The large intestine consists of the cecum; ascending, trans-verse, descending, and sigmoid colon; and the rectum (Fig. 91-1). The function of the large intestine is to receive contents from the ileum, absorb water, and package solid waste for excretion. Absorption of materials occurs in segments of the colon proximal to the middle of the transverse colon, with movement and storage of fecal material in the left colon and distal segments of the colon.
Four major tissue layers, from the lumen outward, form the large intestine: the mucosa, submucosa, muscularis externa, and serosa (Fig. 91-2). Complete replacement of surface epithelial cells occurs every 7 to 10 days with the total number of epithelial cells remaining constant in normal colonic tissue. As patients age, abnormal cells accumulate on the surface epithelium and protrude into the stream of fecal matter, their contact with fecal mutagens can lead to further cell mutations and eventual adenoma formation.16
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