Astriffgekts (Lat. astruu/cre, to bind), agents which have the power to contract the animal tissues, diminish the amount of their fluids, and increase their density. They seem to act partly by a direct coagulation of albuminous and gelatinous structures, and partly by diminishing the size of the blood vessels and consequently the amount of blood. An example of the first mode is seen in the formation of leather by tanning, which, however, is a degree of action far beyond what can take place in the living body. Astringents diminish both the absorbing and secreting functions of mucous membranes, and coagulate the secretions already formed. They excite a peculiar feeling of dryness and puckering in the mouth. They are used to check bleeding and excessive discharges from mucous membranes, to promote the healing of ulcerated surfaces, and to restore lax and flabby tissues to their normal firmness. Some of them are absorbed, and, after passing through the blood, are excreted by the kidneys. - The vegetable astringents, nutgalls, oak and hemlock bark, kino, catechu, rhatany, logwood, crane's-bill, uva ursi, win-tergreen, and a large number of others, contain more or less of the different forms of tannic and gallic acids.
The chief mineral astringents are acetate of lead, the different alums, persalts of iron, nitrate of silver, and the sulphates of copper and zinc. Some astringents, as tannic acid, alum, and lead, find a useful application in the arts of dyeing and tanning.