Digestion in the Large Intestine and Neurohormonal Regulation

The mucosa of the large intestine secretes mucus but no digestive enzymes of its own. Some carbohydrates and proteins that escape digestion in the small intestine do enter the large intestine, where they are broken down by bacteria living there. Bacterial fermentation of leftover carbohydrates produces hydrogen, carbon dioxide and methane gases in the colon, while bacterial breakdown of leftover proteins produces substances such as indole, skatole and hydrogen sulphide, which account for the characteristic odour of faeces. These same bacteria also synthesise several vitamins, including B-vitamins and vitamin K, which the body can absorb.

Digestive secretions are not released continuously but in a coordinated sequence, timed appropriately by neural and hormonal (neurohormonal) control. The sight, smell or even the thought of food can trigger salivary secretion through a neural reflex. The tenth cranial nerve (vagus) and the hormone gastrin (secreted by the stomach) stimulate gastric juice secretion. As acidic chyme enters the duodenum, the intestinal mucosa releases three hormones: secretin, which inhibits further gastric-juice secretion while stimulating the secretion of bile, pancreatic juice and intestinal juice; cholecystokinin (CCK), which brings about a similar stimulating effect and also induces satiety (the …