27.37 : Digestive Functions of the Large Intestine

The large intestine is where the final stages of digestion happen. When the cecum receives chyme, it contains undigested carbohydrates that undergo fermentation. Gut bacteria ferment these carbohydrates to produce short-chain fatty acids that provide some energy and help synthesize essential vitamins.

As the chyme moves to the colon, it triggers two characteristic sluggish contractions - haustral churning and mass peristalsis. Haustral churning involves the rhythmic contraction and relaxation of smooth muscles in individual haustra. In the ascending and transverse colon, the walls contract and squeeze the contents into the next haustrum as the chyme distends a haustrum to capacity. These segmented minute-long movements occur every 30 minutes and allow the mixing for absorption of vitamins, water, and electrolytes, such as sodium and chloride. Additionally, peristalsis occurs at a slower rate of 3-12 contractions per minute to assist haustral churning.

Lastly, wave-like muscular contractions around the middle of the transverse colon, called mass peristalsis, push residual contents over large areas 3 or 4 times daily toward the rectum.