This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
These are substances which cause contraction of the tissues to which they are applied and lessen secretion from mucous membranes.
Vegetable substances containing these acids, e.g.Catechu.
Astringents are usually divided into local and remote.
With the exception of gallic acid and ergot they all coagulate or precipitate albumen. Dilute mineral acids do not coagulate albumen, but precipitate albuminous substances from the alkaline fluids in which they are naturally dissolved in the body.
When applied to a surface from which the epidermis has been removed, the other astringents combine with the albuminous juices which moisten this surface, as well as with the tissues themselves, and form a pellicle more or less thick and dense, which in some measure protects the structures beneath it from external irritation, at the same time that they cause the structures themselves to become smaller and more dense. On a mucous membrane they have a similar action, and they lessen its secretion. It was formerly supposed that their action was partly due to their causing the blood-vessels going to a part of the body to contract, thus lessening the supply of fluid to it, as well as to their effect on the tissues themselves. But experiment has shown that, while nitrate of silver and acetate of lead possess this power, perchloride of iron and alum do not, and that tannic and gallic acids actually dilate the vessels. The astringent action of these latter drugs must therefore be exerted upon the tissues. (Rossbach.)
Astringents may be employed locally in various forms. In the solid form, as a powder, or in various preparations, such as lotions, ointments, plasters, glycerines, etc, they are applied, especially the metallic astringents, to wounds and ulcers for the purpose of reducing the size and increasing the firmness of exuberant granulations, as well as of protecting the surface by forming a pellicle over it. They are used to lessen congestion and diminish the secretion of the various mucous membranes - as a lotion to the eye and mouth; as a gargle or a spray to the throat; in the form of an injection to the nose, urethra, and vagina; and of suppositories to the rectum. Administered internally, several astringents have a powerful effect in checking diarrhoea, and certain of them may have a local action upon the stomach and intestines.
The remote action of such astringents as acetate of lead and gallic acid, when absorbed into the blood, in lessening haemorrhage, is made available in the treatment of haemoptysis, haematemesis, haematuria, and loss of blood from other parts of the body.