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The Human Body:
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Large Intestine
The large intestine is about 1.5 m long. It consist of the caecum, appendix, colon, rectum & anal canal. Its main functions are to absorb some of the nutrients and most of the fluids passed into it from the small intestine. The mobility of the unwanted materials is carried out. It also secretes mucus and ions like K+ is secreted from ileal of a concentration of 9 mEq/L to 75 mEq/L at end of large intestine. But at the apical regions Cl- is secreted by crypt cells.The only digestion is by bacteria (eg. to make vitamin K). Absorption of water and ions in particular are also carried out. The maximum absorption of water is about 2-3 L/day. Most digestion and absorption has been accomplished in the small intestine.
The colon receives indigestible food, unabsorbed biliary components, and the remaining fluid. It extracts water and salt and eliminates the feces. Contractions of the haustrae slowly shuffle the colonic contents back and forth. They are initiated by the autonomous rhythmic contractions of the smooth muscle in the wall of the large intestine. Mass movements propel colonic contents long distances. They drive the feces into the distal part of the large intestine. Material is stored here until eliminated by defecation. This is brought about by the Defecation reflex. Firstly, there is a distension of the rectum, stimulating stretch receptors. There is a short reflex and a long reflex. In the short reflex, the myenteric plexus in the sigmoid colon and rectum are stimulated. That brings about increased peristalsis. In the long reflex, the parasympathetic motor neurons in the sacral spinal cord are stimulated and in turn, stimulate the somatic motor neurons. It brings about 3 actions. The first action, it also increases the peristalsis in the large intestine. The second action is it inhibits the constriction of the internal anal sphincter and allows the feces to move into the anorectal canal. The last action is that it contracts the external anal sphincter. Muscle contractions and the relaxation of two sphincter muscles then eliminate the feces. Constipation occurs when the feces become too dry. There is prolonged distention of the large intestine. The large-intestine secretion is entirely protective. It consists of an alkaline mucus solution which protects the intestinal mucosa.
Intestinal gases are absorbed or expelled.
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