Porth's Essentials of Pathophysiology, 4e

678

Gastrointestinal and Hepatobiliary Function

U N I T 8

Mesentery

Muscularis externa

Epithelium Lamina propria Muscularis mucosa

Longitudinal muscle

Circular muscle

Mucosa

Serosa (mesothelium)

Serosa (connective tissue)

FIGURE 28-3. Transverse section of the gastrointestinal system.

Submucosa

mucosa, composed of smooth muscle cells that can contract and change the shape and surface area of the mucosal layer. The mucosal layer performs numerous functions in its role as an interface between the body and the external environment, including production of the mucus that protects and lubricates the inner lining of the GI tract lumen, secretion of the digestive enzymes and substances that break food down, absorption of the breakdown products of digestion, and maintenance of a barrier to prevent the entry of noxious substances and pathogenic organisms. Lymphatics within the mucosa serve as the body’s first line of immune defense. The epi- thelial cells in the mucosal layer have a rapid turnover rate and are replaced every 4 to 5 days. Because of the regenerative capabilities of the mucosal layer, injury to this layer heals rapidly without leaving scar tissue. Submucosa The submucosal layer, or second layer, consists of dense connective tissue, aggregates of adipose tissue, and occasional glands. This layer contains the large blood vessels that send branches to the mucosa, muscular externa, and adventitia. It also contains the lymphatic vessels, as well as nerves that control motility and the secretory activity of glands in the mucosal layer. Muscularis Externa The muscularis externa consists of two concentric and relatively thick layers of smooth muscle: an inner layer made up of circularly arranged smooth muscle cells and an outer layer of longitudinally arranged smooth muscle. Located between the two muscle layers is a con- nective tissue layer that contains nerves that control smooth muscle movement, as well as blood and lym- phatic vessels. Contraction of the smooth muscle in this

layer mixes and churns the GI contents and facilitates its movement along the GI tract.

Serosa and Adventitia The serosa and adventitia constitute the outermost layer of the GI tract. The serosa is a serous membrane consist- ing of a layer of simple squamous epithelium, called the mesothelium , and a small amount of underlying con- nective tissue. It is equivalent to the visceral peritoneum and is continuous with the mesentery and omentum that enclose and support the abdominal viscera. Instead of the serosa, an adventitia consisting only of connective tissue is found where the wall of the GI tract is directly attached to the body wall. The peritoneum is a continuous transparent serous membrane that lines the abdominopelvic cavity and invests the stomach and intestines. It is the largest serous membrane in the body, having a surface area approxi- mately equal to that of the skin. The peritoneum con- sists of two continuous layers: the visceral peritoneum and the parietal peritoneum , which line the wall of the abdominopelvic cavity. Between the two layers is the peritoneal cavity , a potential space containing fluid secreted by the serous membranes. This serous fluid forms a moist and slippery surface that prevents friction between the continuously moving abdominal structures. The mesentery is the double layer of peritoneum that encloses a portion or all of one of the abdominal vis- cera and attaches it to the abdominal wall (Fig. 28-4A). The mesentery contains the blood vessels, nerves, and lymphatic vessels that supply the intestinal wall (Fig. 28-4B). It also holds the organs in place and stores fat. There are dorsal as well as ventral mesenteries; how- ever, in most places the mesentery is dorsal and attaches to the posterior abdominal wall. The mesentery that encloses the jejunum and ileum is gathered in folds that