Absorption of Nutrients

During digestion in the small intestine, macromolecules— carbohydrates, proteins, and fats — are broken down into their simplest forms.

These simplified macronutrients and other essential substances like vitamins, minerals, and water, are prepared for absorption into the blood.

The mucosa of the small intestinal walls is lined with enterocytes — specialized polar epithelial cells.

These cells have microvilli, tiny finger-like projections at their apical ends to increase the surface area for absorption.

Additionally, tight junctions at the apical surface prevent substances from passing between the enterocytes.

As a result, substances enter an enterocyte through its apical membrane from the lumen and exit through the basolateral membrane into the interstitial fluid on the other side of the cell.

Because the plasma membrane of cells is a lipid bilayer, it allows the passive absorption of nonpolar substances. In contrast, polar substances use active transport processes driven directly or indirectly by energy.

Once in the interstitial fluid, these substances enter the blood capillaries.