Function and Design of the Digestive System

Every body cell needs a constant supply of nutrients. The energy contained in these nutrients is used to do cell work. In addition, the cell rearranges the chemical building blocks of the nutrients to manufacture cellular materials for metabolism, growth, and repair. Food as we take it in, however, is too large to enter the cells. It must first be broken down into particles small enough to pass through the cells’ plasma membrane. This breakdown process is known as digestion. After digestion, the circulation must carry nutrients to the cells in every part of the body. The transfer of nutrients into the circulation is called absorption. Finally, undigested waste material must be eliminated from the body. Digestion, absorption, and elimination are the three chief functions of the digestive system. For our purposes, the digestive system may be divided into two groups of organs:
* The digestive tract, a continuous passageway beginning at the mouth, where food is taken in, and terminating at the anus, where the solid waste products of digestion are expelled from the body.
* The accessory organs, which are necessary for the digestive process but are not a direct part of the digestive tract. They release substances into the digestive tract through ducts. These organs are the salivary glands, liver, gallbladder, and pancreas.
     Before describing the individual organs of the digestive tract, we will pause to discuss the general structure of these organs. We will also describe the large membrane (peritoneum) that lines the abdominopelvic cavity, which contains most of the digestive organs.

The Wall of the Digestive Tract
Although modified for specific tasks in different organs, the wall of the digestive tract, from the esophagus to the anus, is similar in structure throughout. The general pattern consists of four layers:
* Mucous membrane
* Submucosa
* Smooth muscle
* Serous membrane
     Refer to the diagram of the small intestine in Figure 15-1 as we describe the layers of this wall from the innermost to the outermost surface. First is the mucous membrane, or mucosa, so called because its epithelial layer contains many mucus-secreting cells. From the mouth through the esophagus, and also in the anus, the epithelium consists of multiple layers of squamous (flat) cells, which help to protect deeper tissues. Throughout the remainder of the digestive tract, the type of epithelium in the mucosa is simple columnar.
     Many of the cells that secrete digestive juices are located in the mucosa. Figure 15-2 is a microscopic view of a representative section of the digestive tract taken from the small intestine. Mucus-secreting cells (goblet cells) appear as clear areas between epithelial cells. Note that the small intestine’s lining has fingerlike extensions (villi) that aid in the absorption of nutrients, as will be described later. The layer of connective tissue beneath the mucosa is the submucosa, which contains blood vessels and some of the nerves that help regulate digestive activity. In the small intestine, the submucosa has many glands that produce mucus to protect that organ from the highly acidic material it receives from the stomach. The next layer is composed of smooth muscle. Most of the digestive organs have two layers of smooth muscle: an inner layer of circular fibers, and an outer layer of longitudinal fibers. When a section of the circular muscle contracts, the lumen of the organ narrows; when the longitudinal muscle contracts, a section of the wall shortens and the lumen becomes wider. These alternating muscular contractions create the wavelike movement, called peristalsis, that propels food through the digestive tract and mixes it with digestive juices.

Figure 15-1 Wall of the digestive tract. The mucous membrane of the small intestine shown here has numerous projections called villi.

Figure 15-2 Microscopic view of small intestine. The layers of the intestinal wall are visible (except for the serous membrane).

The esophagus differs slightly from this pattern in having striated muscle in its upper portion, and the stomach has an additional third layer of smooth muscle in its wall to add strength for churning food. The digestive organs in the abdominopelvic cavity have an outermost layer of serous membrane, or serosa, a thin, moist tissue composed of simple squamous epithelium and loose connective tissue.

This membrane forms part of the peritoneum. The esophagus above the diaphragm has instead an outer layer composed of fibrous connective tissue.

Figure 15-3 The abdominopelvic cavity. Subdivisions of the peritoneum fold over, supporting and separating individual organs.

The Peritoneum
The abdominopelvic cavity (Fig. 15-3) is lined with a thin, shiny serous membrane that also folds back to cover most of the organs contained within the cavity. The outer portion of this membrane, the layer that lines the cavity, is called the parietal peritoneum; that covering the organs is called the visceral peritoneum. This slippery membrane allows the organs to slide over each other as they function. The peritoneum also carries blood vessels, lymphatic vessels, and nerves. In some places, it supports the organs and binds them to each other. Subdivisions of the peritoneum around the various organs have special names.

Subdivisions of the Peritoneum The mesentery is a double-layered portion of the peritoneum shaped somewhat like a fan. The handle portion is attached to the posterior abdominal wall, and the expanded long edge is attached to the small intestine. Between the two membranous layers of the mesentery are the vessels and nerves that supply the intestine. The section of the peritoneum that extends from the colon to the posterior abdominal wall is the mesocolon. A large double layer of the peritoneum containing much fat hangs like an apron over the front of the intestine. This greater omentum extends from the lower border of the stomach into the pelvic part of the abdomen and then loops back up to the transverse colon. A smaller membrane, called the lesser omentum, extends between the stomach and the liver.