ileum, while the colon is referred to as the large intestine. The lining of the GI tract is invaginated to form blind-ending glandular structures that secrete their contents into the gut lumen, such as Brunner's glands in the duodenum that produce large amounts of HCO3-. In addition, there are glandular organs connected to the GI tract via ducts, such as the salivary glands and pancreas, which release their secretions into the gut lumen.

The major structures along the GI tract have various functions. Storage is an important function of the stomach and colon, which are specialized organs for processed food (chyme) storage. They possess distinctive functional anatomy, shape, size, and control mechanisms that enable efficient storage. The small intestine primarily functions in digestion and absorption, and its major specialization lies in its large surface area for absorption. The colon reabsorbs water and ions to prevent their elimination from the body. Muscle contractions in the GI tract walls propel ingested food along the tract. Sphincters, specialized muscle structures, separate the different regions of the GI tract, ensuring selective retention of contents and preventing backflow.
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.