28.13 : Mineral, Vitamin and Water Absorption

Electrolytes are essential minerals and ions primarily obtained from the diet and absorbed through the gastrointestinal tract. Most electrolytes are absorbed in the small intestine. While the absorption of iron and calcium primarily occurs in the duodenum, calcium is also absorbed in the jejunum and ileum. In these regions, passive diffusion contributes to its absorption alongside active transport mechanisms in the duodenum. These ions can exit the enterocytes through specialized active transporters or passive movement along a concentration gradient to enter the interstitial fluid.

When sodium ions enter the enterocytes, the basolateral sodium-potassium pumps actively transport them into the interstitial fluid. Chloride ions typically follow these sodium ions, while potassium ions use facilitated diffusion to cross the intestinal mucosa.

The small intestine absorbs dietary vitamins, while gut bacteria absorb K and B vitamins in the large intestine. Fat-soluble vitamins A, D, E, and K become a part of micelles for absorption with dietary fats. Water-soluble vitamins such as B and C are primarily absorbed through passive diffusion. However, some exceptions exist; for example, vitamin B12 requires an intrinsic factor for absorption in the ileum, utilizing a receptor-mediated endocytosis mechanism rather than passive diffusion.

Lastly, water is absorbed through osmosis, moving from a region of lower solute concentration to a higher solute concentration across the cell membrane.