糞便の形成と排便

Feces, the end product of digestion, comprises undigested dietary fiber like cellulose, mucus, sloughed-off epithelial cells, and microbes. After 3 to 10 hours in the large intestine, chyme loses considerable water to form this semisolid waste material that the descending and sigmoid colon store before elimination.

The descending and sigmoid colon store feces and use haustral contractions to dry it out while retaining enough water for a semisolid texture. Mass peristalsis then propels the feces into the rectum, where rectal wall distention stimulates stretch receptors that trigger the defecation reflex.

The defecation reflex is triggered when mass peristalsis moves feces into the rectum, distending the rectal walls and stimulating stretch receptors. This involuntary response initiates a coordinated series of neural events involving both the myenteric plexus and parasympathetic motor neurons to facilitate bowel movement.

The myenteric plexus triggers the short reflex to stimulate contractions in the sigmoid colon and rectum. Simultaneously, parasympathetic motor neurons in the sacral region activate the long reflex to enhance mass peristalsis, allowing both reflexes to work together to move feces into the anal canal.

The defecation reflex relaxes the internal anal sphincter involuntarily. The pudendal nerves of the somatic nervous system then inhibit the skeletal muscles of the external anal sphincter, allowing voluntary relaxation and enabling the expulsion of feces from the body during elimination.

Haustral contractions in the descending and sigmoid colon dry out feces while retaining enough water to maintain a semisolid texture. This balance prevents feces from becoming too dry and hard, which would make elimination difficult, while also ensuring adequate water reabsorption occurs.

Chyme spends 3 to 10 hours in the large intestine, during which it undergoes considerable water loss to form feces. This extended transit time allows the colon to absorb water and electrolytes while haustral contractions progressively dry and compact the remaining digestive material.