Nutrient Absorption

Absorption is the transfer of the products of digestion together with minerals, micronutrients, and water from the gut lumen into the blood. The mucosa of the small intestine is adapted structurally to optimize nutrient absorption and enterocytes possess specific transport mechanisms to facilitate transport. The mechanisms responsible for the absorption of lipid molecules are significantly different from those governing the absorption of other nutrients.

Sodium absorption is central to the absorption of the majority of nutrients and is therefore considered first. Indeed, the interdependence of sodium, water, and nutrients, particularly sugars, is an important element in nourishment under pathological conditions, for instance, in considering the rationale for rehydration therapy. The enterocyte membrane possesses carrier proteins for the transport of specific substrates. A specific sodium transporter exists on the enterocyte basolateral membrane exporting sodium from the cell by an energy-dependent process. The effect of the sodium transporter is to maintain the low intracellular sodium concentration.

Different processes permit the passage of the sodium ion to pass from the lumen into the enter-ocyte across the mucosal membrane. Sodium may pass by passive diffusion down its concentration gradient or be transported by a (second) luminal transporter in association with nutrients such as glucose and amino acids. Also, some sodium may diffuse between the cells directly into the interstitial spaces. Water molecules follow sodium ions across the membranes.

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