The small bowel is a part of the human gastrointestinal (GI) tract that is made up of the duodenum, jejunum and ileum. It is often examined during intensive colonoscopy exams to screen for diverticulosis, irritable bowel syndrome (IBS) and signs of cancer. The term “small bowel” encompasses these three areas as a whole, since the differences between each are very particular, with distinctions only at the microscopic level. The ileum is usually paler in color and contains slightly more fat than the jejunum. It is also closely related to the human immune system because it contains more lymphoid tissue than other parts of the GI tract. All parts of what is considered the small bowel are located in between the stomach and large intestines, right before the cecum (the first part of the colon).
Function of the Small Bowel
The primary function of the small bowel is to absorb fats, nutrients, vitamins and salt from contents passing through the GI tract. Folds along the walls of the small bowel are covered with intestinal villi and microvilli—or miniature, capillary-like hairs—that increase the surface area of the small bowel and increase its capability to extract important, nutritional substance before food enters the large intestine. The food moves through the small bowel by being pushed along by waves produced by the involuntary contraction of the bowel muscles. This process is called peristalsis and occurs in mostly all segments of the GI tract.
During human digestion, the villi situated in the small bowel can transfer amino acids and certain sugars to the liver. Villi can also absorb some fatty acids and glycerol, present especially in food items with higher fat content.
Diseases in the Small Bowel
The small bowel is often a site of digestive conditions like inflammatory bowel disease (IBD). Inflammatory diseases such as Crohn’s disease occurs when deep tissue in the small bowel is inflamed, causing symptoms that can range from general abdominal pain to severe diarrhea. Many conditions affecting the small bowel can be screened for during a traditional colonoscopy, which although mostly examines the large intestine, oftentimes is extended to an examination of the small bowel. Occasionally, the specialist may want to perform a ‘push enteroscopy’ which involves examining more of the small bowel either from the mouth or the anus as points of entry. Traditional colonoscopy can examine up to about 20 to 30 centimeters of small bowel without this technique, and several hundred centimeters of small bowel with it. If you are experiencing severe digestive symptoms, please contact a gastroenterologist as soon as possible for a proper diagnosis.
Reviewed 12/16/2011 by David M. Nolan, M.D.
Diplomate of the American Board of Internal Medicine, 2011
Currently a Fellow of Gastroenterology, at UCI 2011-2014