The Digestive Tube (Alimentary Canal) in Mammals Help (page 3)

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By — McGraw-Hill Professional
Updated on Aug 30, 2011

The Colon: Our Large Intestine

The last major section of the digestive tube is the colon ( KOH -lun) or “large intestine.” The colon (large intestine) is a wide-diameter, folded tube, about 6 feet (2 meters) in length in an average-sized human adult (see Figure 19.5). The colon begins with the cecum ( SEE -kum), a “blind” ( cec ) or dead-ended pouch that has the vermiform ( VER -mih-form) appendix or “worm-like attachment” hooked to its base. The vermiform appendix is basically a solid attachment of modified lymphatic tissue  that plays a minor role in the body’s immune or self-defense system.

Liquid chyme from the ileum of the small intestine pushes through the ileocecal ( il -ee-oh- SEE -kul) sphincter. Once within the cecum, the chyme begins to undergo an extensive drying out process, wherein large amounts of water and salt (H 2 O) are absorbed. In addition, there are beneficial bacteria in the colon that produce a variety of B-vitamins, as well as sulfur-containing amino acids, which are also absorbed.

Due to this drying out process, chyme is solidified into feces ( FEE -seez) within the colon. Besides H 2 O, feces also contain a significant percentage of fecal ( FEE -kal) bacteria and dietary fiber (actually undigested cellulose material).

Looking at Figure 19.5, you might well ask yourself, “How does chyme/feces in the cecum, which is way down at the bottom of the large intestine, move up to the other parts of the colon?” Good question! The answer is, by mass peristalsis (pear-ih- STAHL -sis). Peristalsis literally means “a constriction” (-stalsis) “around” (peri-). Formally defined, then, peristalsis is a constriction (narrowing) around a particular point of some tube, due to the contraction of a circular ring of smooth muscle around the tube. Mass peristalsis is thus the simultaneous constriction or narrowing of a large number of directly neighboring points along the large intestine wall, such that the heavy mass of the feces is pushed on to the next section of the colon. [ Study suggestion: Either in your imagination, or for real, get a large tube of toothpaste. Turn it upside down, and remove the cap. Now, make a ring around the upper part of the tube using both your hands. Count one . . . two . . . three . . . . Constrict your fingers hard around the tube! What do you observe? This constriction at a single point is a crude model of peristalsis. Think. How could you use the same basic elements to model mass peristalsis in the colon? If you phoned up several of your friends, what could you do?]

“Getting the Goodies”: Nutrition and the Digestive System The Digestive Tube (Alimentary Canal) in Mammals The Colon: Our Large Intestine

Fig. 19.5 The human colon (large intestine).

After the cecum, next in sequence are the ascending colon (which goes upward), transverse colon (which runs sideways), and the descending colon (which goes downward). Sigmoid ( SIG -moyd) means “S-resembling.” Hence, the sigmoid colon is the S-resembling portion of the large intestine coming right after the descending colon.

The sigmoid colon snakes down into the rectum ( WRECK -tum), a muscular-walled, “straight” (rect) tube that empties feces into the anus. There are two sphincters within the rectum. The higher one, called the internal anal sphincter, is not under our conscious control. The internal anal sphincter tends to open whenever mass peristalsis has moved some feces down into the upper portion of the rectum.

Fortunately, for us, there is also a ring of voluntary striated (cross-striped) muscle, positioned in the lower portion of the rectum. This muscular ring is called the external anal sphincter. Its contraction and relaxation is very much under our conscious control (at least, ever since we were first “potty-trained”)! Therefore, we can usually choose the time and place where we will consciously relax this lower sphincter and carry out defecation (egestion).

Let us end by summarizing in words the parts of the large intestine:

COLON = Cecum + Ascending colon + Transverse colon + Descending colon + Sigmoid colon + Rectum

Practice problems for these concepts can be found at:  Nutrition And The Digestive System Test

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