To their more exact study we must, however, make some preface in the way of general remarks concerning a phenomenon everywhere easily recognized, but as yet incompletely understood. This phenomenon has reference to an undue supply of blood to a part, and is commonly known under two terms which are practically synonymous, namely, congestion and hyperemia. To begin with these, then, we must note, first of all, that congestion and hyperemia may be—
- A. Active; and
- B. Passive.
They may also be spoken of as—
- 1. Acute; and
- 2. Chronic.
Considering first the two latter distinctions, it will be found that the acute hyperemias are met with most often in consequence of sharp mechanical disturbances. The chronic hyperemias, on the contrary, are conditions which in many individuals are more or less permanent. Note accurately here the proper significance of certain terms. Hyperemia means, in effect, an oversupply of blood to the given part; the term should have only a local significance. When the entire body seems to be too well supplied with blood, the condition is known as plethora, the counterpart of which term is usually anemia. The direct counterpart of the term hyperemia should perhaps be ischemia, meaning a perverted blood supply in reduced amount. With plethora and anemia as terms implying general conditions, with hyperemia and ischemia implying local conditions, there should be little room for confusion in phraseology.
The active form of hyperemia used to be called “fluxion,” a term now rarely used. Active hyperemia means an increased supply of arterial blood. In passive hyperemia the oversupply is rather of venous blood. In the former case the condition seems due to overactivity of the heart, with such local tissue changes as permit it to occur. In passive hyperemia the blood current is slower—there is a tendency toward, and sometimes an actual, stagnation; all of which is usually due to obstruction of the return of blood to the heart. The conditions permitting these two results may be widely variant.
Active Hyperemia.
—Active hyperemia may be produced by purely nervous influences, even those of emotional origin. The flushing of the face which is known as “blushing” is, perhaps, the most common illustration of this fact. It is well known also that this is, in some degree at least, the result of division of certain nerves which have to do with the regulation of the blood supply. The cervical sympathetic is the best known and most often studied of these, and the consequences of division of this nerve in the neck are stated in all the text-books on physiology. So also by electrical stimulation of certain nerves the parts supplied by them can be made to show a very active hyperemia, which will subside shortly after discontinuance of stimulation, providing this has not been kept up too long. In active hyperemia there is absolute increase of intra-arterial tension, and under these circumstances pulsation may be noted in those small vessels in which commonly it is not seen nor felt. This is the explanation of the throbbing pain complained of under many actively hyperemic conditions. This hyperemia affords the explanation of the clinical signs to which attention has already been called. The increased heat of the part is the result of greater access of blood, which prevents cooling by radiation and evaporation; the peculiar redness is due to the greater filling of the capillaries with the blood, which gives the peculiar hue to the skin and visible textures; while to the increased pressure upon sensory nerves is also due the pain. The minuter changes occurring within the congested part call for more accurate description. Whether or not there is actual dilatation of capillaries under these circumstances is a matter still under dispute, but of the dilatation of the larger vessels there can be no possible question.
As hyperemia is to such a great extent brought about by action of the nervous system, it is well to divide it more accurately into the hyperemia of paralysis, or neuroparalytic congestion, which is the result of a paralysis of the constrictor fibers of the vasomotor system, and into the hyperemia of irritation, or neurotonic congestion, which is due to the irritation of the dilators (Recklinghausen). Physiologists are fairly well agreed that as between the dilating and the constricting apparatus of the vasomotor system there is ordinarily preserved a certain degree of equilibrium; to which fact is probably due that normal condition of affairs inaugurated after temporary disturbance, since overaction in one direction succeeds reaction in the other. As Warren has illustrated this, our common treatment of frostbite by cold applications is a concession to this fact, since by the cold applications we endeavor to limit the reaction which would otherwise follow after thawing out the frozen part.
The best examples of the hyperemia of paralysis are perhaps to be met with after certain injuries to nerves, as, for instance, flushing of the face and hypersecretion of nasal mucus, tears, etc., after injury to the cervical sympathetic. Such, too, in its essentials is that form of shock known as brain concussion, which is often followed by nutritive disturbances among the brain cells, with consequent perversion of brain function.