The colon has a very high absorptive capacity; for every 2 litres of water entering the colon, the residual water in the stools will be less than 200 ml. The flow of chyme from the ileum to the colon in healthy human beings is 1-2 litres.h-1. The colon is capable of absorbing up to 4 L of water per day and can withstand an infusion rate of 6 ml.min-1 before there is any increase in faecal water8 9. The large capacity of the colon to absorb fluid may, however, be overwhelmed by a large fluid input and unabsorbed solutes, such as bile acids, fatty acids, or carbohydrates can also impair this adaptive capacity, possibly resulting in diarrhoea. Absorption of water and sodium is negligible from the rectum. Solids are consolidated to 200-300g of wet material which is equivalent to 30-40 g of dry matter, which is mainly bacterial in origin but contains undigested organic matter and fibre.
The colon is responsible for the absorption of sodium ions, chloride ions and water from the lumen in exchange for bicarbonate and potassium ions. The absorption of sodium is an active process and involves its diffusion across the apical membrane of epithelial cells via water filled channels. Sodium absorption in the colon is enhanced by the hormone aldosterone. A sodium-potassium exchange pump system in the baso-lateral membrane then moves sodium against steep concentration (14 mM to 140 mM) and electrical (-30 mV to +20 mV) gradients into the intercellular space. This movement of sodium creates an osmotic gradient which causes a net movement of water from the colonic lumen via the epithelial cells, through the tight junctions between epithelial cells into the intercellular spaces.
In healthy individuals, approximately 10 mEq of potassium enters the colon each day whilst 5 to 15 mEq are lost in the faeces during the same time period. Potassium secretion is determined by the luminal concentration of potassium, with concentrations of below 15 mEq leading to net secretion. This is accomplished by passive movement of potassium ions along an electrochemical gradient from plasma to lumen, and is facilitated by the tight junctions between epithelial cells which are highly permeable to potassium ions. The sodium-potassium pump in the basolateral membrane of epithelial cells creates a high intracellular potassium concentration (80 mM), of which only a small proportion is lost to the colonic lumen, since the apical epithelial membrane is essentially impermeable to potassium.
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