Larg Intestine

Large Intestine
The large intestine or bowel, which absorbs water and electrolytes and forms indigestible material into feces, has the following regions: the short cecum, with the ileocecal valve and the appendix; the ascending, transverse, descending, and sigmoid colon; and the rectum, where feces is stored prior to evacuation .The mucosa lacks villi and except in the rectum has no major folds. Less than one-third as long as the small intestine, the large intestine has a greater diameter (6-7 cm). The wall of the colon is puckered into a series of large sacs called haustra (L. sing. haustrum, bucket, scoop). The mucosa of the large bowel is penetrated throughout its length by tubular intestinal glands. These and the intestinal lumen are lined by goblet and absorptive cells, with a small number of enteroendocrine cells.
The columnar absorptive cells or colonocytes have irregular microvilli and dilated intercellular spaces indicating active fluid absorption . Goblet cells producing lubricating mucus become more numerous along the length of the colon and in the rectum.
Epithelial stem cells are located in the bottom third of each gland.
The lamina propria is rich in lymphoid cells and in lymphoid nodules that frequently extend into the submucosa . The richness in MALT is related to the large bacterial population of the large intestine. The appendix has little or no absorptive function but is a significant component of MALT . The muscularis of the colon has longitudinal and circular layers but differs from that of the small intestine, with fibers of the outer layer gathered in three separate longitudinal bands called teniae coli . Intraperitoneal portions of the colon are covered by serosa, which is characterized by small, pendulous protuberances of adipose.
Dr.Hameda abdulmahdi College of Medicine /Dep. of anatomy &
histology 2nd stage
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The distal end of the GI tract is the anal canal, 3-4 cm long. At the rectoanal junction the simple columnar mucosal lining of the rectum is replaced by stratified squamous epithelium . The mucosa and submucosa of the anal canal form several longitudinal folds, the anal columns , in which the lamina propria and submucosa include sinuses of the rectal venous plexus. Near the anus the circular layer of the rectum’s muscularis forms the internal anal sphincter. Defecation involves the action of voluntary muscle comprising the external anal sphincter .
The colon
The colon is the longest part of the large intestine. It contains an ascending colon, transverse colon, descending colon, and sigmoid colon. The colon receives digestive contents from the small intestine and absorbs a large volume of water and electrolytes from the contents. Bacteria in the colon make large quantities of vitamins K and B12, but absorption is limited. The colon also forms and stores feces, the waste matter leftover after the digestion process has been completed. The movement of the contents in the large intestine is slower than in the small intestine, taking 8 to 15 hours to move the chyme (thick, semifluid mass) from the cecum to the rectum, where feces are stored. The mucosa of the colon has a smooth surface (no villi) and contains glands of Lieberkuhn. The submucosa contains blood vessels, lymphatic vessels, and nerve fibers as well as submucosal plexuses, but no glands are present. The muscularis externa includes the circular muscle .The longitudinal muscle is aggregated into three bands called teniae coli . The myenteric (Auerbach) plexuses are located between these two muscle layers.