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.
Textbook of Medical Physiology by Guyton and Hall