Absorption refers to taking dietary nutrients from the intestinal lumen for transportation throughout the body. After digestion in the small intestine, carbohydrates, proteins, and fats are broken down into simpler forms. These essential macronutrients and other vital substances, such as vitamins, minerals, and water, are then prepared for absorption into the bloodstream.

Enterocytes, which are specialized polar epithelial cells, line the mucosa of the small intestinal walls. These cells possess microvilli, or tiny finger-like projections, at their apical ends, which help to increase the surface area for absorption. Tight junctions near the apical region of the lateral surface of enterocytes 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. Since the plasma membrane of cells is a lipid bilayer, it allows the passive absorption of nonpolar substances. On the other hand, polar substances use active transport processes driven directly or indirectly by energy.

Once the substances enter the interstitial fluid, they diffuse into the blood capillaries.