24/07/2022
Inflammatory Bowel Disease
Intestinal Anatomy
Inflammatory Bowel Disease (IBD)To understand IBD, a review of gastrointestinal (GI) tract anatomy is helpful. This explanation is not complete, but it covers the basics.
The digestive system consists of a long tube (alimentary canal) that varies in shape and purpose as it winds its way through the body from the mouth to the a**s (see diagram). The size and shape of the digestive tract varies according to the individual (e.g., age, s*x, size, and disease state). The dimensions mentioned below are for an average adult male.
The upper part of the GI tract includes the mouth, throat (pharynx), esophagus, and stomach. In the mouth, chewing mechanically breaks down and mixes food, while saliva begins to modify it chemically, thus beginning the digestive process. Chewing and swallowing (ingesting) require conscious effort, but once food reaches the esophagus, an automatic, rhythmic motion (peristalsis) takes over, propelling the contents along. A number of body systems provide the chemicals necessary to complete digestion, absorption, and elimination. If all is going well, the passage of food from one area of the intestines to the next is precisely coordinated, so that it stays in each area for just the right amount of time.
Transit time is the duration between when you take a bite of your meal and when leftover waste finally passes out as stool (f***s). A meal could take anywhere from 12-72 hours to travel through the digestive tract. Each person is unique; a normal bowel movement pattern for one person may be very different from those of family members or friends. Some individuals have an irregular pattern, never knowing what to expect. The composition and quantity of dietary intake, the presence of a GI disease or disorder, and other factors influence transit time.
Following the esophagus, located within the left side of the abdominal cavity, is the stomach, a sac-like organ about 25 cm (~10″) long. It increases in diameter as it receives food, holding up to a maximum of about 4 litres (but do not eat so much as to extend the stomach fully). Strong chemicals break food down into smaller components, while the stomach’s thick walls keep these chemicals from entering the body as it squeezes its contents with strong circular and longitudinal muscles.
The lower part of the GI tract includes the small and large intestines. The names of these parts are a bit confusing, as the small intestine is about three times as long as the large intestine. Small and large refer to the diameter, which for the small intestine is about 2.5-3 cm (~1″) compared to 6.5-7 cm (~2.5″) for the large intestine.
The small intestine is a long and narrow coiled tube that extends from the stomach to the large intestine, winding around within the abdomen. It has an enormous internal surface area due to the presence of millions of tiny finger-like protrusions called villi, which are covered in hair-like protrusions called microvilli. This is where most digestion and absorption of food takes place. The first section of the small intestine, the duodenum, is the smallest segment, which is about the same length as the stomach (25 cm/~10″), followed by the jejunum (2.5 m/~8′) and the ileum (3.5 m/~11.5′), for a combined total of a little more than 6 m (~20′). Functions of the small intestine include secreting digestive enzymes and buffers, and absorbing nutrients. Iron is absorbed in the duodenum, and vitamin B12 and bile salts are absorbed in the last part of the ileum (terminal ileum).
The mucosa is the innermost layer of the GI tract wall. In the small intestine, it is responsible for both the secretion of digestive juices and the absorption of nutrients. The second layer is the submucosa, which consists of a dense layer of connective tissue with blood vessels, lymphatic vessels, and nerves. Outside of the submucosa is a layer of muscle, the muscularis externa. As these muscles contract, they propel food along the digestive tract. The outermost layer is the serosa, a thin layer of cells that secrete fluid, which reduces the friction caused by muscle movement.
The first part of the colon, the cecum, connects to the small intestine at the ileocecal valve, in the lower right area of the abdomen. The rest of the large bowel consists of the ascending colon (going upward on the right side of the abdomen), transverse colon (going leftward across the abdomen), and descending colon (going downward on the left side of the body), followed by the sigmoid colon. Altogether, the large intestine (colon) is about 1.25 m (~4′) long. The main function of the colon is to absorb water, form stool, and eliminate waste. The large intestine does not secrete digestive enzymes and does not have villi. The colon contains many varieties of friendly bacteria and yeasts to aid digestion and to prevent proliferation of harmful bacteria and yeasts. The colon ends in the re**um (about 15-20 cm/~6-8″ long), which briefly stores waste material until it is eliminated via the a**s as stool, usually as a conscious choice.
Although not directly part of the digestive tract, the liver, gallbladder, and pancreas serve as accessory organs that are vital to the digestive process.
