The Small Intestine
The small intestine extends from the pyloric valve of the stomach to the ileocecal valve, where it joins the large intestine. It is named for its small diameter (compared to that of the large intestine), but perhaps it should be called the long intestine. The small intestine takes up a large portion of the abdominal cavity, averaging about 6 m (18 ft) in length. All the contents of food-fats, proteins, and carbohydrates- are digested in the small intestine to soluble molecules that can be absorbed. To this end, the small intestine receives secretions from the pancreas and liver and produces intestinal juices. Absorption of nutrients for the body’s cells, such as amino acids and sugars, occurs in the small intestine. It also transports nondigestible remains to the large intestine.
Regions of the Small Intestine
The small intestine has the following regions (Fig. 15.6):
Duodenum The first 25 cm (10 in.) contain distinctive glands that secrete mucus and also receive the pancreatic secretions and the bile from the liver through a common duct. Folds and villi (Fig. 15.6) are more numerous at the end than at the beginning.
Jejunum The next 1 m (3 ft) contains folds and villi, more at the beginning than at the end.
Ileum The last 2 m (6-7 ft) contain fewer folds and villi than the jejunum.
Figure 15.6 Regions of the small intestine. The duodenum is attached to the stomach. The jejunum leads to the ileum, which is attached to the large intestine.
Wall of the Small Intestine
It has been suggested that the surface area of the small intestine is approximately that of a tennis court. Three features contribute to increasing its surface area: circular folds, villi, and microvilli (Fig. 15.7).
The circular folds are permanent transverse folds involving the mucosa and submucosa of the small intestine. The villi (sing., villus) are fingerlike projections of the mucosa into the lumen of the small intestine. The villi are so numerous and closely packed that they give the wall a velvet-like appearance. A villus has an outer layer of columnar epithelial cells, and each of these cells has thousands of microscopic extensions called microvilli. Collectively, in electron micrographs, microvilli give the villi a fuzzy border known as a “brush border” (Fig. 15.7d). Because the microvilli bear the intestinal enzymes, these enzymes are called brush-border enzymes.
Figure 15.7 Anatomy of the small intestine. The wall of the small intestine has folds that bear fingerlike projections called villi. Villi in turn have projections called microvilli. The products of digestion are absorbed by microvilli and they enter the blood capillaries and the lacteals of the villi.
Functions of the Small Intestine
The digestive process is brought to completion in the small intestine. Ducts from the gallbladder and pancreas join to form one duct that enters the duodenum (see Fig. 15.8). The small intestine receives bile from the gallbladder and pancreatic juice from the pancreas via this duct. Bile emulsifies fat-emulsification causes fat droplets to disperse in water. The intestine has a slightly basic pH because pancreatic juice contains sodium bicarbonate (NaHCO3), which neutralizes chyme. The enzymes in pancreatic juice and the enzymes produced by the intestinal wall complete the process of food digestion. The other primary function of the small intestine is absorption of nutrients. The tremendous increase in surface area created by the circular folds, villi, and microvilli makes this an efficient process-the greater the surface area, the greater is the volume of intake in a given unit of time. Also, a villus contains a generous supply of blood capillaries and a small lymphatic capillary, called a lacteal (Fig. 15.7c,d). The lymphatic system, as you know, is an adjunct to the cardiovascular system; its vessels carry a fluid called lymph to the cardiovascular veins. Sugars (digested in part from carbohydrates) and amino acids (digested from protein) enter the blood capillaries of a villus. Glycerol and fatty acids (digested from fats) enter the epithelial cells of the villi, and within these cells are joined and packaged as lipoprotein droplets, which enter a lacteal. After nutrients are absorbed, they are eventually carried to all the cells of the body by the bloodstream. As we noted previously, a third function of the small intestine is movement of nondigested remains to the large intestine. The wall of the small intestine has two types of movements: segmentation and peristalsis. Segmentation refers to localized contractions and constrictions that serve to bring chyme into contact with digestive juices and to encourage absorption. Peristalsis in particular then moves nondigested remains toward the large intestine.
Regulation of Digestive Secretions
The secretion of digestive juices is promoted by the nervous system and by hormones. A hormone is a substance produced by one set of cells that affects a different set of cells, the socalled target cells. Hormones are usually transported by the bloodstream. For example, when a person has eaten a meal particularly rich in protein, the stomach produces the hormone gastrin. Gastrin enters the bloodstream, and soon the stomach is churning, and the secretory activity of gastric glands is increasing. A hormone produced by the duodenal wall, GIP (gastric inhibitory peptide), works opposite to gastrin: It inhibits gastric gland secretion. Cells of the duodenal wall produce two other hormones that are of particular interest-secretin and CCK (cholecystokinin). Acid, especially hydrochloric acid (HCl) present in chyme, stimulates the release of secretin, while partially digested protein and fat stimulate the release of CCK. Soon after these hormones enter the bloodstream, the pancreas increases its output of pancreatic juice, which helps digest food, and the gallbladder increases its output of bile. The gallbladder contracts to release stored bile. Figure 15.8 summarizes the actions of gastrin, secretin, and CCK.
Figure 15.8 Hormonal control of digestive gland secretions. Gastrin (blue), produced by the lower part of the stomach, enters the bloodstream and thereafter stimulates the upper part of the stomach to produce more gastric juice. Secretin (green) and CCK (purple), produced by the duodenal wall, stimulate the pancreas to secrete its juice and the gallbladder to release bile.