Movement and Elimination of Wastes
In order to move the residue from one area of the colon to another, the haustra contracts. These contractions largely occur in the cecum and the ascending colon. A number of stimuli can cause the haustra to contract. These stimuli can cause neurotransmitters and hormones to be released. These hormones and neurotransmitters can influence when the haustra contracts, how often the contractions occur, and how strong the contractions are. For instance, a haustra will contract if it becomes filled with residue. However, when chyme fills into the stomach or duodenum, reflexes often signal the haustra to contract more strongly than they normally would. These contractions result in a mass movement that moves the residue farther than regular contractions would. Reflexes of the neurons can also inhibit the haustra. Neurotransmitters are tiny particles that carry messages from neuron to neuron by going across the space, or synapse, which separates them. Therefore, the neurons and the muscles work together to assure that residue moves throughout the large intestine and is ultimately eliminated as feces.
In order to move the residue from the ascending colon to the transverse colon, the muscles in the haustra squeeze together in one area so that the tube becomes more narrow. The constricted muscle resembles a bottleneck or the neck of an hourglass. This pushes the residue upward. Generally, these contractions will start in a lower area, reach their maximum intensity within 30 seconds, and then taper off. Then they will start again in another part of the ascending colon that is closer to the transverse colon. The contractions usually occur every thirty minutes and push the residue into the transverse colon within 10 to 15 hours.
In the large intestine, most of the electrolytes and fluids in the chyme are absorbed into the body. This process largely occurs in the transverse portion of the colon. When the haustra contracts, they cause kneading motions in the transverse colon. These kneading motions mix the chyme and increase the contact of chyme with the mucosa of the transverse colon. As a result of this contaction, water and electrolytes are absorbed into the colon and the residue becomes semisolid.
Oftentimes, stronger contractions, such as those that occur in a mass movement, will occur when the residue is in the transverse colon, the descending colon, or the sigmoid colon. These contractions may occur one to three times per day, last about fifteen minutes, and push the residue through the colon faster.
This process creates waste matter called feces, which is forced into the rectum by contractions that occur in the transverse colon and the descending colon. Each segment of the haustra in these areas of the colon contracts, which forces the feces toward the sigmoid colon. The contractions become more intense for 30 seconds. They subside for 2 minutes because the haustra relaxes, and then the cycle starts again
Eventually, the feces reach the sigmoid colon. There is a high-pressure area where the sigmoid colon meets the rectum that acts like a sphincter muscle to prevent too much feces from arriving into the rectum at once. This sphincter can either inhibit feces from going into the rectum or allow feces to enter the rectum. If the sphincter is inhibited, more feces will enter the rectum. However, if the sphincter is stimulated, feces will not be allowed to enter the rectum. Thus, the sphincter acts like a bottleneck and prevents too much feces from going into the rectum at once. If the rectum contracts, it causes a reflex that prevents feces from going into it.Last Updated: Saturday, July 16, 2011