Excretion

Excretion.

     Excretion is the discharge of materials which have not been absorbed by the body. For example, dietary fiber are not absorbed by our body. This only helps to add volume and hydration to the feces, thus facilitating their excretion. When the chyme reaches the colon (large intestine), it is virtually devoid of nutrients. There still is some water and minerals that will be absorbed mostly in the ascending and the transverse colon. The other function of the colon is to propel feces toward the anus where they will be defecated.

     At the end of the ileum, the chyme pours into the colon. At the junction of the two intestines, there is a valve (thickened muscle) that prevents reflux into the small intestine. The presence of chyme and gastrin (gastric hormone) in the terminal part of the ileum facilitates relaxation (opening) of that valve. Conversely, the valve will close when the colon has received enough chyme.

     In the colon, the movement, the muscular contractions are not the same. Unlike the peristalsis of the small intestine, the colon pushes the feces by bolus. The propulsion is initiated by a local contraction preventing proximal reflux, followed by another contraction just distal to the first contraction, and a third, and so on, for a length of 20 cm or more. The fecal material is pushed, "by bolus", to the rectum.

     One hour after the meal, the movements of the colon intensify. Stimulated by the distension of the duodenum, there is a secretion of gastrin (gastric hormones) which increases gut motility and facilitate transit. The autonomic nervous system, particularly the parasympathetic system, also participate in facilitating movements of the colon.

Defecation.

     The fecal content reaches the sigmoid colon where the feces will be retained for a longer or shorter time, depending on the volume and consistency. When fecal matter reaches the rectum, this produces a stretch stimulus which is delivered to the spinal cord where a local reflex is initiated. The reflex stimulates the contraction of the sigmoid colon and the rectum, and allows the relaxation of the internal sphincter which normally remains in state of tonic contraction. At the same time, for defecation to occur, there must also be relaxation of the external sphincter, which is also in tonic contraction, but whose relaxation is under voluntary control. There must be a voluntary command from the brain to induce the relaxation of the external sphincter. Voluntary, we can also strengthen the sphincter contraction. At the same time, voluntary contraction of the abdominal muscles increases intra-abdominal pressure which further pushes and stimulates the sigmoid colon and the rectum, causing expulsion of the feces.