In speaking of the chemical relationship of the plasma (see p. 222), the formation of fibrin has been mentioned as the essential item in coagulation, and the relation of fibrin to its probable precursors has been discussed. If the points there explained be borne in mind, and the presence of the corpuscles be taken into account, the various characteristics of the clot which forms when blood is shed into a vessel can be easily understood, and should require no further description.

The great importance of the coagulation of the blood in arresting bleeding, and in certain pathological processes, makes it expedient, however, to consider more closely the steps of the process and to inquire into the various circumstances which facilitate its occurrence after the blood is shed, as well as in the living vessels.

Coagulation Of Shed Blood

Before the formation of a perfect clot, blood may be seen to pass through three stages: i, viscous; 2, gelatinous; 3, contraction of clot and separation of serum.

The first stage is commonly very short, and in thin layers of blood passes immediately into the second. In cold weather considerable quantities of blood, if contained in deep vessels, take a much longer time to stiffen, so that the first stage may occupy from one minute to some hours.

The second stage, when the mass has been turned into a firm jelly, may be arrived at within the varying limits just named, and occupies a corresponding period: only a few minutes if the mass be small, spread out or shaken, but many hours if a large quantity be kept motionless and cool.

The third stage therefore begins sometimes as soon as ten to fifteen minutes, but generally after some hours. Clear drops of serum appear about the clot. After several hours this contracts until it forms but a comparatively small mass floating in the serum. If the jelly-like clot be disturbed, the serous fluid makes its appearance much sooner than the time just stated.

During the formation of the clot under ordinary circumstances the corpuscles are entangled in the meshwork of fibrin, so that the gelatinous mass has throughout a dark-red color.

If the coagulation takes place slowly - as it does in very cold weather, in horses' blood, or in human blood if removed from a person during fever - then the heavier red corpuscles have time to subside to the lower layers of the clotting plasma, while the white cells are caught in the meshes of the fibrin and remain in the upper layer of the clot, which then has the pale color familiar to the physician in the old days of bleeding as the "buffy coat," or crusta phlogistica. This buffy coat contains a greater proportion of the elastic fibrin and soft white cells than the rest of the clot, and encloses but few red corpuscles, therefore the fibrin can contract more completely in this upper layer than in the deeper part of the clot which includes the red corpuscles. The effect of this is, that the upper surface becomes concave, and a "cupped" clot is formed. The contraction of the clot proceeds for days, and in order to see the characters described above, the blood should be kept in a cool place and perfectly motionless.

Reticulum of Fibrin Threads after staining has made them visible.

Fig. 109. Reticulum of Fibrin Threads after staining has made them visible. The network (3) appears to start from granular centres (a). (Ranvier).

The contraction of the fibrin and separation of the serum can be made to take place much more quickly by gentle agitation causing the ends of the fibrin threads to separate from the sides of the vessel, but by thus disturbing the clot during its formation, the corpuscles are displaced and escape into the serum, which is then stained and cannot be seen in its clear, transparent state.

If brisk agitation with a glass rod - or better a bundle of twigs - be commenced the moment the blood is drawn, the fibrin is formed more rapidly; but the corpuscles are not entangled in its meshes, for as quickly as the elastic threads are formed they adhere to and are removed by the rod or twigs. Thus the fibrin is formed very rapidly, and the ordinary stages in the formation of a blood clot, consisting of fibrin and the corpuscles, do not occur, for the fibrin is separated from the corpuscles as quickly as it is formed. We then have what is commonly spoken of as "defibrinated blood," which does not give a blood clot. Not that the coagulation has been prevented, but the material essential for the formation of a clot has been removed as quickly as formed, and instead of catching the corpuscles in the meshes of its delicate fibrils to form the clot in the ordinary way, the stringy shreds of fibrin cling around the beating rod as a jagged mass. The following tables show the relation of the different constituents of coagulated and defibrinated blood respectively: -

Living blood =

Plasma Corpuscles =

Serum (appearing as clear fluid).

Fibrin + Corpuscles

= Blood clot.

Living blood =

Plasma Corpuscles =

Fibrin (removed on the rod).

Serum + Corpuscles

Defibrinated = blood.

Many circumstances influence the rapidity with which a blood clot is formed. Speaking generally, circumstances which tend to injure the corpuscles or the plasma, and give rise to changes resulting in their death, promote coagulation; while, on the other hand, conditions which protect the corpuscles and impede fibrin formation must retard coagulation.

These may be arranged categorically, viz.

(A) Circumstances promoting coagulation: -

1. Contact With Foreign Bodies

Contact With Foreign Bodies is of the first importance in hastening coagulation. The greater the surface of contact with the vessel or the air, the more the corpuscles are exposed to injury, and the more rapid are the destructive chemical changes inducing fibrin formation. Thus a drop or two of blood falling on any surface so as to spread out in a thin layer clots almost instantly.

2. Motion

Motion, by renewing the points of contact between the blood and the moving agent, hastens coagulation. Thus, by whipping fresh blood, all the fibrin can be removed in a few minutes, and the defibrinated blood left without a clot.

3. Moderate Heat

The formation of the fibrin generators and the action of the ferment seem to go on more rapidly at 38°-4o° C. than at any other temperature.

4. A Watery Condition Of The Blood

A Watery Condition Of The Blood causes rapid coagulation but a soft clot. This is seen in repeated bleedings or hemorrhages: the blood which flows last clots first.

5. The Addition Of A Small Quantity Of Water

The Addition Of A Small Quantity Of Water, by setting up rapid changes in the corpuscles, accelerates coagulation.

6. A Supply Of Oxygen

Oxygen is used up in the chemical changes attendant upon the death of the blood, and its presence aids the formation of firm clots, such as are produced in arterial blood. Exposure to the air in a shallow vessel facilitates coagulation, partly by extensive contact and partly by a free supply of oxygen. But exposure to air is not necessary, for blood collected in mercury, without ever coming in contact with the air, coagulates very rapidly.