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In this article we will discuss about the absorption of water and salts in human body.
Absorption of Water:
Very little water is absorbed from the stomach. Water introduced into the stomach almost immediately passes into the small intestine which is the chief seat of water absorption. The intestinal contents near the ileocecal valve contain the same proportion of water as the upper part of jejunum. But their absolute amount is much smaller. The large intestine absorbs all the residual water and forms solid stool.
Absorption of water is not influenced by total water content of the body. There is no limit to the amount of water absorbed. Large quantities of fluid may be ingested without developing liquid stool, showing complete absorption. If more water is taken, more will be excreted through the kidney. If less is taken, thirst will appear. So that absorption depends upon intake, whereas intake is guided by the water content of the body.
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The main physico-chemical forces which help water absorption are hydrostatic pressure, endosmosis and osmotic pressure. Although the hydrostatic pressure and endosmosis do not play any significant part but the osmotic pressure plays an important role in the absorption process.
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It is interesting to note that water absorption is influenced by the presence of salts in it. Pure distilled water is absorbed only up to 59%, but if given in the form of increasing strengths of saline the rate of absorption gradually rises and becomes 95% with a solution containing 0.4 – 0.7% NaCl. But beyond that strength the rate falls, because, increased osmotic pressure draws out fluid from the wall.
It is also to be noted here that, along with water, salts are also absorbed. It is known that for the excretion of salts through the kidney a minimum volume of water is required. So it is reasonable to think that for the absorption of salts from the intestine a minimum volume of water is also necessary. Probably this principle will explain why more water is absorbed in presence of salts. Water ad salts are absorbed passively and actively.
During passive absorption, water and small water-soluble substances traverse the intestinal mucosa along osmotic or electrochemical gradient, the energy moving them being derived from whatever processes established the gradients in the first place. Besides this, water and sodium can also be absorbed against osmotic and electrochemical gradient (active transport). The mechanism is still not known.
Absorption of Salts:
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Stomach and large intestine absorb a little amount, but small intestine is the main site for salt absorption. Salts are chiefly absorbed through the blood stream of the portal system. Salts vary in their rate of absorption.
Wallace and Cushny have divided the salts into four main groups according to their rate of absorption:
i. Sodium chloride, bromide, iodide, formate, acetate, propionate, butyrate, valerianate, caprate.
ii. Nitrate, lactate, salicylate, etc.
iii. Sulphate, phosphate, ferrocyanide, caprylate, malonate, succinate, malate, citrate tartrate
iv. Oxalate and fluoride.
The first group is most rapidly absorbed and the fourth group is not absorbed at all. Moreover, the members of the third and fourth groups —all form insoluble compounds with calcium. This may be one of the reasons for their slow rate of absorption.
Although salt absorption depends upon physical factors, yet it is probable that the intestinal epithelium has some power of differential absorption. The salts are selectively absorbed according to their degree of requirement by the body.
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Experimental evidence indicates that the intestinal epithelium does not behave as a passive membrane during absorption of salts. On the other hand, it performs considerable amount of work. The sodium ions after being absorbed inside the epithelial cells produce an electric potential which helps in the movements of chloride ions. Due to this electric potential, chloride ions may move inside the epithelial cells, against the high osmotic pressure Calcium is absorbed in presence of vitamin D, protein and lactose.
Calcium absorption is effected by active transport mechanism which uses high energy phosphate generated by aerobic metabolism. Iron is absorbed in combination with a special protein called ferritin. Absorption is promoted by ascorbic acid which keeps it reduced. The duodenum and upper jejunum are the sites of iron absorption. Iron absorption is regulated by the amount of iron stored in the body.
Phosphate ions are absorbed by all segments of the small intestine but most effectively from the ileum.
