Intestinal Function

Abstract

The small intestine is a highly convoluted, 6–8 m long tube and its main functions are absorption and motility (peristalsis). Peristalsis has a number of important roles—it helps mix chyme with digestive enzymes, helps disperse nutrients in the lumen to enable chyme to contact with the epithelium where complete enzymatic digestion and absorption take place, it moves chyme down the digestive tube, making way for the next load, and it eliminates indigestible and perhaps toxic substances. Small intestines absorb almost 90% of the nutrients after a meal. After complete digestion of food components by digestive enzymes in the lumen and on the brush border, enterocytes absorb degradation products—monosaccharides, amino acids and peptides, free fatty acids, and also micronutrients and vitamins. The intestine cells play a critical role in balancing the ratio of body water to acid-base by absorbing and secreting water and electrolytes. Different pathophysiological processes can impair digestion and/or absorption, leading to the malabsorption syndrome. Small intestines are innervated by the autonomic central nervous system but also have their own nervous system called the enteric nervous system. The enteric nervous system is embedded in the wall of the intestines and contains thousands of ganglia, the great majority of which are found in two plexuses—the myenteric (Auerbach’s) plexus and the submucosal (Meissner’s) plexus.