The longest part of the digestive tract is the small intestine, which is 6.7-7.6m (22-25 feet) long. Although it is longer than the large intestine it has a smaller diameter, this is why it is called the small intestine. The small intestine is divided into three sections: the duodenum, the jejunum and the ileum.
The duodenum is the first 20cm (0.6 feet) of the small intestine and is the thickest section. It receives secretions from the numerous glands along its walls and from the liver and pancreas. This lowers the pH of the chyme and continues to break it down with chemical secretions. Most of the chemical breakdown in the small intestine occurs in the duodenum.
The jejunum is from 1.5- 2.5m (5-8 feet) long and forms two fifths of the remaining length of the small intestine. The other three fifths is the ileum which is 2.5-3.5m (8-12 feet) long. Unlike the duodenum the jejunum and the ileum are both attached to the abdominal wall by a thin mucous membrane and can move about within the body.
The three parts of the small intestine work together to break down the chyme ever further and absorb the nutrients from the food into the body. Chyme passes through the small intestine slowly, and so the enzymes have time to act on the food and raise the pH of the acidic chyme.
The digestive juices found in the small intestine are bile from the liver, pancreatic juice from the pancreas and intestinal juice from the intestine itself. All three secretions are necessary for digestion to occur, if one is missing serious problems can occur.
The chyme is spread over the surface of the small intestine and mixed with digestive fluids by contractions. Similar to the muscular contractions in the stomach, peristalsis moves the semiliquid solution backwards and forwards and kneads it.
Occasionally a stronger wave moves the most liquid of the chyme further down the small intestine, leaving behind the more solid remains to be digested further. Peristalsis does not happen at regular intervals, the waves appear and disappear randomly. They travel varying distances from a few centimetres to a metre (3 feet) and occasionally the whole length of the small intestine. The strength is also different for each muscular wave, sometimes crushing any solids remaining, other times barely causing the chyme to move.
The small intestine is able to absorb nutrients efficiently because of its large surface area. Although the width of the small intestine is 3-4 cm (1.2-1.6 in) it has a total absorptive area is 4 500 square meters (5 400 square yards). The enormous absorptive surface is due to the folds in the intestine wall which run parallel to each other. Each fold is 5-6 cm (2-2.4 in) in length and 3.2 millimetres (1/8 inch) thick.
They are found along the whole length of the small intestine, most are found in the duodenum and the upper part of the jejunum. They become smaller and gradually disappear in the lower section of the ileum. There are about 800 folds which increase the surface area by 5 to 8 times the outside area of the small intestine.
Another feature which increases the surface area is the mucosal villi. They are found along the walls and are tiny finger like projections which absorb nutrients. They are small 0.5-1 millimetres (0.02-0.04 inch) in height and are usually 1/8 - 1/3 their height. The surface of the villi has more tiny projections called microvilli which are tiny at 1/10 a micron (there are 25,000 microns in an inch) in length and one micron in height.
The villi are mostly found in the duodenum where they look like tiny leaves. In the jejunum and ileum their appearance and shape is different to that in the duodenum. Once the villi absorb the nutrients they enter the circulatory system though the blood and lymph vessels found at the base of the villi. From here they are transported to the liver to be stored or passed around the body by the bloodstream.
It takes the small intestine 6 hours to digest a normal meal. After this time all the nutrients in the food have been absorbed into the bloodstream leaving only water and indigestible substances to be passed into the large intestine. At the connection between the small and large intestine is the ilocaecal, which are two liplike folds which allow liquid chyme to be passed into the large intestine but stop it from returning.
