Large Intestine,Rectum And Anal Canal
The large intestine is about 1.5 metres long, beginning at the calcum in the right iliac fossa and terminating at the rectum and anal canal. Its lumen is about 6,5 cm in diameter, larger than that of the small intestine. It forms an arch round the coiled-up small intestine For descriptive purposes the large intestine is divided into the caecum, colon, rectum and anal canal.
This is the first part of the large intestineIt
is a dilated region that has a blind end inferiorly and is continuous with the ascending colon superiorly. Just below the junction of the two, the ileocaecal valve opens from the ileum. The vermiform appendix (meaning ‘worm-like’, and
often known simply as ‘the appendix’) is a fine tube, closed at one end, which leads from the caecum. It is about 8-9 cm long and has the same structure as the walls of the large intestine but contains more lymphoid tissue. The appendix
has no digestive function but can cause significant problems when it becomes inflamed.
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This is a slightly dilated section of the large intestine and is about 13 cm long. It leads from the sigmoid colon and terminates in the anal canal.
This is a short passage about 3.8 cm long in the adult and leads from the rectum to the exterior. Two muscular sphincters control the anus: the internal sphincter, consisting of smooth muscle, is under the control of the autonomic
nervous system, and the external sphincter, formed by skeletal muscle, is under voluntary control
The colon has four parts, which have the same structure and functions:
- The ascending colon passes upwards from the caecum to the level of the liver, where it curves acutely to the left at the hepatic flexure to become the transverse colon.
- The transverse colon extends across the abdominal cavity in front of the duodenum and the stomach to the area of the spleen, where it forms the splenic flexure and curves acutely downwards to become the descending colon.
- The descending colon passes down the left side of the abdominal cavity, then curves towards the midline. At the level of the iliac crest it is known as the sigmoid colon.
- The sigmoid colon is an S-shaped curve in the pelvic cavity that continues downwards to become the rectum.
The contents of the ileum that pass through the ileocaecal valve into the caecum are still fluid, even though a large amount of water has been absorbed in the small intestine. In the large intestine, absorption of water, by osmosis, continues until the familiar semisolid consistency of faeces is achieved. Mineral salts, vitamins and some drugs are also absorbed into blood capillaries from the large intestine.
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The large intestine is heavily colonised by certain types of bacteria, which synthesise vitamin K and folic acid. They include Escherichia coli, Enterobacter aerogenes, Streptococcus faecalis and Clostridium perfringens. These microbes are commensals, ie. are normally harmless, in humans.However, they may become pathogenic if transferred to another part of the body; for example, E, coli may cause cystitis if it gains access to the urinary bladder. Gases in the bowel consist of some of the constituents of air, mainly nitrogen, swallowed with food and drink Hydrogen, carbon dioxide and methane are produced by bacterial fermentation of unabsorbed nutrients, especially carbohydrate. Gases pass out of the bowel as flatus (wind).
The large intestine does not exhibit peristaltic movement as do other parts of the digestive tract. Only at fairly long intervals (4-6 times a day in adults) a wave of strong peristalsis sweeps along the transverse colon, forcing its contents into the descending and sigmoid colons. This is known as mass movement and it is often precipitated by the entry of food into the stomach. This combination of stimulus and response is called the gastrocolic reflex.
The rectum is usually empty, but when a mass movement forces the contents of the sigmoid colon into the rectum the nerve endings in its walls are stimulated by stretch. In infants, defecation occurs by reflex (involuntary) action. However, during the second or third year of life, children develop voluntary control of bowel function. In practical terms, this acquired voluntary control means that the brain can inhibit the reflex until it is convenient to defaecate. The external anal sphincter is under conscious control through the pudendal nerve. Thus, defecation involves involuntary contraction of the muscle of the rectum and relaxation of the internal anal sphincter. Contraction of the abdominal muscles and lowering of the diaphragm during a forced expiration (Valsalva’s mancuvre) increase the intra-abdominal pressure and so assist defaecation. When the need to pass faces is voluntarily postponed, it tends to fade until the next mass movement occurs and the reflex is initiated again. Repeated suppression of the reflex may lead to constipation (hard faces) as more water is absorbed.