BIOLOGY Volume 1 - A Guide to General Biology - 2004

8. HETEROTROPHIC NUTRITION

8.3. The Human Alimentary Canal

Digestion AND ABSORPTION in humans take place within the alimentary canal, or gastrointestinal tract—or, more simply, the gut—extending from the Mouth to the anus. Because the wall of the alimentary canal is a continuation of the body's outer surface, the food contained within it is, in effect, outside the body. Food can only be assimilated by the Organism after it has been mechanically broken down by the Teeth and the Muscles of the intestinal walls (mechanical digestion), followed by the enzymatic breakdown of its complex macromolecules into sufficiently small, simple molecules (chemical digestion) that can be absorbed by the Cells of the intestinal lining. From here, nutrients enter the Blood or Lymph, which transports them to the cells of various body Tissues, where they are finally assimilated. Undigested food residues are expelled through the anus. At this stage, it is helpful to recall the basic processes that make up Nutrition (Sec. 8.1.1).

The human alimentary tract is divided into specialized regions (segments), each playing a specific role in the overall process of digestion and absorption. These regions and a brief description of their Functions are shown in Fig. 8.11.

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Fig. 8.11. General layout of the Organs of the human alimentary tract and their functions.

8.3.1. General Structure of the Human Alimentary Tract

Each region of the gut possesses certain morphological and physiological characteristics, but all of them share the General structural plan shown in Fig. 8.12. The wall of the alimentary canal consists of four distinct layers: the mucosa, the submucosa, the muscularis, and the serosa.

Fig. 8.12. General structural plan of the alimentary canal (transverse section).

Mucosa (Mucous Membrane)

The mucosa is the innermost layer of the alimentary canal and consists of the epithelial lining, lamina propria, and muscularis mucosae. It is this layer that contains A large number of secretory cells and is the site of absorption.

The cells of the epithelial lining secrete large quantities of mucus, which coats the food and thereby facilitates its passage through the digestive tract. Mucus also prevents the gut wall from being digested by its own Enzymes. Some epithelial cells bear microvilli on their surface, which contain membrane-bound enzymes. Microvilli can be seen under a Light Microscope as a thin layer with a characteristic striation perpendicular to The Cell surface; because of this appearance, the layer is referred to as the brush border. The epithelial cells rest on a basement membrane, beneath which lies the lamina propria. The latter consists of a supporting layer of Connective Tissue containing Blood and Lymphatic vessels. Many Regions of the lamina propria also contain glands formed by epithelial invaginations. External to the lamina propria lies the muscularis mucosae—a thin layer of smooth Muscle. The primary function of this layer is to form folds in the mucosa and submucosa in certain PARTS OF THE digestive tract.

Submucosa

The submucosa consists of connective tissue containing nerve plexuses, blood and Lymphatic vessels, and Collagen and elastic fibers. In the submucosa of the duodenum, there are mucous glands that secrete their products via ducts onto The surface of the mucous membrane.

Muscularis (External Muscle Layer)

The muscularis consists of two layers of smooth muscle: an inner circular layer and an outer longitudinal layer. The activity of smooth muscle is controlled by the Autonomic Nervous system; its movements are involuntary, meaning they are not under conscious control by the Brain. The coordinated contraction of these two layers produces wave-like peristaltic Movements of the gut walls that help propel the food forward. These same movements also ensure the mixing of food (Sec. 8.3.5).

Located between the circular and longitudinal muscle layers is the myenteric plexus (Auerbach's plexus). Auerbach's plexus consists of autonomic nerves that control peristalsis. Impulses traveling along sympathetic nerves lead to the relaxation of the digestive tract muscles and the contraction of sphincters, whereas impulses traveling along parasympathetic nerves stimulate the contraction of the gut walls and the relaxation of sphincters. Situated between the circular muscle layer and the submucosa is another nerve plexus, the submucosal plexus (Meissner's plexus), which controls the secretion of the glands in the wall of the alimentary tract.

In several regions of the alimentary canal, the circular muscle layer thickens to form structures known as sphincters. The relaxation and contraction of sphincters control the movement of the food bolus from one part of the digestive tract to another. Sphincters are located at the junctions between the Esophagus and The Stomach (cardiac sphincter), the stomach and the duodenum (pyloric sphincter), the ileum and the cecum, and surrounding the anus.

Serosa

The serosa forms the outermost layer of the alimentary canal. It consists of loose Fibrous connective tissue.

The outer surface of the alimentary canal throughout its entire length (except for the esophagus) is covered by the Peritoneum. The peritoneum also lines the Abdominal cavity, which houses the main part of the digestive tract, and forms the mesentery, which Supports and suspends the stomach and intestines from the posterior body wall. The mesentery is formed by two layers of peritoneum and contains nerves, blood and lymphatic vessels running to and from the intestine. The surface of the peritoneum is moist, which reduces friction generated when the intestinal loops rub against one another and against other organs.