The large intestine comprises the caecum, ascending, transverse, descending and sigmoid colon, rectum and anal canal (Fig. 41a). It is approximately 1.2 m in length and between 6 and 9 cm in diameter. Approximately 1.5 L of chyme enters the large intestine per day through a sphincter called the ileocaecal sphincter. Distension of the terminal ileum results in the opening of the sphincter and distension of the caecum causes it to close, thereby maintaining the optimum rate of entry to maximize the main function of the large intestine, which is to absorb most of the water and electrolytes. The initial 1.5 L is reduced to about 150 g of faeces consisting of 100 mL of water and 50 g of solids.
The muscle layers of the large intestine are slightly different from those found in the rest of the gastrointestinal (GI) tract. It still has a powerful circular muscle layer, but its longitudinal muscle layer is concentrated into three bands called the taeniae coli. The caecum and the ascending and transverse colon are innervated by para sympathetic branches of the vagus; the descending and sigmoid colon, rectum and anal canal are innervated by parasympathetic branches of the pelvic nerves from the sacral spinal cord. These parasympathetic fibres innervate the intramural plexuses. The sympathetic nerves via the superior mesenteric plexus, and via the inferior mesenteric and the superior hypogastric plexuses, innervate the proximal and distal parts of the large intestine, respectively. The rectum and anal canal are innervated via the inferior hypogastric plexus. Stimulation of the parasympathetic fibres causes segmental contraction, whereas stimulation of the sympathetic fibres stops colonic activity. The internal and external anal sphincters usually keep the anal canal closed and are controlled both reflexly and voluntarily. The internal sphincter is made up of circular smooth muscle, and the more distal external sphincter is composed of striated muscle which is innervated by motor fibres from the pudendal nerve.
Movement of the chyme through the large intestine involves both mixing and propulsion. However, as the main function is to store the residues of food and to absorb water and electrolytes from it, the movements are slow and sluggish (approximately 5–10 cm/h). Chyme usually remains in the colon for up to 20 h. The mixing movement is called haustration and the sac-like compartments in the colon caused by this process are called haustra. The contents of the haustra are often shunted back and forth from one to another in a process called haustral shuttling. This aids the exposure of chyme to the mucosal surface and helps the reabsorption of water and electrolytes. In the distal parts of the colon, the contractions are slower and less propulsive, and eventually the faeces collect in the descending colon.
Several times a day there is an increase in activity within the colon, in which there is a vigorous propulsive movement, the mass movement. This results in the emptying of a large proportion of the content of the proximal colon into the more distal parts. This mass movement is initiated by a complex series of intrinsic reflex pathways started by the distension of the stomach and duodenum soon after the consumption of a meal.
When a critical mass of faeces is forced into the rectum, the desire for defecation is experienced. This sudden distension of the rectum walls produced by the final mass movement leads to a defecation reflex. This reflex comprises a contraction of the rectum, relaxation of the internal anal sphincter and, initially, contraction of the external anal sphincter. This initial contraction is soon followed by a reflex relaxation of the sphincter initiated by an increase in the peristaltic activity in the sigmoid colon and pressure in the rectum. The faeces are then expelled. This reflex relaxation can be overridden by higher centre activity, leading to a voluntary control over the sphincter which can delay the expulsion of faeces. The prolonged distension of the rectum then leads to a reverse peristalsis, which empties the rectum into the colon and removes the urge to defecate until the next mass movement and/or a more convenient time.
The chyme that enters the large intestine is isotonic; however, in the colon more water than electrolytes is absorbed, leading to water being absorbed against a concentration gradient. The process is con- trolled by Na+–K+ ATPases located in the basolateral and lateral membranes of the epithelial cells that line the walls (Fig. 41b). The mucosal surface of the large intestine is relatively smooth with no villi (only microvilli); however, crypts are present and the majority of cells are columnar absorptive cells with a large number of mucous- secreting goblet cells. Na+ is extruded by the membrane pumps into the extracellular spaces. Tight junctions at the luminal side of the cells prevent the diffusion of Na+ and Cl− from the extracellular spaces into the lumen; this leaves a hypertonic solution close to the lumen, causing water to diffuse from the contents of the lumen. The electrolytes are absorbed by a variety of mechanisms similar to those described for the small intestine. Essentially, there is a net movement of K+ and HCO3 ions from the blood into the large intestine because of the potential difference set up by the asymmetrical absorption of Na+ and Cl− across the cell wall.
Most of the bacteria that are present in the GI tract are found in the large intestine, because the acid environment in the rest of the tract destroys most of the so-called microflora. Ninety-nine per cent of the bacteria are anaerobic and most are lost in the faeces (which is said to contain 1011 bacteria per gram). The bacteria are involved in the synthesis of vitamins K, B12, thiamine and riboflavin, the breakdown of primary to secondary bile acids and the conversion of bilirubin to nonpigmented metabolites, all of which are readily absorbed by the GI tract. The bacteria also break down cholesterol, some food additives and drugs.