Q. Write Short Notes on the Functional Adaptations of the Small Intestines [GI. Phys.]
The Stomach as a Poor Absorptive Area of the Gastrointestinal Tract
Along the length of the GI tract, the stomach is a poor absorptive area because it lacks the typical villus type of absorptive membrane and because the junctions between the epithelial cells are tight junctions. Only a few highly lipid-soluble substances, such as alcohol and some drugs like aspirin can be absorbed in small quantities.
The small intestine is remarkably well adapted for its primary role in absorption
All products of carbohydrate, protein, and fat digestion, as well as most of the ingested electrolytes, vitamins, and water, are normally absorbed by the small intestine indiscriminately.
Most absorption occurs in the duodenum and jejunum; very little occurs in the ileum, and not because the ileum does not have absorptive capacity but because most absorption has already been accomplished before the intestinal contents reach the ileum.
Functional Adaptations that Increase the Small Intestine’s Surface Area
Folds of Kerckring, Villi, and Microvilli Increase the Mucosal Absorptive Area by Nearly 1000-Fold
The following special modifications of the small intestine mucosa greatly increase the surface are available for absorption
- Presence of Folds of Kerckring: The inner surface of the small intestine is thrown into circular folds that are visible to the naked eye and increase the surface area threefold. In other words, the absorptive surface of the small intestinal mucosa shows many folds called valvulae conniventes (or fold of Kerckring), which increase the surface area of the absorptive mucosa about threefold, as stated earlier. These folds extend circularly most of the way around the intestine and are especially well developed in the duodenum and jejunum, where they often protrude up to 8 millimeters into the lumen.
- Presence of Villi: Projecting from this folded surface are microscopic finger-like projections known as villi, which give the lining a velvety appearance and increase the surface area by another 10 times. The surface of each villus is covered by epithelial cells interspersed occasionally with mucus cells. The villi lie lose to one another in the upper small intestine that they touch in most areas, but their distribution is less profuse in the distal small intestine.
- Presence of Microvilli: Even smaller hair-like projections known as microvilli (or the brush border) arise from the luminal surface of these epithelial cells, increasing the surface area another 20-fold. Each epithelial cell has as many as 3,000 to 6,000 of these microvilli, which are visible only with an electron microscope. It is within the membrane of this brush border that the small intestine enzymes perform their functions. Actually, each intestinal epithelial cell on each villus is characterized by a brush border, consisting of as many as 1000 microvilli 1 micrometer in length and 0.1 micrometer in diameter protruding into the intestinal chyme.
Altogether, the fold, villi, and microvilli provide the small intestine with a luminal surface area about 1,000 times greater than it would have been it were a tube of the same length and diameter lined by a flat surface. In fact, if the surface area of the small intestine were spread out flat, it would cover an entire tennis court.
Extending from the epithelial cell body into each microvillus of the brush border are multiple actin filaments that contract rhythmically to cause continual movements of the microvilli, keeping them constantly exposed to new quantities of intestinal fluid.
Malabsorption (impairment of absorption) may be caused by damage to or reduction of the surface area of the small intestine. One of the most common causes is gluten enteropathy. In this condition, the person’s small intestine is abnormally sensitive to gluten, a protein constituent of wheat and some other grains. Through an unknown mechanism, exposure to gluten damages the intestinal villi: the normally luxuriant array of villi is reduced, the mucosa becomes flattened, and the brush border becomes short and stubby. Because this loss of villi decreases the surface area available for absorption, absorption of all nutrients is impaired. The condition is treated by eliminating gluten from diet.