Absorption in the Intestine

Absorption in the GI tract entails the movement of nutrients, water, and ions from the intestinal lumen to the blood for delivery to the tissues. The intestines have special anatomical features that increase the surface area for absorption. Circular folds, aka plicae circulares, are permanent ridges of submucosa and mucosa that project into the lumen; they are especially prominent in the duodenum and jejunum and increase the surface area of the small intestine threefold. Villi: The circular folds are covered with villi, which are 1mm projections arising from the mucosa; villi increase the surface area 10-fold. Villi are covered with special epithelial cells, called enterocytes, and goblet cells, which secrete mucus. Microvilli arise from the enterocytes of the villi; the microvilli comprise the brush border and increase the surface area of the intestines 20-fold. The brush border produces digestive enzymes that help break down nutrients in preparation for absorption. Absorption takes place via two pathways: In the transcellular pathway, substances pass through the intestinal cells; this is the most common pathway that we'll see in this tutorial. In the paracellular pathway, substances pass between cells; this is likely reserved for small ions and is beyond the scope of this tutorial. Monosaccharides are the end-products of carbohydrate digestion. First, we draw the GI lumen with glucose, galactose, and fructose; we show a GI epithelial cell, with apical and basolateral sides, and interstitial space and a capillary. Sodium-potassium pumps on the basolateral side of the GI cell creates a concentration gradient for sodium. This gradient drives cotransporters SGLT-1 to move sodium with glucose and galactose across the apical membrane. Fructose enters the cell via facilitated diffusion through GLUT-5 transporters. On the basolateral side, each monosaccharide exits the cell via facilitated diffusion, and makes its way to the bloodstream. We re-draw the GI structures, and show amino acids and dipeptides and tripeptides in the lumen. Single amino acids enter the enterocyte via sodium-linked transporters by facilitated diffusion; they exit via facilitated diffusion. Cotransporters (Peptide Transporter 1) bring hydrogen ions and oligopeptides across the apical membrane. In the enterocyte, peptidases further digest oligopeptides into free amino acids, which diffuse across the basolateral membrane. We re-draw our GI tract and interstitial space, but with a nearby lacteal instead of a capillary. In the GI lumen, show a micelle, which comprises bile salts arranged around monoglycerides. The monoglycerides separate from micelles and cross the apical membrane via simple diffusion (the bile salts are returned to the liver for "recycling"). Within the endoplasmic reticulum of the enterocyte, monoglycerides assemble to form triglycerides; these are released from ER as chylomicrons. Lipoproteins and long-chain fatty acids fuse with chylomicrons, and travel to basolateral surface. At the basolateral surface, the chylomicrons bud off from the cell and enter nearby lacteals; they eventually join the systemic circulation via thoracic duct near the neck. "Fluids" includes not only the fluids we ingest, but also the large volume of fluids we secrete in digestive juices. Water follows sodium (and other ions) via osmosis; nearly 80% of the fluid that reaches the intestines is absorbed in the small intestine; the rest is absorbed in the large intestine. Water-soluble vitamins are transported across the GI wall via vitamin-specific carriers.