This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
Administered in moderate dose and dilute solution, hydrochloric acid has two main effects - (1), it augments the acidity of the gastric juice, and (2), after absorption, it gives rise to extra-formation of chloride of sodium. It is first eupeptic, and then assists haematosis; it also aids the solution of useful substances which would be otherwise inert, such as phosphate and carbonate of lime, metallic iron, oxides, etc. (Rabuteau).
The augmented acidity of the gastric juice, as a rule and within certain limits, improves the digestive power and increases formation of peptones; it is accompanied by greater secretion of saliva, and a sense of warmth at the stomach, but if carried to excess causes irritation. A great part of the interest connected with the study of this acid depends upon the question whether it really forms an integral part of the normal secretion of the gastric glands, and there is certainly a large amount of evidence in the affirmative. Besides the older observations of Prout ("Philosophical Transactions," 1824), of Lassaigne (Journal de Chimie, t. x.), and others, we have the more recent analyses of Schmidt ("Die Verdau-ungssafte," 1852), and of Gautier ("Chimie Appliquee," 1874), who even calculate the proportion of free hydrochloric acid as 3.05 per 1,000. It is not denied that lactic, acetic, and butyric acids may also be found in the gastric juice, as described by Cl. Bernard, Lehmann, and other eminent authorities, but it is almost certain that they result from chemical changes during the digestion of foreign substances. Enderlin, examining the quite fresh gastric secretion of an executed criminal, could find no trace of lactic acid, nor could any organic acid act on fluoride of calcium as gastric juice does (Melsens). Further, Rabuteau claims to have demonstrated by an original process the existence of hydrochloric acid in the secretion of fishes (Comptes Rendus, 1873) and of dogs ("Elements de Therapeutiques, 1875, p. 429). After a fast of twenty-four hours he gave to two animals some bits of tendon, and about an hour afterward divided their medulla. The very acid gastric juice was collected, filtered, saturated with pure quinine, evaporated, and divided into two portions. One part was exhausted by benzine, which can dissolve hydrochlorate and lactate of quinine (though not alkaline chlorides), and on evaporating the benzine, hydrochlorate of quinine was easily recognized. The other part was treated by amylic alcohol, which was then evaporated, and the residue treated by chloroform, which took up a salt proved to be solely hydrochlorate of quinine without trace of lactate. He estimated the quantity of free acid at 2.5 per 1,000 - not very different from the results of Schmidt, and we may fairly presume that the acid is derived from the chloride of sodium circulating in the blood. Lactate of soda is not likely to circulate, inasmuch as it would, very soon after absorption, be changed into bicarbonate. "A free acid always exists in gastric juice, which is usually hydrochloric, rarely lactic acid alone, not unfrequently a mixture of both acids" (McKendrick's "Physiology," 1878).
If, then, hydrochloric acid be the normal acid of the gastric juice, it would seem to be the one most easily assimilated by the stomach, and should be preferred, as a rule, when acid is indicated. It is scarcely necessary to state that if administered undiluted this acid causes irritant poisoning with symptoms similar to those described fully under sulphuric acid.
1 Souligoux attaches much importance to it, as altering galvanic reactions within the system.
As the blood and lymph, and almost all the secretions of the body have an alkaline reaction, it becomes interesting and important to inquire what effect is produced upon such alkalinity by the administration of acids. Some observers, as Eylandt, Wilde, and Gaeth-gens have concluded that any altered relation of acids and bases within the body occurs, if at all, within very narrow limits.1 Hoffmann held that an excess of free acid can pass through the blood to the urine, but this is probably incorrect. Miguel, after giving sulphuric acid, found the alkaline salts of the urine increased in amount - implying that the acid combined with alkali in the blood, and thus removed from that fluid for excretion an unusual proportion of such alkali. Salkowski arrived at a similar conclusion, and Lassar asserted, from analyses of blood, that its alkalinity was much lowered under the use of acids. But the estimation of urinary ingredients does not give a satisfactory answer to the question, and alkalimetry, as applied to the blood, is exceedingly difficult, hence another and an ingenious method of analysis has been adopted by F. Walter (op. cit., April, 1875). Starting from the highly probable supposition that the carbonic acid contained in the blood must be almost wholly in combination with alkalies, and that its amount must therefore be proportional to, and be an index of, the amount of alkali contained in that liquid, he analyzed the gas-contents of blood withdrawn from animals under acid-treatment, as compared with that of animals in a normal condition. Most of his experiments were made with hydrochloric acid, because it required less water for dilution than other acids. From 1 to 3 grammes of acid were given diluted, in three doses, by the stomach tube, in the course of twenty to forty hours. The blood was drawn from veins after decided symptoms of acid-poisoning had set in, and when compared with normal blood it showed a remarkable lessening of the carbonic acid, and (by inference) of combined alkalies. This was especially the case in rabbits (herbivora). While normal rabbit-blood showed an average percentage of 25 volumes CO2, that drawn after 1.22 grammes of acid gave 16, and after 2.44 grammes of acid, only 3 volumes of the gas; this blood was dark, and coagulated with difficulty, but was decidedly, though weakly, alkaline in reaction. In dogs (carnivora) the difference was not so great, but a diminution of about 10 per cent. in the amount of CO2 occurred under the influence of hydrochloric acid. This curious difference between the effects of the acid on the two classes of animals was first pointed out by Salkowski, and it was found that dogs have a certain immunity as to the general symptoms of acid-poisoning, so that they can take much larger doses than the herbivora without ill results. (This has been accounted for by the increased formation of ammonia compounds in the latter class of animals under the influence of the acid, causing its neutralization, to some extent). The experiments of Walter prove, however, that it is possible, by means of the internal administration of acids, to withdraw alkalies from the vital fluids, and this to such an extent as even to cause death from their deprivation.
With regard to the influence of hydrochloric acid on the general circulation, it was noticed by early observers - Boerhaave and others - that even moderate doses accelerate the pulse and cause flushing of the face; and full doses produce some excitement of brain-function, so that the symptoms have been compared to those caused by alcohol (Deutsch). Bobrick took 18 min. diluted with 5 oz. of water, and within half an hour noted an increase of pulse by six beats. This continued for an hour, but was succeeded by a fall of four beats below the normal frequency. He noticed excitement of similar character after internal and external applications of the acid to frogs, and concluded that it was produced through the nervous system, for it did not appear after destruction of the nerve-centres.