Any large loss of fluid to the body, as by sweating, diarrhea, hemorrhage, gives rise to an intense thirst that has its natural reaction in increased intake of water by the mouth. On the other hand, the property possessed by the alimentary canal of absorbing water and weak saline fluids contained in its interior is very little influenced by the state of depletion, or otherwise, of the water contained in the body. It is practically impossible, in the opinion of Starling (controverted later), however large the quantities of fluid ingested, to evoke the production of fluid motions, the greater part of the ingested fluid being absorbed on its way through the alimentary canal. Thus a man may keep himself in perfect health and maintain the water content of his body constant whether he take one liter (half a gill over a quart) or six of water daily. The whole process of regulation, apart from that determined by the appetite, is carried out at the other end of the cycleby the kidneys. As concerns absorption of water, there is no chemical solidarity between the alimentary (intestinal) surface and the rest of the body. Whenever water is presented to this surface it is absorbed and passed into circulation.
The absorption of water in the stomach may be regarded as nil. The chief absorption occurs in the small intestine. The intestinal contents at the ileocecal valve (the junction point of the small intestine and colon) contain relatively nearly as much water as they do at the upper part of the jejunum (middle section of the small intestine). Their absolute bulk, however, is much smaller, so that only a small proportion of the water that has been taken in by the mouth remains to be absorbed in the colonan amount probably much less than that which has been added to the contents of the small intestine in the form of secretion by the stomach, liver, pancreas and intestinal tubules.
P. B. Hawk, a physiological chemist, has made some careful studies of the effect of water drinking on health, extending back for upwards of two decades, which he has published from time to time under the general title Studies in Water Drinking. He was particularly anxious to discover if the popular notion that water should not be drunk in quantity during meals for the reason that it dilutes the gastric juice is a sound one. His conclusion in one of his first papers, published in the Journal of Experimental Medicine in 1910, was that this notion is wholly erroneous. He then concludes that what took place in his experimental water drinking, greatly in excess of ordinary practice, up to 1 liter during the progress of each meal (one liter 1.056 U. S. quarts), as far as the digestive mechanism was concerned was that the entrance of the large amount of water into the stomach stimulated the flow of gastric juice, thus facilitating the digestion of the protein constituents of the diet. The strongly acid chyme (semi-fluid stomach contents) passing into the intestine caused a pronounced stimulation of the pancreatic secretion (discharged into the small intestine from the pancreas). The products of incomplete gastric digestion were consequently more rapidly and thoroughly digested. The coursing of the large volume of water through the small intestine so stimulated the absorption that the products of intestinal digestion were absorbed with increased rapidity and thoroughness. The intestinal bacteria, therefore, found less pabulum at hand and in consequence these micro-organisms were less nourished than usual, a fact demonstrated by a decrease in the output of bacteria and of bacterial nitrogen. The effect of this stimulated digestion, absorption and assimilation was noted at once in the increased body weight of the subject’
During the preliminary period the subject excreted 105 grams of fresh feces a day. Under the influence of the water drinking the daily output was reduced to 74 grams, whereas the final period was 97 grams a day. Calculated to a dry basis there were 23 grams of dry excretion per day during the preliminary period against 17.5 grams for the water period and 20.4 for the final period (this final period was the 17th, 18th and 19th days, when the normal nitrogen elimination by the urine had been reestablished)
It has been calculated that about 80 fluid ounces, 2/ quarts, of water pass the ileo-cecal valve into the colon every 24 hours.
Hawk says in another paper, 1911, that the amount of water in the colon output during the water period (a liter with each meal) was less than during the preliminary or final periods, showing that even with the large amounts of water sent into the intestine the amount absorbed was actually more than the excess administered. The findings obtained in this experiment, he says, show that during the periods when large amounts of water were taken with meals the total amount of dry matter and moisture was less than without these unusual amounts of water, and that more or less permanently better utilization of carbohydrates accompanied the water drinking.
An interesting feature in the large intestine is the marked absorption of water. In the small intestine water is absorbed, no doubt, in large quantities, but its loss is evidently made good by osmosis, or the secretion of water into the intestine (through the intestinal wall), since the contents at the ileocecal valve are “quite as fluid as at the pyloris” (discharging end of the stomach). In the large intestine the absorption of water is not compensated by a secretion. The material loses water rapidly while in the ascending colon, and before it reaches the descending colon it has acquired the ordinary consistency.