Mineral, Vitamin and Water Absorption

Essential elements, including iron, calcium, sodium, chloride, and bicarbonate, are primarily consumed as dietary minerals or ions.

While most electrolytes are absorbed throughout the small intestine, iron is absorbed in the duodenum and calcium in the duodenum and jejunum.

All these ions can exit the enterocytes using specialized active transporters or passively along a concentration gradient to enter the interstitial fluid.

For example, after sodium ions enter the enterocytes, the basolateral sodium-potassium pumps actively transport them into the interstitial fluid. Typically, chloride ions passively follow these sodium ions. In contrast, potassium ions use facilitated diffusion to cross the intestinal mucosa.

The small intestine absorbs dietary vitamins, and the large intestine absorbs vitamins K and B from the gut bacteria.

Fat-soluble vitamins A, D, E, and K become part of micelles for absorption with dietary fats. 

Conversely, water-soluble vitamins B and C, except B₁₂, are absorbed via passive diffusion.

Lastly, water is absorbed via osmosis across the cell membrane.