The skin is very liable to variations of its blood supply. An active Hyperaemia hardly occurs as a pathological condition except as part of an inflammation. Passive hyperaemia, on the other hand, is exceedingly common as a result of general venous engorgement, especially in disease of the heart. From the blue colour assumed by the skin in passive hyperemia the condition is designated Cyanosis.

Local variations in the circulation also occur by stimulation of the vaso-motor nerves, the resulting conditions being named Angio-neuroses. These imply an excessive irritability of the vaso-motor nerves of the skin, and a proclivity on slight stimulation to contraction chiefly of the arteries, but also of the veins. The most exaggerated example of this is afforded by Raynaud's disease, where from insignificant external causes spasm of arteries and sometimes of veins comes about. If both arteries and veins are contracted then there is anaemia of the part (a condition called by Raynaud local syncope), whilst if only the arteries are contracted there is a passive hyperemia or cyanosis (called by Raynaud local asphyxia). In either case necrosis may occur, taking the form of Symmetrical Gangrene. Short of these extreme cases we find, in persons whose vaso-motor nerves are sensitive, that exposure to cold leads to a local syncope. On the other hand, in persons in health, the arteries alone contract, and cold produces a cyanosis or blueness of the skin.

Besides these more direct examples of angioneuroses there are Local cedemas and other lesions which are related to the vaso-motor nerves but not in such a simple fashion. Urticaria is a local oedema or accumulation of fluid in the lymphatics of the corium, resulting in limited swellings. It always implies an abnormal sensitiveness of the vessels. Thus in many persons a flea-bite induces a local cedema, characterized by the wheals or quaddels of urticaria. In others the stroking of the skin with the edge of the nail or a pencil causes ridges of cedema to rise so that words can be produced in raised letters (Urticaria factitia). But this special sensitiveness may be induced either by irritants in the blood or by reflex irritation. Thus a more or less generalized urticaria is brought about in some persons by eating shell-fish or certain other kinds of food. In these, however, there is, as in urticaria factitia, always a local irritation as well as a general sensitiveness, as is proved by the fact that the urticaria develops on parts which are exposed to friction, of the clothes or otherwise. There is thus a graduation in these lesions towards inflammation.

This applies even more to those eruptions which Unna has included along with general urticaria under the name of Erythanthema, in which the exudation of fluid leads to elevation of the cuticle and the formation of vesicles and bullae.

Haemorrhages are frequent and of various kinds. The skin is liable to haemorrhage by traumatic rupture of its vessels, but still more, perhaps, by alterations in the state of the blood affecting its vessels. In scurvy, in purpura, in small-pox, in typhus, etc., the skin is the seat of haemorrhage much more frequently than any other structure.

The blood escaping from the vessels collects in the serous spaces of the corium for the most part, but may pass to the subcutaneous tissue, where the fat is sometimes considerably infiltrated. When the blood has escaped from a small vessel and infiltrates a limited area so that a bluish spot is seen on viewing the surface, the term Petechia is given. As these small haemorrhages depend on the state of the blood, the petechiae are nearly always multiple. When the blood infiltrates a larger area then the term Ecchymosis is used. Sometimes the blood collects between the corium and epidermis, and then a hemorrhagic Vesicle is the result; but this can only happen if the deeper layers of the epidermis have been killed so as to allow of their separation from the corium. When there are numerous small haemorrhages forming a large number of petechiae, then it is customary to use the term Purpura, or to speak of a purpuric condition.

The blood effused in the skin undergoes changes like those already described (see Haemorrhage). The affected area is first dark blue, and the colour does not disappear on pressure. After a time, as the blood-colouring matter is dissolved, the colour becomes fainter and changes in hue, while the discoloration becomes more extended. If blood has collected between the layers of epidermis, it comes to the surface and is disposed of as the epidermis is shed.

General oedema of the skin, apart from inflammation, is chiefly related to diseases of the heart and of the kidneys. Its general pathology has been already considered. The exuded fluid collects in the lymph spaces and is carried off by the lymphatics. Myxoedema has also been described.