After food has been partly digested in the stomach it passes through a valve called the pylorus, into the rest of the digestive tract – the intestines. These are clearly divided into two: the small and large intestines. Each of these, in turn, has further subdivisions.
The small intestine, comprising the duodenum, jejunum and ileum, lies coiled in the middle of the abdominal cavity. If stretched out the small intestine would measure approximately six metres, but in some individuals it may as long as ten metres. Its function is to break down the chyme from the stomach and absorb the basic constituents of food. In order to increase the total absorptive surface area, the internal surface area of the small intestine is gathered up into folds. These, in turn, possess numerous intestinal villi – tiny finger-like projections – and a ‘brush border’ made up of the even smaller microvilli. This multiplying means that the area available for digestion and absorption becomes about 550 square metres – the size of a tennis court.
The duodenum is a C-shaped piece of bowel 25cm long; the first part lies freely in the abdomen, but the remainder adheres to the near abdominal wall by the all-enveloping layer of peritoneum – the lining membrane of the abdominal cavity. The pancreatic and bile ducts open into the second part of the duodenum by way of the sphincter of Oddi. The duodenum is the first part of the small bowel and it receives partly digested food from the stomach. Because this food, or chyme, is acidic, the duodenum is prone to damage that often results in ulcers, especially in its first part. Under normal circumstances the entrance of acid chyme in the duodenum stimulates the release of secretin. This hormone acts as a trigger for an immediate release of sodium bicarbonate by the pancreas, thus neutralizing the acidity of the chyme. In this way the potentially damaging influence of the chyme from the stomach is counterbalanced. Neutralizing chyme is also necessary because digestive enzymes from the pancreas and from the wall of the small intestine are not able to work in an acid environment. Secretin also indirectly influences the acidity of the chyme. It reduces the release of gastrin in the stomach with an accompanying reduction in the production of hydrochloric acid.
Jejunum and ileum
The jejunum is the part of the small intestine, just over 3 metres in length, that immediately follows the duodenum. While there is no true division between this and the ileum, the remaining 4.5 metres, there is a marked difference between the beginning of the jejunum and the end of the ileum. The initial part of the jejunum is approximately 3.5 cm in diameter and has more mucosal folds, making it thicker than the ileum. Both the jejunum and ileum are supported by the small intestine mesentery, a pleated membrane which also suppliesfrom branches of the superior mesenteric artery. The mesentery also allows the small intestine a certain amount of movement which is important if digestion is to proceed smoothly. The ileum leads into the large intestine via the ileo-caecal valve (valva ileocaecalis),.a muscular thickening of the end part of the ileum that protrudes into the caecum, the sac-like first part of the large intestine. This valve regulates the flow of chyme into the large intestine and prevents return, or reflux, of faeces into the srrrall intestine. This is important because faeces in the large intestine contain potentially harmful bacteria. Also, by limiting the flow of chyme into the caecum, the time it spends in the small intestine, and therefore the time available for absorption of digested food, is increased.
The main section of the large intestine, the colon, is approximately one and a half metres long. It begins at the sac-like caecum, in the right-hand lower part of the abdominal cavity, travels upwards as a section called the ascending colon, across to the left as a section called the transverse colon, and down on the left side as the descending colon. The convoluted segment of the large intestine, the sigmoid colon, leads directly into the rectum. The colon is supported at the sides by the peritoneum. The functions of the colon are mainly those of storage (faeces) and of excretion (large quantities of mucus). The absence of villi implies that food particles have been almost completely absorbed. Only water and dissolved mineral salts are absorbed to a greater extent. Some of the bacteria in the large intestine can produce vitamin K, needed for normalcoagulation, and vitamin B„ needed for normal growth of skin structures and mucosa.
The chyme entering the caecum is fluid, and the faecal material in the ascending colon is also fluid. By the time the faeces have reached the transverse colon, they are semi-solid. With most of the water absorbed, faeces are usually solid by the time they reach the descending colon. If there is some irritation of the lining of the colon, the contents are propelled forwards more quickly, less water is absorbed and the result can be diarrhoea.
The appendix is a worm-like section of bowel ranging from a few centimetres long in some people to more than 15cm in others. Its opening lies at the bottom of the caecum near the ileocaecal valve. The appendix normally lies behind the caecum, but it can be found at any point along the large intestine. The internal diameter of the appendix, or lumen, is relatively wide in infants, but in later life appears much smaller and is more easily obstructed by solidified lumps of faeces. This is the usual cause of acute appendicitis .
Rectum and anus
The rectum, at just over 12cm long, runs over the base of the spine, behind the bladder, and in women behind the vagina and uterus. It leads into the anal canal, which is nearly four centimetres long. The upper half of this is lined by bowel mucosa, whereas the lower half is lined by skin. The wall of the anal canal is formed by the circular layer of muscle which thickens to form the internal anal sphincter. The external anal sphincter is a separate ring of muscles with a separate nerve supply under voluntary control. Defecation is a reflex process but its initiation is under voluntary control. The rectum is usually empty, with faeces being stored in the sigmoid colon. When normal digestive processes move the faeces into the rectum, the sensory part of the anus detects whether it is solid, liquid and gas and the appropriate action is taken. During defecation the abdominal muscles are contracted, breath is held, the sphincters are relaxed and a peristaltic wave empties the last section of the colon. Soft bulky stools, from a diet high in fibre, cause less wear and tear on the bowel and need less effort. Because of this a high-fibre diet reduces the likelihood of diseases of the colon.