The gastrointestinal tract begins at the mouth and ends at the anus. In health, the time taken from ingestion to elimination in the Western world is about 1–3 days (Tortora and Derrickson, 2014). This transit time is longer for people with constipation and shorter for people with diarrhoea.
Anatomy and physiology of the GI tract
Mouth and stomach
The mouth contains teeth, required for chewing food, and saliva, which keeps the mouth moisturised and supple as well as being responsible for the first part of the chemical breakdown of food. Once food has been ingested and chewed, it passes to the stomach where chemical breakdown continues with the gastric juices. The stomach is acidic; this helps to prevent infection caused by any ingested bacteria. The stomach's strong muscle walls mechanically churn the food to help to break it down into the various components that are required by the body, such as amino acids, which are broken-down proteins. Food stays in the stomach for several hours while breakdown occurs. Little is absorbed directly from the stomach (Watson, 2011).
Small bowel
When the chyme from the stomach passes into the small bowel, it travels to the duodenum. In the duodenum, the contents are made less acidic by the secretion of pancreatic enzymes and bile released from the gallbladder. The bile, which is produced by the liver, also helps to break down fats.
As the chyme passes from the duodenum into the jejunum and ileum, it is moved along by muscle contractions of the small bowel termed peristalsis. Most nutrients, such as amino acids, are absorbed in the small bowel. Additionally, several litres of fluids from ingestion and secretions are also absorbed. Specific areas of the small bowel absorb different nutrients; for example, the terminal ileum is responsible for absorbing vitamin B12 (Watson, 2011).
Colon
As waste from the small bowel passes into the colon, about a litre of fluid is absorbed and the faeces become more formed. Also in the colon, sodium is absorbed and the bacteria there produce vitamin K and flatus (Waugh and Grant, 2006).
Passage through the colon is slower than through the small bowel, with mass movements that are triggered by eating, for example. When the faeces reach the end of the colon, they enter the rectum where, in health, more faeces are added until the rectal wall distends and triggers the sensation to defecate. If convenient, the anal sphincters will relax and the faeces are eliminated into the toilet.
Diseased GI tract
Many parts of the GI tract can be problematic in disease. An ulcerated mouth can make ingestion painful and this may result in anorexia or weight loss. A diseased small bowel might result in a reduced absorption capacity, which might lead to deficiencies in vitamins, such as vitamin B12 or water-soluble vitamins. If absorption within the intestines is poor, there will be a shorter transit time and the faeces will be looser and more acidic.
A faster transit time also means that stool volume will be increased, resulting in an increased number of defecations each day. Increased frequency of defecation as well as a more acidic faecal output can result in painful perianal skin.
Another problem associated with loose and liquid stool is the greater risk of faecal incontinence, which can reduce quality of life (National Institute for Health and Care Excellence (NICE), 2014). Rectal bleeding may result from ulceration within the gut.
Inflammatory bowel disease
Inflammatory bowel disease (IBD) is a chronic relapsing and remitting autoimmune disease, causing inflammation of the gastrointestinal tract. IBD is a term that incorporates both ulcerative colitis (UC) and Crohn's disease.
IBD affects approximately 400 000 people in UK—about 1 in every 240 people—and arises in men and women equally. Every 30 minutes, someone in UK is diagnosed with IBD (Crohn's and Colitis UK, 2019). This article focuses on people with UC.
Ulcerative colitis
UC is a long-term condition affecting only the colon. Part or all the inner mucosal lining of the rectum and colon becomes inflamed and ulcerated (Mowat et al, 2011). The condition is characterised by superficial and diffuse mucosal inflammation beginning in the rectum and extending in continuity proximally (Kemp et al, 2018). Patients with UC often experience periods of disease relapse and remission.
According to the Montreal classification, three main areas of the colon are affected by UC (Whayman et al, 2011) (Box 1). At diagnosis, in approximately one in three UC patients, disease is limited to the rectum. In another third, the disease extends to the splenic flexure. The remaining third will have pancolitis (Seibold et al, 2014; Ananthakrishnan et al, 2017).
Montreal classification
E1 | Ulcerative proctitis (inflammation involving only the rectum) |
E2 | Left-sided ulcerative colitis (distal to splenic flexure) |
E3 | Extensive or pancolitis (proximal to splenic flexure) |