The large intestine is the part of the alimentary tract extending from the ileocecal valve to the anus. It is approximately five feet long and has an average diameter of 2 1/2 inches. The large intestine can be divided into five regions: the cecum. ascending, transverse, descending and sigmoid colon. The cecum. is a blind pouch which receives chyme from the ileum through the ileocecal valve. It has the largest diameter (3 to 3.5 inches) of the large intestine and is the least motile, but does occur in various abdominal positions. A small, wormlike tube, the vermiform appendix is attached to the cecum. Inflammation of this structure is referred to as appendicitis.
The ascending colon, approximately 8 inches long, arises from the cecum on the right side of the abdomen and extends upwards to a position behind the liver. It curves to the left at the hepatic flexure and becomes the transverse colon (approximately 16-20 inches long) which extends across the abdomen. This region exhibits the most colonic movement. It extends inferiorly at the splenic flexure to the level of the iliac crest. This is the descending (approximately 10-12 inches long). The colon turns toward the midline at the iliac crest in a form resembling the letter S. This turning point is the sigmoid flexure and the region is that of thesigmoid colon (approximately 16 inches long). The sigmoid colon empties into the last part of the large intestine, the rectum (approximately 4 to 7 inches long). Three semilunar valves (Valves of Houston) are present in the mucosal layer of the rectum which slows fecal movement through this region. It is not designed to serve as a reservoir, but normally is empty unless defecation is occurring. Generally the descending and sigmoid colon evacuate at the same time. The last two to three inches of the rectum is referred to as the anal canal. Its lining is not of mucus but squamous epithelium. Several blood vessels are present in this region and their inflammation is known as piles or hemorrhoids. An external sphincter muscle is located at the distal end of the canal and an internal sphincter at its proximal end. The former is under voluntary control while the latter is an involuntary muscle. The opening of the anal canal to the exterior is the anus, through which fecal matter is voided. Levator ani muscles assist the above sphincters during defecation.
Intestinal movements are of four types, haustral churning, peristalsis, mass peristalsis, and antiperistalsis. Haustral churning involves the filling, distension and contraction of a haustrum. This moves chyme from one haustrum to the next. Peristalsis is the slow wavelike contractions of the longitudinal and circular muscles which also aids in the forward movement of the chyme. Mass peristalsis refers to strong peristaltic contractions in the sigmoid colon which pushes its contents into the rectum, prior to defecation. Bastedo and others have observed a rare yet fourth type of intestinal movement. It is described as antiperistalsis, i.e., peristaltic contractions in the direction of the cecum. It is hypothesized that this holds back colon contents for a while so that nutrients may be absorbed in the cecum, ascending colon and small intestine. The large intestine is bigger in diameter. It contains several "pockets" (haustra), and it has more longitudinal muscle bands (taeniae coli). The small intestine has villi and plicae circulares which are absent in the large intestine.
Most of the digestion and absorption is complete by the time the chyme reaches the cecum. The major function of the large intestine is the formation and elimination of feces. Feces consists of undigested food residues, intestinal epithelium and mucus, bacteria, and a large amount of water. Ranklin, Bargen and Buie consider the colon to be divisible into right and left halves. The right portion is concerned with digestion and absorption while the left portion is concerned with storage of intestinal debris. The ileocecal valve permits unabsorbed material to enter the cecum and prevents colon bacteria from entering the ileum.
Colon bacteria ferment starches, releasing hydrogen, carbon dioxide and methane gases, and convert any left over protein into amino acids. These products are then converted to indoles and hydrogen sulfide which give the feces its odor. Bilirubin which entered the intestine in the bile is broken down into other pigments by bacteria and is responsible for the normal coloration of feces (i.e., brown). Light, clay colored stools indicate biliary deficiency or blockage. The amount, composition, color and odor of feces depends on the food ingested. A diet consisting of many vegetables will produce a large amount of feces, generally dark yellow. A high protein diet will produce a small amount of very dark feces. In a mixed diet the bulk will vary but the feces should generally be brown. Very black stools indicate upper intestinal pathology. The black is from blood which has traversed a long distance through the intestine.
Bacteria are also involved in the production of several important vitamins which are absorbed by the large intestine and stored in the liver (K and B vitamins). Besides vitamins, the colon absorbs large amounts of water (cecum and ascending colon). Unfortunately, toxins of bacterial metabolism are not prevented from entering the circulatory system as well. These (indol, skatol, cresol, phenol and others) are treated by the liver and excreted by the kidneys. Hydrogen and methane gasses absorbed in the colon are excreted through the lungs. Thus, a very foul breath is often a symptom of a stagnant and fermenting colon. Besides lubrication, mucus is secreted by the large bowel in an effort to mechanically prevent the entrance of toxins into the blood.