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.
The observable physiological effects of astringents are, beside the general condensation of tissue referred to, or rather as a part or result of it, shortening of fibres; diminished caliber of the arteries, veins, capillaries, absorbents, and ducts; diminished secretion, exhalation, and absorption; constipation of the bowels; and increased firmness along with contraction of the pulse. The blood becomes more coagulable, in consequence, probably, of the same influence exerted by them on the organized constituents of this fluid as on the solids. It is supposed also to be less disposed to putrefaction after death. The astringents are said to increase the appetite, and invigorate digestion. This effect they undoubtedly have, in debilitated states of the function, connected with relaxation of tissue. It is reasonable to suppose that, even in health, with a very moderate degree of their peculiar influence, they may produce some slight increase of the functions. Their effect is to bring the molecules more closely together; which may thus be rendered capable of a more energetic vital reaction. But. whatever may be the case with the perfectly pure astringents, it is certain that many of the medicines ranked in this this do exercise a tonic influence; and for this reason, that they unite positive tonic powers with their astringent property. Such is the case with the vegetable astringents, which, beside their characteristic ingredient, not unfrequently contain a bitter principle also, and with the preparations of iron, which are essentially and powerfully tonic, while they are in some degree astringent.
It must be obvious, upon a little consideration, that, though astringents are stimulant to the organic contractility, they may really prove sedative to the healthy functions, when employed too freely, or continued too long. The digestive function is necessarily impaired in consequence of the diminished secretion of gastric juice, the restrained peristaltic-movement of the stomach and bowels, and the impeded absorption. As a result of this defective digestion, if from no other cause, the circulation is enfeebled, nutrition suffers, emaciation takes place, and a general reduction is experienced in the forces and functions of the system.
The above results flow from an excess of the proper astringent influence. But a still greater abuse of this class of medicines leads to other and very different effects. When applied to delicate surfaces in great excess, instead of acting simply as astringents, they become irritants. In the denuded skin they excite inflammation, and, taken into the stomach, cause gastric and intestinal pains, nausea, vomiting, and sometimes diarrhoea; their astringent influence being either prevented or overwhelmed by the irritation.
As already stated, some of them, in very great excess, overcome the vital resistance of the tissue to their chemical affinity for one or more of its constituents; and disorganization, with the death of the part, ensues. It is said that, under such circumstances, putrefaction does not readily take place, being prevented in part by the previous expulsion of the liquids, but probably in chief by a direct preservative or antiseptic effect, arising from the union of the astringent with the animal principles