This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
It has long been known that iron is a normal constituent of the blood; and comparatively recent researches have shown that it exists exclusively in the colouring matter of the red corpuscles, of which it forms an essential constituent, and, as is generally believed, in definite proportion.* Without it, the red corpuscles could not exist, nor life be supported. In what state of combination it is contained in the colouring matter, has not been ascertained. It is certainly held by a powerful affinity; for it is altogether insensible to the tests by which iron can be detected in all other combinations. The probability is, that it is united, in an elementary condition, either directly with the other elements of the colouring matter, or with some peculiar organic principle, having for it an affinity beyond that of any other body in nature, and capable of being overcome, through chemical agency, only by the destruction of that principle. Hence, it may be found either by burning blood, in which case it is left in the ashes, or by the instrumentality of chlorine, which destroys the animal principle referred to. The part which iron acts in the economy is wholly unknown. The theory of Liebig, which supposes it to be a carrier of oxygen from the lungs to all parts of the system, entering the respiratory organs as a protocarbonate, there becoming sesquioxidized with the escape of the carbonic acid, and afterwards carried away with the arterial blood to supply oxygen to all the functions, and be reconverted into the protocarbonate, though very beautiful, has not been authenticated, and at present must be regarded as at best a plausible conjecture. That iron is the real colouring matter of the blood, has long been regarded as highly probable; nor did the discovery of hematosin as a distinct principle serve in any degree to invalidate this opinion; but if it be true, as asserted by M. Sanson, that hematosin can be deprived of iron without losing its colour, the opinion must be abandoned. Nevertheless, M. Le Canu, on repeating the process of M. Sanson, did not succeed in obtaining the same result; so that it may be considered as a yet unsettled point, whether iron is or is not the colouring principle. (Quevenne, Arch, de Physiol, Oct. 1854, p. 147.) That it has the property, when taken internally, of increasing the redness of the blood, is a familiar fact. A reference to the above physiological points seemed to be a necessary introduction to the consideration of iron as a therapeutic agent.
* According to M. Le Canu, about 1 part of it exists in 4400 of blood, and 7 parts in 100 of hematosin. For the blood, the proportion is necessarily variable, as that of hematosin itself is variable; for the hematosin, it is fixed, the combination being definite (Archives de Physiol., de Therap., et d' Hyg. de Bouchardat, Oct. 1854, pp. 144 and 147).
Whether metallic iron has any direct influence on the system is not certainly known; for, though very decided effects follow its introduction into the stomach, it is supposed that the metal is oxidized and combined with an acid before it operates. Nevertheless, it is by no means certain that the finely powdered metal may not find, among the constituents of the gastric juice, the organic principle with which it is combined in hematosin, and, uniting with this, enter into the blood at once in a state suitable for the part it has to play in the formation of the red corpuscles. This idea does not by any means preclude that of its absorption, and existence in the serum in other forms of combination. In fact, it serves to explain the double action of the metal; that of a tonic in the blood, like any other absorbed medicine of the same class; and that of a reconstructive agent, serving to increase the red corpuscles, by affording an essential constituent in a due state of preparation.
The operation of the chalybeates, or preparations of iron, must be considered in reference, first, to their local effects upon the tissue with which they are primarily brought into contact; and secondly, to their effects upon more or less distant parts, or on the system generally.
1. In regard to their local operation, when in any degree soluble, or capable of being rendered so by the reagencies to which they are exposed, they act as excitants and astringents; the degree in which they produce such effects being very different in the different preparations, and in some measure proportionate to their solubility. If used in small doses, they produce on the stomach only that degree of excitation which is called tonic, increasing the appetite, and invigorating digestion; while their astringent operation is evinced by a tendency to constipation, and the smaller, dryer, and harder stools, which are apt to follow. If carried too far, they cause irritation of the stomach and bowels, with a sense of heat, weight, or uneasiness in the epigastrium, sometimes nausea and vomiting, and not unfrequently griping pains and diarrhoea. Some of the soluble salts, as the sulphate, sesquinitrate, and chloride, are capable of producing even dangerous if not fatal inflammation, and thus acting as irritant poisons. The local effects of the chalybeates, applied externally, either to the mucous surfaces or the skin, are of the same nature; but, in the latter instance, the cuticle serves as a protection, and the irritant influence is much less felt.
A phenomenon resulting from the use of iron, though not a part of its physiological operation, is the black colour of the stools which almost invariably attends it. This is caused probably in part by the combination of the sesquioxide with the tannic acid often contained in the food, as in tea and coffee for example, and in part by the formation of the sul-phuret of iron through reaction with suphuretted hydrogen in the bowels, or one of the soluble sulphurets. A knowledge of this fact is important; as otherwise wrong inferences might be drawn from the colour of the stools, and lead to improper practice. Instances of this I have myself witnessed. Not unfrequently also the preparations blacken the tongue, in consequence probably of the simultaneous use of astringent substances; and care must be taken not to confound this appearance with the blackness of tongue resulting from disease.