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Large intestine

Anatori Sealife Comments 0 11th April 2019
Large intestine. Cleaning of the intestine before the X-ray

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 consist of five regions: the cecum, ascending, transverse, descending and sigmoid colon. The cecum is a blind pouch that 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.

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), extending across the abdomen. This region exhibits the most colonic movement. It extends inferiorly at the splenic flexure to the level of the iliac crest. It is 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 the sigmoid 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 faecal movement through this region. It is not designed to serve as a reservoir but typically is empty unless defecation is occurring. Generally, the descending and sigmoid colon evacuate at the same time.

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We refer to the last two to three inches of the rectum 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 haemorrhoids. An external sphincter muscle location is 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. Levator ani muscles assist the above sphincters during defecation.



Large intestine and different organs

Intestinal movements are of four types, haustral churning, peristalsis, mass peristalsis, and antiperistalsis. Also, haustral churning involves the filling, distension and contraction of a haustrum. It moves chyme from one haustrum to the next. Peristalsis is the slow wavelike contractions of the longitudinal and circular muscles. It also aids in the forward movement of the chyme. Mass peristalsis refers to muscular peristaltic contractions in the sigmoid colon. Thus, it pushes its contents into the rectum before defecation.

Bastedo and others have observed a rare yet fourth type of intestinal movement. Hypothetically, it holds back colon contents for a while to absorb nutrients in the cecum, ascending colon and small intestine. The large intestine is more expansive 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 primary function of the large intestine is the formation and elimination of faeces. Faeces consists of undigested food residues, intestinal epithelium and mucus, bacteria, and 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 part is concerned with the storage of intestinal debris. The ileocecal valve permits unabsorbed material to enter the cecum. Moreover, it prevents colon bacteria from entering the ileum.



Colon bacteria

Colon bacteria ferment starches, releasing hydrogen, carbon dioxide and methane gases, converting any leftover protein into amino acids. These products then convert to indoles and hydrogen sulfide, which give the faeces its odour. Bilirubin which entered the intestine in the bile, is broken down into other pigments by bacteria and is responsible for the normal colouration of wastes (i.e., brown). Light, clay coloured stools indicate biliary deficiency or blockage. The amount, composition, colour and odour of faeces depends on the food ingested.

A diet consisting of many vegetables will produce a large number of excrements, generally dark yellow. A high protein diet will create a small amount of very dark faeces. In a mixed diet, the bulk will vary, but the faeces should generally be brown. Black stools indicate upper intestinal pathology. The black is from the blood which has traversed a long distance through the intestine.

Bacteria are also involved in producing several essential 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 from bacterial metabolism can also enter the bloodstream without hindrance. These (indol, skatol, cresol, phenol and others) are treated by the liver and excreted by the kidneys.

Our body excretes hydrogen and methane gasses absorbed in the colon through the lungs. Thus, very foul breath is often a symptom of a stagnant and fermenting colon. Besides lubrication, mucus is secreted by the large bowel to prevent the entrance of toxins into the blood mechanically.