Small Intestine

The small intestine is a tubular organ that extends from the pyloric sphincter to the beginning of the large intestine. With its many loops and coils, it fills much of the abdominal cavity.

As has been mentioned, this portion of the alimentary canal receives secretions from the pancreas and liver. It also completes the digestion of the nutrient in chyme, absorbs the various products of digestion, and transports the remaining residues to the large intestine.


Parts of the Small Intestine

The small intestine consists of three portions, the duodenum, jejunum, and ileum.

The duodenum, which is about twenty-five centimeters long and five centimeters in diameter, lies behind the parietal peritoneum and is the most fixed portion of the small intestine. It follows a C-shaped path as it passes in front of the right kidney and upper three lumbar vertebrae.

The remainder of the small intestine is mobile and lies free in the peritoneal cavity. The proximal two-fifths of this portion is called the jejunum, and the remainder is the ileum. These portions are suspended from the posterior abdomnial wall by a doubele-layeredfold of peritoneum, called mesentery. This supporting tissue contains the blood vesels, nerves, and lympathic vessels that supply the intestine wall.

Although there is no distinct seperation between the jejunum and ileum, the diameter of the jejunum tends to be greater, and its wall is thicker, more vascular, and more active than that of the ileum.


Structure of the Small Intestinal Wall

Throughout its length, the inner wall of the small intestine has a velvety appearence. This is due to the pressence of innumerable tiny projections of the mucous membrane, called intestinal villi. These structures are most numerous in the duodenum and the proximal portion of the jejunum. They project into the passage way, or lumen, of the alimentary canal, where they come in contact with the intestinal contents. The villi increase the surface area of the intestinal lining, and they play an important role in the absorption of digestive products.

Each villus consists of a layer of simple columnar epithelium and a core of connective tissue containing blood capularies, a lymphatic capillary, and nerve fibers.

The blood and lymph capillaries function to carry away substances absorbed by a villus, and the nerve fibers act to stimulate or inhibit its activites.

Between the bases of adjacent villi are tublar intestinal glands, which extend downward into the mucous membrane.


Secretions of the Small Intestine

In addition to the mucous-secreting goblet cells, which occur extensivly throughout the mucosa of the small intestine, there are many specialized mucus-secreting glands within the submucosa of the proximal duodenum. These glands secrete large quanities of thick, alkaline mucus in response to various stimuli.

The intestinal glands at the bases of the villi secrete great amounts of a watery fluid. The villi rapidly reabsorb this fluid, and it provides a vehicle for moving digestive products into the villi. The fluid secreted by the intestinal glands has a pH that is nearly neutral, and it seems to lack digestive enzymes. The epithelial cells of the intestional mucosa, however, have digestive enzymes embedded in the surfaces of their microvilli. These enzymes break down food molecules just before absorption takes place. The enzymes include peptidases, which split peptides into amino acids; sucrase, maltase, and lactase, which split the double sugars (disaccharides) sucrose, maltose, and lactose into the simple sugars (monosaccharides) glucose, fructose, and galactose, and intestinal lipase, which splits fats into fatty acids and glycerol.

Regulation of Small Intestinal Secretions

Secretions from the goblet cells and intestinal glands are stimulated by direct contact with chyme, which provides both chemical and mechanical stimuli, and by reflexes triggered by distension of the intestinal wall. The reflex actions involve parasympathetic motor impusles that cause secretory cells to increase their activities.


Absorption in the Small Intestine

Because the villi greatly increase the surface area of the intestinal mucosa, the small intestine is the most important absorbing organ of the alimentary canal. In fact, the small intestine is so effective in absorbing digestive products, water, and electrolytes that very little absorbable material reaches its distal end.

Carbohydrate digestion begins in the mouth with the activity of salivary amylase. It is completed in the small intestine by enzymes from the intestinal mucosa and pancreas. The resulting monosaccharides are absorbed by the villi and enter the blood capillaries. Even though small quantities of these simple sugars may pass into the villi by diffusion, most of them are absorbed by active transport or facilitated diffusion.

Protein digestion begins in the stomach as a result of pepsin activity and is completed in the small intestine by enzymes from the intestinal mucosa and pancreas. During this process, large protein molecules are converted into amino acids. These smaller particles then enter the villi by active transport and are carried away by the blood.

Fat molecules are digested almost entirely by enzymes from the intestinal mucosa and pancreas. The resulting fatty acids and glycerol molecules diffuse into the epithelial cells of the villi. They are re-synthesized into fat molecules by action of the endoplasmic reticulum. The re-synthesized fat molecules are similar to those previously digested. These fats are encased in protein to form tiny droplets, called chylomicrons, which make their way to the lacteals of the villi. Lymph in the lacteals and other lymphatic vessels carries the chylomicrons to the blood.

On the other hand, some fatty acids with relatively short carbon chains may be absorbed directly into the blood capillary of a villus without being converted back into fat.

In addition to absorbing the products of carbohydrate, protein, and fat digestion, the intestinal villi absorb various electrolytes and water.


Movements of the Small Intestine

Like the stomach, the small intestine carries on mixing movements and peristalsis. The mixing movements include small, ringlike contractions that occur periodically, cutting the chyme into segments and moving it to and fro.

The chyme is propelled through the small intestine by peristaltic waves. These waves are usually weak, and they stop after pushing the chyme a short distance. Consequently, food materials move relatively slowly through the small intestine, taking from three to ten hours to travel its length.

Sometimes, however, stimulation of the small intestinal wall by over distension or by irritation may elicit a strong peristaltic rush that passes along the entire length of the small intestine. This type of movement serves to sweep the contents of the small intestine into the large intestine relatively rapidly.

At the distal end of the small intestine, where the ileum joins the cecum of the large intestine, is a sphincter muscle called they ileocecal valve. Normally this sphincter remains constricted, preventing the contents of the small intestine from entering. At the same time, it prevents the contents of the large intestine from backing up into the ileum. After a meal however, a reflex is elicited, and peristalsis in the ileum is increased. This action forces some of the contents of the small intestine into the cecum.