An adult human takes in roughly 1 to 2 litres of dietary fluid every day. In addition, another 6 to 7 litres of fluid is received by the small intestine as secretions from salivary glands, stomach, pancreas, liver and the small intestine itself. By the time the ingesta enters the large intestine, approximately 80% of this fluid has been absorbed. The absorption of water is absolutely dependent on absorption of solutes, particularly sodium.
Within the intestine, there is a proximal to distal gradient in osmotic permeability. Further down the small intestine, the effective pore size through the epithelium decreases, hence the duodenum is much more "leaky" to water than the ileum and the ileum more leaky than the colon. However, the ability to absorb water does not decrease, but water flows across the epithelium more freely in the proximal compared to distal gut because the effective pore size is larger. The distal intestine actually can absorb water better than the proximal gut. The observed differences in permeability to water across the epithelium is due almost entirely to differences in conductivity across the paracellular path as the tight junctions vary considerably in "tightness" along the length of the gut.
Regardless of whether water is being secreted or absorbed, it flows across the mucosa in response to osmotic gradients. In the case of secretion, two distinct processes establish an osmotic gradient that pulls water into the lumen of the intestine. Firstly, the increases in luminal osmotic pressure resulting from influx and digestion of food cause water to be drawn into the lumen. Chyme when passed into the small intestine from the stomach is slightly hyperosmotic, but as its macromolecular components are digested, osmolarity of that solution increases dramatically. For example, starch which is a large molecule, will only contributes a small amount to osmotic pressure when intact. As it is digested, thousands of molecules of maltose are generated, each of which is as osmotically active as the parent molecule. Thus, as digestion proceeds the osmolarity of the chyme increases dramatically and water is pulled into the lumen. Then, as the osmotically active molecules are absorbed, osmolarity of the intestinal contents decreases and water is then reabsorbed.
Secondly, crypt cells actively secrete electrolytes, which leads to water secretion. The apical or luminal membrane of crypt epithelial cells contain a cyclic AMP-dependent chloride channel known also as the cystic fibrosis transmembrane conductance regulator or CFTR because mutations in the gene for this ion channel result in the disease cystic fibrosis. This channel is responsible for secretion of water. Elevated intracellular concentrations of cAMP in crypt cells activate the channel which results in secretion of chloride ions into the lumen. The increase in concentration of negatively-charged chloride anions in the crypt creates an electrical potential which attracts sodium, pulling it into the lumen across the tight junctions. The net result is secretion of sodium chloride into the crypt which creates an osmotic gradient across the tight junction, hence water is drawn into the lumen. Abnormal activation of the cAMP-dependent chloride channel in crypt cells has resulted in the deaths of millions of people. Several types of bacteria produce toxins, the best known of which is the cholera toxin, that strongly and often permanently activate the adenylate cyclase in crypt enterocytes. This leads to elevated levels of cAMP, causing the chloride channels to essentially become stuck in the "open" position". The result is massive secretion of water which produces the classic symptom of severe watery diarrhoea.
The most important process which occurs in the small intestine which makes absorption possible is maintenance of an electrochemical gradient of sodium across the epithelial cell boundary of the lumen. To remain viable, all cells are required to maintain a low intracellular concentration of sodium. In polarized epithelial cells like enterocytes, a low intracellular sodium concentration is maintained by a large number of sodium pumps or Na+/K+ ATPases embedded in the basolateral membrane. These pumps export 3 sodium ions from the cell in exchange for 2 potassium ions, thus establishing a gradient of both charge and sodium concentration across the basolateral membrane. In rats, there are about 150,000 sodium pumps per small intestinal enterocyte, which allows each cell to transport about 4.5 billion sodium ions out of each cell per minute7. This flow and accumulation of sodium is ultimately responsible for absorption of water, amino acids and carbohydrates. The transport of water from lumen to blood often occurs against an osmotic gradient, allowing the intestine to absorb water into blood even when the osmolarity in the lumen is higher than osmolarity of blood. The proximal small intestine functions as a highly permeable mixing segment, and absorption of water is basically isotonic. That is, water is not absorbed until the ingesta has been diluted to just above the osmolarity of blood. The ileum and especially the colon are able to absorb water against an osmotic gradient of several hundred milliosmoles.
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