25.5 Small Intestine

small intestine

the site of nearly all chemical digestion and nutrient absorption and the longest part of the digestive tract

duodenum

first portion of the small intestine; receives stomach contents, pancreatic juice, and bile; neutralizes stomach acid, emulsifies fat, inactivates pepsin, pancreatic enzymes take over digestion

jejunum

second portion of the small intestine; very rich in blood supply and the location where most digestion and nutrient absorption occurs

ileum

last and largest portion of small intestine; compared to previous, it’s thinner, less muscular, less vascular, and paler in color

aggregated lymphoid nodules

prominent clusters on side opposite the mesenteric attachment; visible to the naked eye and become larger near the large intestine

ileal papilla

formed by thickened muscularis and regulates passage of food residue into the large intestine

villi

fingerlike projections that increase the surface area for absorption

microvilli

small plasma membrane extensions that form a fuzzy brush border on the apical border of each enterocyte

brush border enzymes

contained in plasma membrane of microvilli and carry out some of the final stages of enzymatic digestion

contact digestion

chyme must contact the brush border for digestion to occur

duodenal glands

secrete an abundance of bicarbonate-rich mucus; neutralize stomach acid and shield mucosa from its erosive effects

intestinal juice

secreted by intestinal crypts especially in response to acid, hypertonic chyme, and distension of intestine, pH of 7.4-7.8; contains water, mucus, and little bit of enzyme

intestinal contractions

mix chyme with intestinal juice, bile, and pancreatic juice; churns chyme and brings it into contact with mucosa for contact digestion; moves residue towards large intestine