How is the small intestine adapted for absorption?
The small intestine is adapted for absorption by being long, highly vascularized, and having a large internal surface area.
The small intestine is responsible for completing digestion and absorbing the major organic nutrient monomers: monosaccharides; amino acids; and fatty acids. It achieves this with remarkable efficiency due to three factors: its great length, it large degree of vascularization, and its high internal surface area.
The small intestine is typically 5-7 meters in length, which provides a very long contact time for the partially digested chyme from the stomach.
The internal surface of the intestine, the mucosa, is highly folded into individual, finger-like projections called villi. Each cell of this layer has a highly-folded apical membrane called microvilli, which forms the brush border of the small intestine and creates an enormous surface area to enhance absorption.
Finally, along the entire length of the small intestine is a fine capillary network of blood vessels within the mesentary tissue that feeds blood directly to the liver via the hepatic portal vein.
These structural features all owe to the highly efficient absorptive power of the small intestine.