This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
The Small Amount Of Fibrin In The Liquor Sanguinis, compared with the quantity of albumen, will attract the student's attention. The proportion of fibrin present varies in different parts of the circulation, and it is not easy to determine the measure of its variation; but there is one circumstance which makes it seem probable that the fibrin is not used for the manufacture of tissue, but is a product resulting from the changes effected in the blood by circulating among the tissues; and that is, that the blood emerging from the liver, after being subjected to the action of that organ, is no longer spontaneously coagulable, and only yields a small amount of fibrin after violent whipping with rods (Beclard).
The fibrin remains fluid while the blood circulates in the body, yet it coagulates almost immediately when withdrawn from its vessels, and still more speedily when stirred than when kept at rest, unless it be kept fluid by reducing the temperature to the freezing point, or by addition of certain foreign matters. This fluidity of the blood within the vessels, and coagulation when removed from them, has long been a puzzle to physiologists, and is not even yet fully explained. But there is one point which is certain, namely, that coagulation is the result of the mixing together of two different substances, both of them albuminoids, and only one of them present in the liquor sanguinis, while the other, which is required in comparatively very small quantity, is contained in the red corpuscles. The fibrinous element of the liquor sanguinis is, on this account, sometimes termed fibrinogen, while the element furnished by the corpuscles, known as paraglobulin, gets also the title of fibrinoplastin, or is said to exercise a fibrinoplastic action. The necessity for the mixing of two elements before coagulation can take place may be illustrated by tying a large vein of an ox at two places, and removing the included portion filled with blood. If this portion of vein be hung up, the contained blood will remain fluid, but the corpuscles will fall to the bottom. If, after that, the vein be opened, so as to allow the pure liquor sanguinis to run out, it will be found that the liquid so obtained will continue fluid for any length of time in any vessel, and however much it may be stirred; but when a few red blood corpuscles are mixed with it, it coagulates at once.
This experiment also illustrates another point, namely, that while contact with foreign bodies causes the red corpuscles to part with their paraglobulin, the wall of a bloodvessel has no such effect. The blood will remain fluid for days in the veins of a sheep's trotter got from the butcher, and yet will coagulate immediately when the veins are ripped open with scissors (Lister). We do not know the explanation of this, and we do not know to what the formation of fibrinogen from albumen in the liquor sanguinis is due; but what has been said is sufficient to show that coagulation is not a vital process, as was once supposed, but is a change of a chemical description.