Large Intestine
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.
Defecation
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.
Gut microflora
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.
