The lower digestive tract is made up of the small and large intestine. The small intestine is 22 feet long and is comprised of the duodenum, jejunum, and ileum which form the passageway between the stomach and the large intestine. Once food is broken down by the stomach, it moves into the duodenum to be broken down even further with bile and enzymes and is then carried through the jejunum to the ileum. It is in the ileum that most of the nutrients from food are absorbed through osmosis, filtration, and diffusion. Mucosal folds give the small intestine a circular shape and increase the surface area available for absorption. These folds contain villi, which are tiny projections covered in cells that aid in absorption, and each cell has hair-like microvilli that increase absorption ability (Children’s Hospital of Pittsburgh [UPMC], 2019).
Once through the small intestine, digested liquids move into the 5-foot long large intestine. It is here that water is reabsorbed, and salts are absorbed if needed. Gut bacteria feed on the liquid mixture to help with decomposition of undigested food residue, carbohydrates, amino acids, cell debris, and dead bacteria. The first portion of the large intestine is the pouch-like cecum, which passes liquid into the colon for reabsorption. Any leftover wastes are passed to the rectum to be expelled (UPMC, 2019).
Distribution in the GI tract of where patients experience CD symptoms (John Hopkins Medicine, 2013)
Individuals who develop Crohn’s disease (CD), an idiopathic inflammatory bowel disease, usually experience transmural and intermittent inflammation along these areas of the gastrointestinal tract (McCance & Huether, 2019). Patients can also experience these symptoms along any part of the GI tract from the mouth to the anus, but the lower GI tract is most commonly affected. The percentages of where CD affects patients is pictured and includes the gastroduodenal area, small intestine, and most commonly the right colon and distal ileum (Johns Hopkins Medicine, 2013). The onset of CD can be triggered by environmental factors that disturb the mucosal barrier, abnormally stimulate the gut immune response, and/or alter the balance of gut microbiota (Boyapati, Satsangi, & Ho, 2015). It is hypothesized that many who develop CD have a genetic susceptibility, and while many genes may be involved, the NOD2 (nucleotide-binding-oligomerization-domain-containing protein 2) gene has the strongest association to CD (Boyapati et al., 2015). When NOD2 is functioning normally, it codes for a protein receptor which identifies bacterial wall fragments with the use of intestinal epithelial cells (McCance & Kuether, 2019). If the NOD2 gene is defective, which can happen in instances such as a frameshift mutation, bacteria are able to continually grow and microbial dysbiosis occurs (McCance & Huether, 2019). This leads to reduced levels of defensins, Th1-mediated inflammation, cytokine and leukocyte recruitment, and triggers the release of reactive oxygen species, proteases, leukotrienes, and nitric oxide which can cause injury and exacerbate inflammation (McCance & Huether, 2019). Progression of CD is then caused by neutrophil infiltration of crypts and abscess formation (McCance & Huether, 2019). If inflammatory cells are able to invade the intestinal mucosa and travel deeper to the serosa then ulcers, noncaseating granulomas, and fistulas can form (McCance & Huether, 2019). Strictures may also develop that result in obstruction (McCance & Huether, 2019).
Pathogenesis of Crohn’s disease (John Hopkins Medicine, 2013)
Clinical presentations include severe diarrhea, abdominal pain, rectal bleeding, weight loss, anemia, and hypoalbuminemia (McCance & Huether, 2019). More severe symptoms may include inflammation of the skin, eyes, mouth, and joints (McCance & Huether, 2019). Symptoms will vary based on the location of the disease. In addition to genetic susceptibility, other risk factors are an altered gut microbiome, cigarette smoking, age less than 40, urban residency, females, and Jewish ethnicity (McCance & Huether, 2019). Crohn’s disease is diagnosed by process of elimination, as a healthcare provider will rule out other causes of symptoms before diagnosis. A colonoscopy or endoscopy may be performed to search for the presence of granulomas or ulcers (McCance & Huether, 2019). Pelvic imaging can also help to get a detailed view of the bowel and pelvic regions in order to search for a cobblestone appearance cause by the intermittent inflammation (McCance & Huether, 2019). In order to better remember the features of Crohn’s disease, clinicians can use the pneumonic CHRISTMAS which stands for: cobblestones, high temperature, reduced lumen, intestinal fistulae, skip lesions, transmural, malabsorption, abdominal pain, and submucosal fibrosis.
(McCance & Huether, 2019)