3) Ileum
About ½ m long. Absorption of the majority of nutrients
The wall of the small intestine is composed of the same 4 layers that make up most of the GI tract: mucosa, submucosa, muscularis, and serosa. The epithelial layer of the small intestinal mucosa contains many types of cells. Absorptive Cells digest and absorb nutrients Goblet cells secrete mucus. The small intestine mucosa also contains deep crevices lined with glandular epithelium. Cells line the crevices and form Intestinal Glands that secrete Intestinal Juice . The intestinal glands contain: Paneth cells which secrete lysozyme , a bactericidal enzyme, and are capable of phagocytosis. Paneth cells may also have a role in regulating the microbial population in the small intestine. 3 types of enteroendocrine : S cells, CCK cells, and K cells The lamina propria of the small intestine mucosa has an abundance of MALT . Lymphatic nodules are most numerous in the distal part of the ileum Peyer’s Patches Submucosa of the duodenum contains Duodenal (Brunner’s) Glands which secrete an alkaline mucus that helps neutralize gastric acid in the chyme. Structural features include Circular Folds, Villi, and Microvilli. (~ 200 million per square millimeter of the small intestine). Microvilli form a fuzzy line called the Brush Border . Present in the villi are lacteals which are required for the absorption of lipids.
Role of Intestinal Juice and Brush-Border Enzymes
intestinal juice (a clear yellow fluid) contains water and mucus and is slightly alkaline (pH 7.6) Together pancreatic and intestinal juices provide a liquid medium aiding absorption of substances from chyme. Absorptive cells synthesize several digestive enzymes called Brush-Border Enzymes and insert them into the plasma membrane of the microvilli. Brush-Border Enzymes include: 4 carbohydrate-digesting enzymes called α – Dextrinase, Maltase, sucrase, and Lactase; protein-digesting enzymes called Peptidases; and 2 nucleotide-digesting enzymes. As absorptive cells slough off into the lumen of the small intestine, they break apart and release enzymes that help digest nutrients in chyme.
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