The Natural Arrest of Hæmorrhage and the Repair of Blood Vessels

Primary Hæmorrhage.—The term primary hæmorrhage is applied to the bleeding which follows immediately on the wounding of a blood vessel. The natural process by which such hæmorrhage is arrested varies with the character of the wound in the vessel and may be modified by accidental circumstances.

(a) Repair of completely divided Artery.—When an artery is completely divided, the circular fibres of the muscular coat contract, so that the lumen of the cut ends is diminished, and at the same time each segment retracts within its sheath in virtue of the recoil of the elastic elements in its walls, the tunica intima curls up in the interior of the vessel, and the tunica externa collapses over the cut ends. The blood that escapes from the injured vessel fills the interstices of the tissues, and, coagulating, forms a clot which temporarily arrests the bleeding. That part of the clot which lies between the divided ends of the vessel and in the cellular tissue outside, is known as the external clot, while the portion which projects into the lumen of the vessel is known as the internal clot, and it usually extends as far as the nearest collateral branch. These processes constitute what is known as the temporary arrest of hæmorrhage, which, it will be observed, is effected by the contraction and retraction of the divided artery and by clotting.

The permanent arrest takes place by the transformation of the clot into scar tissue. The internal clot plays the most important part in the process; it becomes invaded by leucocytes and proliferating endothelial and connective-tissue cells, and new blood vessels permeate the mass, which is thus converted into granulation tissue. This is ultimately replaced by fibrous tissue, which permanently occludes the end of the vessel. Concurrently and by the same process the external clot is converted into scar tissue.

If a divided artery is ligated at its cut end, the tension of the ligature is usually sufficient to rupture the inner and middle coats, which curl up within the lumen, the outer coat alone being held in the grasp of the ligature. An internal clot forms and, becoming organised, permanently occludes the vessel as above described. The ligature and the small portion of vessel beyond it are subsequently absorbed.

In course of time the collateral branches of the vessel above and below the level of section enlarge and their inter-communication becomes more free, so that even when large trunks have been divided the vascular supply of the parts beyond may be completely restored. This is known as the development of the collateral circulation.

Imperfect Collateral Circulation.—While the development of the collateral circulation after the ligation or obstruction from other cause of a main arterial trunk may be sufficient to prevent gangrene of the limb, it may be insufficient for its adequate nourishment; it may be cold, bluish in colour, and there may be necrosis of the skin over bony points; this is notably the case in the lower extremity after ligation of the femoral or popliteal artery, when patches of skin may die over the prominence of the heel, the balls of the toes, the projecting base of the fifth metatarsal and the external malleolus.

If, during the period of reaction, the blood-pressure rises considerably, the occluding clot at the divided end of the vessel may be washed away or the ligature displaced, permitting of fresh bleeding taking place—reactionary or intermediary hæmorrhage (p. 272).

In the event of the wound becoming infected with pyogenic organisms, the occluding blood-clot or the young fibrous tissue may become disintegrated in the suppurative process, and the bleeding start afresh—secondary hæmorrhage (p. 273).

(b) If an artery is only partly cut across, the divided fibres of the tunica muscularis contract and those of the tunica externa retract, with the result that a more or less circular hole is formed in the wall of the vessel, from which free bleeding takes place, as the conditions are unfavourable for the formation of an occluding clot. Even if a clot does form, when the blood-pressure rises it is readily displaced, leading to reactionary hæmorrhage. Should the wound become infected, secondary hæmorrhage is specially liable to occur. A further risk attends this form of injury, in that the intra-vascular tension may in time lead to gradual stretching of the scar tissue which closes the gap in the vessel wall, with the result that a localised dilatation or diverticulum forms, constituting a traumatic aneurysm.

(c) When the injury merely takes the form of a puncture or small incision a blood-clot forms between the edges, becomes organised, and is converted into cicatricial tissue which seals the aperture. Such wounds may also be followed by reactionary or secondary hæmorrhage, or later by the formation of a traumatic aneurysm.

Conditions which influence the Natural Arrest of Hæmorrhage.—The natural arrest of bleeding is favoured by tearing or crushing of the vessel walls, owing to the contraction and retraction of the coats and the tendency of blood to coagulate when in contact with damaged tissue. Hence the primary hæmorrhage following lacerated wounds is seldom copious. The occurrence of syncope or of profound shock also helps to stop bleeding by reducing the force of the heart's action.

On the other hand, there are conditions which retard the natural arrest. When, for example, a vessel is only partly divided, the contraction and retraction of the muscular coat, instead of diminishing the calibre of the artery, causes the wound in the vessel to gape; by completing the division of the vessel under these circumstances the bleeding can often be arrested. In certain situations, also, the arteries are so intimately connected with their sheaths, that when cut across they were unable to retract and contract—for example, in the scalp, in the penis, and in bones—and copious bleeding may take place from comparatively small vessels. This inability of the vessels to contract and retract is met with also in inflamed and œdematous parts and in scar tissue. Arteries divided in the substance of a muscle also sometimes bleed unduly. Any increase in the force of the heart's action, such as may result from exertion, excitement, or over-stimulation, also interferes with the natural arrest. Lastly, in bleeders, there are conditions which interfere with the natural arrest of hæmorrhage.

Repair of a Vessel ligated in its Continuity.—When a ligature is applied to an artery it should be pulled sufficiently tight to occlude the lumen without causing rupture of its coats. It often happens, however, that the compression causes rupture of the inner and middle coats, so that only the outer coat remains in the grasp of the ligature. While this weakens the wall of the vessel, it has the advantage of hastening coagulation, by bringing the blood into contact with damaged tissue. Whether the inner and middle coats are ruptured or not, blood coagulates both above and below the ligature, the proximal clot being longer and broader than that on the distal side. In small arteries these clots extend as far as the nearest collateral branch, but in the larger trunks their length varies. The permanent occlusion of those portions of the vessel occupied by clot is brought about by the formation of granulation tissue, and its replacement by cicatricial tissue, so that the occluded segment of the vessel is represented by a fibrous cord. In this process the coagulum only plays a passive rôle by forming a scaffolding on which the granulation tissue is built up. The ligature surrounding the vessel, and the elements of the clot, are ultimately absorbed.

Repair of Veins.—The process of repair in veins is the same as that in arteries, but the thrombosed area may become canalised and the circulation through the vessel be re-established.