Faecal elimination is the expulsion of the residues of digestion (faeces) from the digestive tract (Patton [190]). The act of expelling faeces is called defaecation.

This section considers the normal structure and function of the bowel, which includes the small and large intestines (Figure 6.2). The small intestine begins at the pyloric sphincter of the stomach; it then coils through the abdomen and opens into the large intestine at the ileocaecal junction. It is approximately 6 m in length and is divided into three segments: the duodenum (25 cm), jejunum (2.5 m) and ileum (3.5 m) (Patton [190]). The mucosal surface of the small intestine is covered with finger‐like processes called villi, which increase the surface area available for absorption and digestion. A number of digestive enzymes are secreted by the small intestine (Tortora and Derrickson [237]).

Movement through the small bowel is divided into two types – segmentation and peristalsis – and is controlled by the autonomic nervous system. Segmentation refers to the localized, concentric contraction of the intestine, aided by constant lengthening and shortening of the villi; this results in mixing the intestinal contents and bringing particles of food into contact with the mucosa for absorption. Once the majority of a meal has been absorbed through this process, intestinal content is then pushed along the small intestine by repeated peristaltic wave‐like actions. Intestinal content usually remains in the small bowel for 3–5 hours (Tortora and Derrickson [237]).

The total volume of fluid, including ingested liquids and gastrointestinal secretions, that enters the small intestine daily is about 9.3 L. The small intestine is responsible for absorbing around 90% of the nutrients, electrolytes and water by diffusion, facilitated diffusion, osmosis and active transport (Tortora and Derrickson [237]). Water is able to move across the intestinal mucosa in both directions but is influenced by the absorption of nutrients and electrolytes. As various electrolytes and nutrients are actively transported out of the lumen, they create a concentration gradient, promoting water absorption, via osmosis, in order to maintain an osmotic balance between intestinal fluid and blood. This ultimately leads to only about 1 L of effluent passing through into the colon (Patton [190], Tortora and Derrickson [237]).

From the ileocaecal sphincter to the anus, the colon is approximately 1.5–1.8 m in length and 4–6 cm in diameter. Its main function is to eliminate the waste products of digestion by the propulsion of faeces towards the anus. In addition, it produces mucus to lubricate the faecal mass, thus aiding its expulsion. Other functions include the absorption of fluid and electrolytes (including sodium and potassium), the storage of faeces and the synthesis of vitamins B and K by bacterial flora (Patton [190], Tortora and Derrickson [237]).

Faeces consist of the unabsorbed end‐products of digestion: bile pigments, cellulose, bacteria, epithelial cells, mucus and some inorganic material. They are normally semi‐solid in consistency and contain about 70% water (Tortora and Derrickson [237]). The colon absorbs about 2 L of water in 24 hours, so if faeces are not expelled they will gradually become hard (due to dehydration) and more difficult to expel. If there is insufficient roughage (fibre) in the faeces, colonic stasis occurs, which leads to continued water absorption and further hardening of the faeces. Faeces will, therefore, vary in consistency, as illustrated in the Bristol Stool Chart (Figure 6.3).

The movement of faeces through the colon towards the anus occurs via mass peristalsis, a gastrocolic reflex initiated by the presence of food in the stomach. Mass peristalsis begins at the middle of the transverse colon and quickly drives the colonic contents into the rectum. This mass peristaltic movement generally occurs three to five times a day (Perdue [198]). In response to this stimulus, faeces move into the rectum (Tortora and Derrickson [237]). This rectal distension triggers the desire to defaecate, also known as the ‘call to stool’.