The small intestine is about 2.5 cm (1 in) in diameter and 4 to 5 m (12-15 ft) in length. It begins at the pyloric sphincter of the stomach, fills much of the abdominopelvic cavity, and empties into the large intestine. Most of the digestive processes and absorption of nutrients occur in the small intestine.

Structure

There are three sequential segments composing the small intestine. The duodenum (du-o-de ‘-num) is a very short section, about 25 to 30 cm long, that receives chyme from the stomach. The middle section is the jejunum (je-ju’-num), and it is about 160 to 200 cm long. The last and longest segment is the ileum (il’-e-um), which is about 170 to 215 cm long. The ileum joins with the large intestine at the ileal (il-e-al) orifice.

The small intestine is suspended from the posterior abdominal wall by the mesentery (mes ‘-en-ter-e), double folds of the peritoneum that provide support but allow movement. Blood vessels, lymphatic vessels, and nerves serving the small intestine are also supported by the mesentery.

The mucosa of the small intestine is modified to provide a very large surface area. The distinctive velvety appearance of the intestinal mucosa results from the presence of i ntestinal villi, tiny projections from the mucosa that are extremely abundant. Each villus is covered by simple columnar epithelium and contains a centrally located lacteal, a lymphatic capillary. A blood capillary network surrounds the lacteal. At the bases of the villi are tiny pits that open into intestinal glands, which secrete intestinal juice.

The mucosal surface area in contact with chyme and digestive fluids is further increased by the presence of numerous microvilli forming a “brush border”.

Intestinal Juice

The fluid secreted by the intestinal glands is known as intestinal juice. It is slightly alkaline and contains abundant water and mucus. Intestinal juice provides an appropriate environment for the action of bile salts and pancreatic digestive enzymes within the small intestine. Recall that trypsin in pancreatic juice is activated only after being mixed with intestinal secretions.

Regulation of Intestinal Secretion

The presence of chyme in the small intestine provides mechanical stimulation of the mucosa that activates the secretion of intestinal juice and enzymes. Chyme also causes an expansion of the intestinal wall, triggering a neural reflex that sends parasympathetic nerve impulses to the mucosa. The nerve impulses stimulate an increase in the rate of intestinal secretions.

Digestion And Absorption

Vigorous segmentation within the small intestine mixes chyme with bile, pancreatic juice, and intestinal juice. The emulsification of fats by bile and the continued digestion of carbohydrates, fats, and proteins by pancreatic and brush border enzymes occur within the small intestine. Brush border enzymes are embedded within the brush border of the small intestine mucosa. These actions complete the digestive process within the small intestine.

There are three brush border enzymes that split disaccharides into monosaccharides. (1) Maltase converts maltose into glucose; (2) sucrase converts sucrose into glucose and fructose; and (3) lactase converts lactose into glucose and galactose.

Brush border enzymes acting on proteins are also present. Various peptidases split peptides into amino acids.

Summary of The Major Digestive Enzymes And Their Actions

Enzyme Substrate Product
Saliva
Salivary amylase Starch and glycogen Maltose
Gastric Juice
Pepsin Proteins Peptides
Pancreatic Juice
Pancreatic amylase Starch and glycogen Maltose
Pancreatic lipase Triglycerides Monoglycerides* and fatty acids*
Trypsin Proteins Peptides
Brush Border Enzymes
Maltase Maltose Glucose*
Sucrase Sucrose Glucose* and fructose*
Lactase Lactose Glucose* and galactose*
Peptidases Peptides Amino acids*

Carbohydrate digestion begins in the mouth and concludes in the small intestine. The end products of carbohydrate digestion are monosaccharides, the simple sugars glucose, fructose, and galactose. These sugars are absorbed across the epithelium and into the capillaries of the villi through both facilitated diffusion and active transport.

Fat (triglyceride) digestion primarily occurs in the small intestine. The end products are monoglycerides and fatty acids. Very small, or short-chain, fatty acids are absorbed by simple diffusion across the epithelium and into the capillaries of the villi. All other lipids require an alternate means of absorption. Bile salts interact to form structures called micelles, small transportation spheres that are hydrophilic on their surface and hydrophobic in their core. Micelles absorb large fatty acids, monoglycerides, cholesterol (a steroid), phospholipids, and lipid-soluble vitamins into their core and transport them to the intestinal brush border. The contents of the micelles move by simple diffusion into the epithelial cells. Once inside the epithelial cells, the fatty acids and monoglycerides recombine to form triglycerides. The triglycerides combine in small clusters with phospholipids, steroids, and lipid-soluble vitamins. These clusters are coated with protein and form structures known as chylomicrons (ki-lo-mi ‘-krons). The protein coat makes chylomicrons water-soluble. Chylomicrons move out of the epithelial cells by exocytosis and enter the lacteals of the villi. They are carried by lymphatic vessels to the left subclavian vein, where lymph from the intestine enters the blood.

Protein digestion begins in the stomach and concludes in the small intestine. The end products are amino acids, which are actively absorbed across the epithelium and into the capillaries of villi.

In addition to the end products of digestion, other needed substances are absorbed in the small intestine. For example, water, minerals, and water-soluble vitamins are absorbed into the capillaries of villi. Materials absorbed into the blood are carried from the intestines to the liver via the hepatic portal vein. After processing by the liver, appropriate concentrations of nutrients are released into the general circulation to serve the needs of tissue cells. In this way, the liver contributes to the overall homeostasis of the body.