INTRODUCTION
The large intestine is about 1.5 meters long, beginning at the caecum in the right iliac fossa and terminating at the rectum and anal canal deep within the pelvis. Its lumen is about 6.5cm in diameter, larger than that of the small intestine. It forms an arch round the coiled-up small intestine. For description purposes the large intestine is divided into the caecum,, colon sigmoid , rectum and anal canal.
The caecum
This is the first part of the large intestine . It is a dilated region which 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 is a fine tube, closed at one end, which leads from the caecum. It is about 8-9cm 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 problem when it becomes inflamed.
The colon
The colon has four parts which have the same structure and functions.
The ascending colon.This 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. This part 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.This 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.This part described as S-shaped curve in the pelvic cavity that continuous downwards to become the rectum.
THE RECTUM
This is a slightly dilated section of the large intestine about 13cm long. It leads from the sigmoid colon and terminates in the anal canal.
THE ANAL CANAL
This is a short passage about 3.8cm long in the adult and leads from the rectum to the exterior. Two sphincter muscles control the anus; the internal sphincter, consisting of smooth muscles, is under the control of the autonomic nervous system and external sphincter, formed by skeletal muscle, is under voluntary control.
STRUCTURE
The four layers of tissue described in the basic structure of the gastrointestinal tract are present in the caecum, colon, the rectum and anal canal. The arrangement of the longitudinal muscle fibres is modified in the caecum and colon. They do not form a continuous layer of tissue but are instead collected into three bands, called taeniae coli, which run lengthways along the caecum and colon. They stop at the junction of the sigmoid colon and the rectum. As these bands of muscle tissue are sightly shorter than the total length of the caecum and colon they give it a sacculated or puckered appearance.
In the rectum the longitudinal muscle fibres spread out as in the basic structure and this layer therefore completely surrounds the rectum and anal canal. The anal sphincter are formed by thickening of the circular muscle layer.
In the submucosal layer there is more lymphoid tissue than in any other part of the alimentary tract, providing non-specific defence against invasion by resident and other potentially harmful microbes. In the mucosal lining of the colon and the upper region of the rectum are larges number of mucus-secreting goblet cells within simple tubular glands. They are not present beyond the junction between the rectum and anal canal.
The lining membrane of the anal canal consists of stratified squamous epithelium continuous with the mucous membrane lining of the rectum above and which Marges upper section of the anal canal the mucous membrane is arranged in 6-10 vertical folds, the anal columns. Each column contains a terminal branch of the superior rectal artery and vein.
FUNCTION OF LARGE INTESTINE, RECTUM AND ANAL CANAL
Absorption
The contents of the ileum which pass through the ileocaecal valve into the caecum are fluid, even though a large amount of water has Been absorbed in the the small intestine. In the large intestine absorption of water, by osmosis, continuous 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.
Microbial activity
The large intestine is heavily colonised by certain types of bacteria, which synthesis vitamin K and folic acid. They include Escherichia coli, Enterobacter aerogenes, streptococcus faecalis and Clostridium perfringens. These microbes are commensals, i.e. normally harmless, inferred to another part of the body, e.g. E.coli may cause cystitis if it gains access to the urinary bladder. Gases in the bowel consists of some of the constituents of air, mainly nitrogen, swallowed with food and drink. Hydrogen, carbon dioxide and methane are produced by bacteria fermentation of unabsorbed nutrients, especially carbohydrate. Gases pass out of the bowel as flatus.
Mass movements
The large intestine does not exhibit peristaltic movement as in other part of the digestive tract. Only at fairly long intervals does a wave of strong peristalsis sweep along the transverse colons, this is known as mass movement and it is often precipitated by the entry of food into the stomach. This combination stimulus and response is called the gastrorocolic reflex.
Defaecation
Usually the rectum is 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, defaecation occurs by reflex 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 exhibit the the reflex until it is convenient to defaecate. The external anal sphincter is under conscious control through pudendal nerve. Thus, defaecation involves involuntary contraction of the muscle of the rectum and relaxation of the internal anal sphincter.contraction of the abdominal muscle and lowering of the diaphragm increase the intra-abdominal pressure and so assist defaecation. When the need to pass faeces is voluntarily postponed, it tends to fade until the next mass movement occurs and the reflex is initiated again. Repeated suppression of the reflex lead to constipation as more water is absorbed.
Constituents of faeces
The faeces consists of a semisolid brown mass. the brown colour is due to the presence of stercobilin.
Even though absorption of water takes place in the small and large intestine, water still makes up about 60-70% of the weight of the faeces. The reminder consists of:
BLOOD SUPPLY
Arterial supply is mainly by the superior and inferior mesenteric arteries. The superior mesenteric artery supplies caecum, ascending and most of the transverse colon. The inferior mesenteric artery supples the reminder of the colon and the proximal part of the rectum. The middle and inferior rectum arteries, branches of the internal iliac arteries, supply the distal section of the rectum and the anus.
Venous drainage is mainly by the superior and inferior mesenteric veins which drain blood from the parts supplied by arteries of the same names. These veins join the splenic and gastric veins to form the portal vein. Veins draining the distal part of the rectum and anus join the internal iliac veins, meaning that blood from tis region returns directly to the inferior cava, by passing the portal circulation.
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