This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
There is a remarkable tendency on the part of the blood to maintain an equilibrium as regards its own composition, volume, and density. When a large supply of water is received in the alimentary canal and absorbed by the blood vessels, the blood is momentarily diluted and the blood pressure slightly raised, although the latter effect will depend upon the facility with which the blood vessels are dilated. The blood immediately distributes the water thus absorbed, and the slightly increased pressure, as well as the diluted character of the blood, hastens the elimination of water from the various gland surfaces. The kidneys are particularly sensitive in this respect, and when in normal condition are the great regulators of the composition of the blood and, indirectly, of blood pressure by means of the elimination of water. Perspiration is increased. There is a tendency also for all the tissues to keep abundantly supplied with water; a large amount of aqueous vapour is exhaled from the surface of the lungs, and the digestive secretions are increased in volume. The blood may subsequently become more dense than before, owing to the increased functional activity of different organs.
More nutriment is absorbed and more carbonic acid is exhaled, and urea and uric acid may be slightly increased.
If very large quantities of water, or any fluids consisting chiefly of water, are imbibed throughout a long period, they tend to overwork the kidneys and produce various alterations in the tissues. Practically, however, it seldom happens, excepting in some forms of gastric or intestinal disorder, and other instances mentioned above, that too much water is taken. When drunk in such fluids as beer, or diluted liquors, the resulting disturbances of the system are attributable rather to other ingredients.
Laymen are usually more willing to ascribe obesity to supposed excessive consumption of fluids than to overeating. They often say that they suppose water is " fattening." It is so only in the sense that it promotes tissue change or metabolism and washes away waste matter, not in the sense that it is itself a storage substance, as fat is.