DAE - Intestines Lesson
Intestines
Small Intestine
The small intestine is a tube-like continuation of the alimentary canal that starts at the stomach and ends at the large intestine. In adults, the small intestine is about 23 feet long. This organ is the site where almost all digestion and absorption of nutrients and minerals from food take place.
The duodenum is the first and shortest section of the small intestine. It begins at the pyloric sphincter and ends at the start of the jejunum. The duodenum is where most chemical digestion using enzymes takes place.
The jejunum is the middle section of the small intestine. It has a lining that is designed for the absorption of nutrients. The mucous membrane of the jejunum is covered in projections called villi, which increase the surface area of tissue available to absorb nutrients. In fact, villi make the inner surface area of the small intestine 1,000 times larger than it would be without them. The entire inner surface area of the small intestine is about as big as a basketball court because of these tiny projections! The epithelial cells which line these villi possess even larger numbers of microvilli. Here nutrients such as carbohydrates, amino acids, small peptides, vitamins, and most glucose are absorbed into the bloodstream by the many tiny blood vessels within the villi.
The ileum is the final and longest section of the small intestine. It is thicker and more vascular than the other sections and extends from the end of the jejunum to the ileocecal sphincter at the beginning of the cecum (first section of the large intestine). The ileum functions to absorb vitamin B12, bile salts, and any products of digestion that were not absorbed by the jejunum. The walls of the ileum are made up of folds, each of which contains villi. Like the jejunum, the ileum has an extremely large surface area both for the adsorption of enzyme molecules and for the absorption of products of digestion. Layers of circular and longitudinal smooth muscle enable the digested food to be pushed along the ileum by peristalsis. The undigested food (waste and water) is sent through the ileocecal sphincter to the colon. It takes about 3 to 5 hours for all of the chyme to leave the small intestine.
Large Intestine
The large intestine is a wide tube-like continuation of the alimentary canal that extends from the ileocecal sphincter to the anus. The main function of this organ is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body. The large intestine absorbs about 90% of the water you ingest! Waste enters the large intestine from the small intestine in a liquid state. As the waste moves through the large intestine, excess water is absorbed from it. After the excess water is absorbed, the remaining solid waste is called feces. The large intestine takes about 16 hours to finish up the remaining processes of the digestive system.
As compared to the small intestine, the walls of the large intestine have no circular folds or villi and contain more intestinal glands. These glands largely contain goblet cells and enterocytes. Goblet cells function to secrete mucus that eases the movement of feces and protects the intestine from the effects of acids and bases. Enterocytes absorb water and salts and vitamins produced by intestinal bacteria. The large intestine can house over 700 species of symbiotic bacteria (intestinal flora) that perform a variety of functions including aiding in digestion. These bacteria can produce gas as a by-product of fermentation which is referred to as flatus. Each day, up to 1500 mL of flatus is produced in the colon. More is produces when you eat foods such as beans, which are rich in otherwise indigestible sugar and complex carbohydrates.
As with the small intestine, the large intestine can be divided into several sections. The large intestine consists of the cecum, colon, rectum, and anal canal. The cecum is the first part of the intestine. It is a sac-like structure that is suspended below the ileocecal sphincter. The appendix is attached to the inferior surface of the cecum. It contains some lymph tissue which scientists believe gives it a role in immunity; however, it is generally considered vestigial. Appendicitis is the result of a blockage that traps infectious material inside of the appendix. This organ can be removed with no apparent damage or consequence to the patient.
The cecum blends seamlessly with the colon. Upon entering this structure, food residue travels up the ascending colon on the right side of the abdomen. The colon then bends and travels across the abdomen. This section is called the transverse colon. The colon bends once more and food residue must travel down the left side of the abdomen in the descending colon. After entering the pelvis, it becomes the S-shaped sigmoid colon. The large intestine is approximately 5 ft. long, which is about one-fifth of the whole length of the intestinal canal.
Feces are stored in the rectum until they are eliminated from the body through the anus due to contraction and relaxation of the circular muscles in the anal sphincter. When these muscles relax, they let the feces pass out of the body through the anus. After feces pass out of the body, they are called stool. Releasing the stool from the body is referred to as defecation. If defecation is delayed for an extended time, additional water is absorbed, making the feces firmer and potentially leading to constipation. On the other hand, if the waste matter moves too quickly through the intestines, not enough water is absorbed, and diarrhea can result. In general, diet, health, and stress determine the frequency of bowel movements. The number of bowel movements varies greatly between individuals, ranging from two to three per day to three to four per week.
Review the structure of the intestines in the learning object below:
Review
Now that you have completed the initial assessments for this module, review the lesson material with the practice activities and extra resources.
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