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From biology classes I remember that water from food we eat is mostly 'sucked in' (absorbed into blood) in the colon. I guess the same is true for water we drink. But if I eat something, it will reach the colon after say 4 hours, and if I drink a lot of water, I'd want to go to bathroom earlier than in 4 hours.

So is water absorbed not only in colon?

If it's absorbed in other organs, what % of it would be absorbed in each organ?

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    $\begingroup$ The internet is unanimous on this: some absorption takes place in the stomach, but most of the water is absorbed in the small intestine. Try searching Google: water absorption stomach $\endgroup$ – Alan Boyd Jul 2 '17 at 11:26
  • $\begingroup$ @AlanBoyd - The internet is not unanimous on anything as far as I know. Which site the information is obtained from matters a lot! (I agree with the small intestine, though.) $\endgroup$ – anongoodnurse Jul 2 '17 at 14:03
  • $\begingroup$ @anongoodnurse perhaps unanimous was a little strong, since I didn't check the entire internet (still working on it), but I think you'll find it difficult to find a relevant site that doesn't provide this information. $\endgroup$ – Alan Boyd Jul 3 '17 at 18:21
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Role of the stomach in water absorption:

The absorption of much of the ingested water from a hypotonic food already starts in the stomach due to osmotic reasons.

If we study the mechanism of secretion of HCl by the oxyntic cells of the stomach, HCl secretion favours water osmosis from the blood to the lumen both for protective reasons as well as to maintain the osmotic balance of the gastric juice. This is predominantly the phenomenon in a filled stomach.

However, in the empty stomach, absorption of water is relatively more perceptible in the reverse direction upon water consumption.

Furthermore, the pyloric sphincter is usually open to the passage of water and electrolytes to empty to the duodenum. Thus, they may travel to the large intestine for absorption even before chyle. But this too is dependent on the empty or filled condition of the stomach, in which an empty stomach would pass the water and electrolytes in 5-6 minutes whereas in the filled stomach it could take 15 minutes or even more. Thus these factors may cause the early micturition to ensue.

Role of the small intestine in water absorption:

The small intestine, besides the transcellular pathway (through the cells), also facilitates water transport via the paracellular pathway. Thus it is more permeable to water osmosis than the stomach. Furthermore, the small intestinal segments duodenum, jejunum and ileum are differentially permeable to the absorption of different solutes. This facilitates the simultaneous absorption of water according to the osmotic gradient resulting from this differential absorption of the solutes, maintaining isosmoticity with the blood plasma.

However, this also has the implication that, when the digested products from the stomach (chyme) are hyperosmotic, water may flow from the blood in the villus to the intestinal lumen rather in the opposite direction until chyme is rendered isosmotic to the plasma.

Role of the large intestine in water absorption:

The large intestine is the key centre for water reabsorption rather than the stomach and the small intestine because of the following reasons:

a) It prevents most of the paracellular flow of water and electrolytes because of tight junctions, unlike in the small intestine. This prevents the back flow of electrolytes and water from the chyle to the blood.

b) The proximal colon is where most of the water reabsorption occurs as a consequence of major electrolyte intake. This region is highly active in the reabsorption of Na+ and Cl-.

c) It is mainly involved in concentrating the fecal matter, so reabsorption of water and electrolytes becomes its principle function.

For reference:

Textbook of Medical Physiology by Guyton and Hall

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  • $\begingroup$ Can you recheck your references? Though water absorption does occur from/in the stomach, it is far from the primary site. Also, there is more to the GI tract that the stomach and intestines. There are the duodenum, jejunum, ileum, ascending, transverse and descending colon. $\endgroup$ – anongoodnurse Jul 2 '17 at 14:02

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