Lipid absorption causes a greater increase in intestinal blood flow, a condition known as absorptive hyperemia, and oxygen consumption than either carbohydrate or amino acid absorption. During absorption of all three classes of nutrients, the mucosa releases adenosine and CO2 and oxygen is depleted. The hyperosmotic lymph and venous blood that leave the villus to enter the submucosal tissues around the major resistance vessels are also major contributors to absorptive hyperemia. By an unknown mechanism, hyperosmolality resulting from NaCl induces endothelial cells to release NO and dilate the major resistance arterioles in the submucosa. Hyperosmolality resulting from large organic molecules that do not enter en-dothelial cells does not cause appreciable increases in NO formation, producing much less of an increase in blood flow than equivalent hyperosmolality resulting from NaCl. These observations suggest that NaCl entering the en-dothelial cells is essential to induce NO formation.
The active absorption of amino acids and carbohydrates and the metabolic processing of lipids into chylomicrons by mucosal epithelial cells place a major burden on the mi-crovasculature of the small intestine. There is an extensive network of capillaries just below the villus epithelial cells that contacts these cells. The villus capillaries are unusual in that portions of the cytoplasm are missing, so that the two opposing surfaces of the endothelial cell membranes appear to be fused. These areas of fusion, or closed fenestrae, are thought to facilitate the uptake of absorbed materials by capillaries. In addition, intestinal capillaries have a higher filtration coefficient than other major organ systems, which probably enhances the uptake of water absorbed by the villi (see Chapter 16). However, large molecules, such as plasma proteins, do not easily cross the fenestrated areas because the reflection coefficient for the intestinal vasculature is greater than 0.9, about the same as in skeletal muscle and the heart.
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