No more fitting illustration of the paramount importance of mineral salts in the human economy can be found than the influence exercised by chloride of sodium on tissue metabolism and the composition of the fluids of the body. The accumulated results of recent researches go to show that when consumed in quantities above the physiological limit, common salt is retained in the tissues, and that as this cannot take place in the form of crystals of sodium chloride, but only in solution, water must perforce be retained in proportion to the amount of salt. Hence the tissues become more watery and the body-weight increases.

Chloride of sodium is found almost universally throughout nature, and is a constituent of all animal and vegetable tissues, although the latter are relatively poor both in sodium and chlorine, as they contain only a quarter as much as animal organisms. In the human body it displays a preference for the fluids, blood serum containing about 6.75 grams per 1,000 and cerebro-spinal fluid almost as much. Chlorine is also an important constituent of the red blood corpuscles, but in them it is found in association with potassium, there being as much as 3 67 grams of chloride of potassium in the erythrocytes of an adult. Chlorine is found less abundantly in the tissues, although the body of a man weighing 70 kilograms contains on an average about 200 grams of chloride of sodium, with somewhat less chloride of potassium, the former being found chiefly in the fluids and the latter mainly in the tissues, especially the blood corpuscles, muscles, and the nervous tissue.

The quantity consumed in the food varies greatly in different nations, as much as 20 grams daily being quite common in most European countries. This is rapidly excreted in normal circumstances by the kidneys, and in the sweat, tears, and faeces, whilst pathologically it appears in the expectoration, vomited matter, and diarrhoeal stools. During health the body excretes the chlorides of the food with great rapidity, and so the proportion in the tissues and the fluids remains fairly constant.

As the chlorides of the body circulate in a state of solution, all gains or losses of chlorides involve a gain or loss of water, and as chloride of sodium passes through the body without undergoing any chemical change, it is thus in the position of being able to control osmotic changes within the economy. Hence it follows that the blood and tissue fluids may be isotonic, hypotonic, or hypertonic in relation to each other, in accordance with the content of chloride of sodium. In addition to this all-important function, it is stated to exercise an antitoxic influence, protecting the tissue cells against the action of poisons.

Only a small daily dose is actually necessary to renew the quantity contained in the body. Bichat says this should not exceed 2 grams, although in health it may sometimes be of advantage to take a little more, and von Noorden suggests 3 or 4 grams as the minimum amount. When less is habitually excreted than is ingested, then retention takes place and a state of hyperchlorination exists. When more is excreted than is ingested, we have the somewhat rare condition of hypochlorination. This can only be induced slowly by a dechlorinated diet, and is accompanied by loss of weight, emaciation, polydipsia, and polyuria.