The cellular architecture of the vertebrate gut follows a common plan throughout its length: Four major layers of different cell types form the wall of the gut (Figure 50.11). These layers differ somewhat from compartment to compartment, but they are always present.
Starting in the cavity, or lumen, of the gut, the first tissue layer is the mucosa. Mucosal cells have secretory and absorptive functions. Some secrete mucus, which lubricates and protects the walls of the gut; others secrete digestive enzymes, and still others secrete hormones. Mucosal cells in the stomach secrete hydrochloric acid (HCl). In some regions of the gut, nutrients are absorbed by mucosal cells. The plasma membranes of these absorptive cells have many folds that increase their surface area (see Figure 50.9c)
At the base of the mucosa are some smooth muscle cells, and just outside the mucosa is the second tissue layer, the sub-mucosa. Here we find the blood and lymph vessels that carry absorbed nutrients to the rest of the body. The submucosa also contains a network of nerves; the neurons in this network are both sensory (responsible for stomach aches) and regulatory (controlling the various secretory functions of the gut).
External to the submucosa are two layers of smooth muscle tissue responsible for the movements of the gut. Innermost is
50.11 Tissue Layers of the Vertebrate Gut In all compartments of the gut, the organization of the tissue layers is the same, but specialized adaptations of specific tissues characterize different regions.
Nerve net between muscle layers
Nerve net between muscle layers the circular muscle layer, with its cells oriented around the gut. Outermost is the longitudinal muscle layer, with its cells oriented along the length of the gut. The circular muscles constrict the gut, and the longitudinal muscles shorten it. Between the two layers of smooth muscle is another network of nerves, which controls and coordinates the movements of the gut. The coordinated activity of the two smooth muscle layers moves the gut contents continuously toward the rectum.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.