Digestive System

Despite its name, the small intestine is the largest part of the digestive tract.  Even though the small intestine is only about one inch in diameter, the small intestine is approximately 20 to 23 feet long. Because the small intestine is long, the small intestine has a lot of surface area. This large surface area helps to ensure that the nutrients are absorbed.

The small intestine has three parts called the duodenum, the jejuneum, and the ileum. As mentioned previously, the duodenum begins at the pyloric sphincter.  It then wraps around the head of the pancreas and ends at a turn called the duodenojejuneal flexure. It then becomes the jejuneum. The jejuneum coils around the upper left of the lower half of the body before it winds toward the lower right half of the body. When it reaches the lower right half of the body, it is called the ileum. The small intestine ends where the ileum joins the cecum, the first part of the large intestine.

The mucosa layer on the inside of the small intestine has projections called villi, which look like tiny fingers. These villi are largest in the duodenum and become smaller as the tube progresses away from the stomach. These villi make the walls of the small intestine look like a shag carpet.  On the inside of the villi is a network of blood vessels and lymphatic vessels called a lacteal. The nutrients are picked up and transported into the bloodstream there. Between the villi are tiny pores that lead to glands called intestinal crypts. These crypts extend as far as the muscularis mucosae layer of the wall of the intestine. These crypts secrete a highly alkaline intestinal juice that contains mostly water and mucus. They also secrete lysozyme, phoshpolipase, and defensins, which serve to protect the body from infections by bacteria.

On the cells that make up the villi are smaller projections called microvilli.  These projections form a border called a brush border, which helps to increase the surface area of the small intestine. The brush border also contains enzymes called brush border enzymes, such as enterokinase. These enzymes are important to the final stages of digestion, but they are not released into the cavity of the small intestine. Chyme must come into contact with the mucosa of the small intestine in order for the brush border enzymes to break down the nutrients in it.

As mentioned earlier, the small intestine is where most of the nutrients are absorbed.   Some of these nutrients include glucose, amino acids, vitamins, minerals, acids like those found in citrus drinks or vinegar, and peptides.  Sometimes, food products that do not serve as fuel or building blocks for cells and chemical compounds that are considered foreign to the body are also absorbed. As chyme passes through the small intestine, the microvilli catch many different nutrients in the chyme and absorb them. How the nutrient is absorbed depends on what the nutrient is. For instance, nutrients that do not mix well with water or that mix well with fats are caught by the microvilli and pass directly through the cell. Other nutrients, such as proteins, are absorbed through the microvilli and wrapped in tiny structures called vesicles, which are similar to bubbles, so that they can travel through the cell. Once the nutrients pass through the outer layer of cells in the wall of the small intestine, they are picked up by the blood and taken to other cells in the body that need them. Cholesterol in food is also absorbed by the microvilli in the small intestine.

Although the microvilli are vital in helping to absorb nutrients, not all nutrients that are taken in through the small intestine are absorbed by microvilli. In between the place where two cells that have microvilli meet is a tiny hole. Smaller nutrients that can dissolve well in water, such as minerals, are thought to pass through these holes. Although the holes can allow smaller nutrients to pass through, they are too small to let in larger substances, such as germs or proteins that might produce an allergic reaction.

Even though germs or other harmful substances may not be able get in between the spaces where the cells meet, they can still be absorbed by the microvilli. Thus, the cells in the wall of the small intestine also have ways of protecting the body against toxic substances or harmful germs. If a substance absorbed by the microvilli is toxic, the cell will send enzymes that will make it nontoxic. The cell also has a means of forcing toxic compounds out of it so that these compounds do not go into the body. Similarly, if the microvilli absorb germs, the cell will send compounds such as antibodies and peptides that can fight the microbes and destroy them.

The cells in the walls of the small intestine also use molecules to transmit messages from the outside of the cell to the inside. Cells in the wall of the small intestine can recognize whether a substance in the intestine is food and secrete hormones to the side of the cell that faces the bloodstream. The cells that absorb the nutrients also have receptors that can detect whether a substance is made of molecules that would indicate that it is bacteria. The presence of bacteria causes a chain of events that results in cytokines arriving at the site. These cytokines tell white blood cells made by the thymus to fight the bacteria.

Even if the germs are able to pass through the mucosa layer of the small intestine, the small intestine has ways of defending against them. The lamina propria sub-layer and the submucosa layer of the small intestine have nodules that are part of the lymphatic system. These nodules contain white blood cells that destroy germs. They are less numerous in the duodenum and become more numerous as the small intestine progresses toward the large intestine. In the ileum section of the small intestine where they are most numerous, these nodules are called Peyer patches.

In order to increase contact of the microvilli with the chyme, the circular sub-layer of the muscularis externa layer of the small intestine contracts. These contractions are ringlike and serve to mix the chyme with the pancreatic juice, the bile, and the intestinal juice. Mixing the chyme with these juices serves to decrease the acidity of the chyme. Mixing the chyme also brings the chyme into contact with the brush border enzymes and promotes absorption by the microvilli. The ringlike contractions also push the chyme slowly toward the large intestine.

When the intestinal walls have absorbed most of the nutrients, the ringlike contractions stop and peristaltic contractions push the chyme towards the large intestine. These contractions occur in waves, starting in one section of the small intestine only to stop and resume in a section closer to the large intestine. At the place where the small intestine meets the large intestine is a sphincter muscle called the ileocecal valve. Because the muscularis layer in the ileocecal valve is thickened, the ileocecal valve protrudes into the cecum. The ileocecal valve is usually closed, but the presence of food in the stomach triggers the release of gastrin and a reflex arc called the gastroileal reflex. The release of gastrin and the reflex arc increase the ringlike contractions in the small intestine and cause the sphincter to relax. As a result, the residue enters the cecum. The pressure that results from the build-up of residue in the cecum causes the sphincter to close and prevents the residue from going back into the small intestine.