Approximately 1500 ml of chyme moves into the large intestine through the ileocaecal sphincter each day. The major functions of the large intestine are the formation and storage of faeces consisting of unabsorbed dietary constituents and secreted metabolites. However small but significant amounts of digestion and absorption also occur in the large intestine.
The wall of the large intestine is smooth (i.e. no there are no villi) and the simple columnar epithelial cells lack microvilli. There are however large numbers of Goblet cells that secrete a particularly viscous form of mucus that helps lubricate faeces during defecation.
A. MOTILITY
The walls of the colon are formed into large pockets known as haustra (shown in x-ray opposite). The most common type of movement observed in the large intestine are haustral contractions that occur approximately every 30 minutes and essentially propel the contents of one haustra to the next. The process is largely regulated by local a local reflex involving mechanosensitive neurones of the enteric nervous system that detect the distension produced by the chyme and initiate contraction of the muscularis externa.
The other types of movements that are observed in the colon are are long, powerful waves of contractions that accelerates the movement of its contents towards the rectum. These contractions are known as mass movements and usually occur just after a meal. The control of these mass movements appears to involve a long reflex initiated by activation sensory neurones in the stomach by distension that results in enhanced parasympathetic tone to the colon.
Mass movements are significantly attenuated by opiate analgesics such as morphine and codeine but enhanced by the presence of undigested dietary fibre (or roughage).
B. DIGESTION
Most of the digestion that goes on in the large intestine is performed by the bacteria that live there in a symbiotic relationship with their host. These bacteria digest cellulose and some lipids to release small amounts of fatty acids. However the most noticeable consequence of this digestion is the production of around 500 ml of methane and hydrogen sulphide that form a smelly gas known as flatus. 
C. ABSORPTION
The large intestine plays a fairly minor role in absorption of nutrients. The fatty acids produced by the actions of its bacteria are absorbed and sodium ions are actively transported from the lumen into the bloodstream. The movements of these substances reduces the osmolarity of the chyme and (as in the small intestine) water is absorbed by osmosis. This process results in around 1000 ml of water being reabsorbed and is fairly important in converting the chyme into semi-solid faeces as a precursor to defaecation. Anything that increases in the rate of movement through the large intestine will lead to less water removed and the unpleasant consequences of diarrhoea.
D. DEFECATION
Defecation is the process by which faeces that are stored in the rectum are transferred to the outside world through the anus.
As the rectum is progressively filled by mass movements in the colon this distension is detected by sensory neurones in its walls.
These neurones synapse with parasympathetic preganglionic neurones in the spinal cord that trigger contraction of the rectum and relaxation of the smooth muscle of the internal anal sphincter. This forces the faeces towards the anal canal.
However, because the external anal sphincter is still contracted, defecation can’t occur.
However the increased pressure in the anal canal is detected by sensory neurones that make us aware of the need to defecate.
If it is socially acceptable to do so, we consciously relax the skeletal muscle of the external anal sphincter.
Further contraction of the rectum then expels the faeces which are rapidly flushed in the Brisbane River, out in Moreton Bay and end up on the beaches of the Gold Coast.
In young children the spinal cord pathways that control the external anal sphincter have not yet developed so they exhibit incontinence of faeces.