This article, the fourth in a series on the human gastrointestinal (GI) tract, describes the form and function of the small intestines.
VOL: 102, ISSUE: 09, PAGE NO: 24
Marion Richardson, BD, CertEd, DipN, RGN, RNT, is senior lecturer/programme leader, emergency nursing, Department of Nursing and Midwifery, University of Hertfordshire
Soup-like, acidic chyme (the food partially digested in the stomach) enters the small intestine through the pyloric valve from the stomach. The first part of the small intestine, the duodenum, is a common site of ulcers due to the acidity of its contents. The convoluted small intestine between the pyloric valve to the ileocaecal valve is about 7m (20ft) long and connects the stomach to the large intestine.
The small intestine is only about 3.5cm in diameter, hence the name ‘small’, and it lies in the central and lower abdomen. Food moves through it relatively slowly, over a period of hours, allowing time for the actions of digestion and absorption for which this part of the GI tract is designed.
Peristaltic waves move the contents only a short distance at a time. There are three regions within the small intestine: the short, C-shaped duodenum; the jejunum; and the ileum.
The lining of the small intestine is unlike that in the rest of the GI tract. It has multiple circular folds called plicae, which are covered with thousands of tiny finger-like projections called villi, each about 0.5mm long. Inside each villus is a dense network of capillaries that absorb the products of protein and carbohydrate digestion, and a lymphatic vessel that absorbs fats from the passing chyme.
The surfaces of the villi are covered with up to 3,000 tiny microvilli, giving them a brush-like appearance (the brush border). The folds and villi increase the surface area to about 200m2 - 600 times that of a plain tube of the same length (Smith, 2005). This increases the efficiency of absorption from the small intestine.
The small intestine is the main site of digestion and absorption of foodstuffs in the body and most of this activity takes place in the duodenum. Digestive juices are produced throughout the small intestine and in the pancreas, and bile is produced in the liver. Bile and pancreatic juices empty into the middle of the duodenum via the sphincter of Oddi. Digestion involves the breakdown by enzymes of food molecules into their simple component molecules. Absorption is the transportation of these molecules, along with vitamins, minerals and ingested water, from the lumen of the gut into the blood or lymph (McGeown, 2002).
Intestinal digestive juice
Up to 3L of intestinal juice is secreted daily by cells in the walls of the duodenum, and to a lesser extent the jejunum and ileum (Smith, 2005). The intestinal juice contains much mucus and some digestive enzymes, which assist with digestion.
Pancreatic digestive juice
The pancreas lies behind the stomach and is the most important source of the enzymes that digest all three of the major types of food (see Digestion, below). Pancreatic juice also contains sodium bicarbonate, an alkali, which neutralises the hydrocholoric acid from the stomach, and large amounts of water. The secretions into the duodenum are alkaline (pH 7.8-8.4).
Pancreatic secretion is strongly stimulated by food entering the duodenum. This triggers the release of two hormones - secretin and cholecystokinin - which activate the production of the digestive enzymes and secretion of the pancreatic juice.
Bile is produced in the liver, which secretes approximately 600-1,000ml a day into the bile ducts and thence into the duodenum. Bile is concentrated and stored in the gall bladder, which lies just beneath the liver.
Bile does not contain any digestive enzymes but does contain bile salts, which transform lipids (fats) into microscopic particles in a process known as emulsification. The resultant small particles provide a large total surface area of fats so that the pancreatic lipases can digest them more easily (Godfrey, 2002). McGeown (2002) notes that without emulsification there would simply be an oil slick in the small intestine. Bile also contains sodium bicarbonate, the bile pigments bilirubin and biliverdin, cholesterol and lecithin (McGeown, 2002) and aids the absorption of fat-soluble vitamins and iron as well as lipids.
The bile pigments bilirubin and biliverdin are made from the breakdown of red blood cells and give bile its yellow/green colour. Some of these pigments are reabsorbed from the gut and the remainder colour the faeces and are excreted with them (see part 5 of this series).
After a meal is consumed, the gall bladder contracts under the influence of the hormone cholecystokinin, produced in the walls of the duodenum, and bile is secreted into the duodenum. If the bile duct becomes blocked by a gallstone, then pain and jaundice will result.
Most chemical digestion occurs in the duodenum where the digestive enzymes - pepsin from the stomach and amylases, lipases and proteases from the pancreas - are all available. The cells of the mucosal epithelium of the small intestine also produce enzymes that help complete digestion, particularly of proteins.
Naturally, the secretion of digestive enzymes must be coordinated to occur when food is present in the small intestine or the juices would simply digest the GI tract itself. This coordination is achieved by the parasympathetic nervous system, particularly the vagus nerve, and hormones, especially cholecystokinin and secretin.
Foodstuffs are broken down into their constituents. Proteins are broken down by pepsin and proteases to become amino acids, amylases digest carbohydrates into the monosaccharides glucose, fructose and galactose, and fats are transformed by lipases into an alcohol called glycerol and fatty acids. These molecules can be absorbed along the length of the small intestine.
The small intestine is adapted to absorb substances across the intestinal mucosa. Materials are absorbed across the epithelial membrane and into the blood or lymph vessels in the villi by a number of methods - diffusion, osmosis, active transport and pinocytosis (Godfrey, 2005).
In addition to digested food molecules, the small intestine absorbs electrolytes and water. Each day approximately 9L of water arrive in the small intestine - about 1.5L of this is drunk and the remainder is secreted into the intestine (McGeown, 2002). Only about half a litre of this water reaches the large intestine - the rest is absorbed throughout the small intestine and this is closely linked to the absorption of sodium chloride.
Malabsorption syndromes such as Crohn’s disease and coeliac disease occur if the small intestine fails to absorb nutrients properly. Symptoms include weight loss, nausea, abdominal cramps, anaemia and fatigue.
As they pass through the small intestine, the contents of the GI tract are changed from soup-like chyme to faecal matter and most of the nutrients are digested and absorbed. The matter that is left at the end of the terminal ileum passes through the ileocaecal valve and into the large intestine.
- This article has been double-blind peer-reviewed.
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