It is to be understood that we have to deal only with local hyperemia - congestion, so called.

It consists in an excessive amount of blood in the capillaries of an organ; that is, in an injected condition of this latter, exceeding what experience has shown to be its average. This is not possible without dilatation of the vessels, nor can we admit the existence of congestion with coarctation of the vessels, and a consequent accelerated passage of the blood through them, which some have designated as active congestion.

A simple comprehensive view of the development of hyperaemia is not feasible, the conditions not being always the same. It will, perhaps, suffice to distinguish between active, passive, and mechanical hyperaemia. We shall here, however, not consider these individually - more especially, the two former - further than may be requisite for the establishment of a principle. Their closer investigation will be more appropriate under the head of Inflammation.

1. Active Hyperaemia

Active Hyperaemia is the result of external or internal stimuli acting immediately upon an organ, or reflected to it from other organs, - irritating the sensitive nerves, and thereby causing antagonistic palsy of the nervi vasorum or (according to another theory), evoking an increased afflux of blood, a preternatural affinity of the parenchyma for the blood.

To this category belong, for the most part, those hyperaemiae, dependent upon external or internal causes, which precede inflammations; most of those habitual, constitutional hyperaemia, rooted directly in the nervous system or in the blood; again, those hyperaemiae occasioned by augmented activity, by overwrought function, or concurrent with excited conditions of the organs.

2. Passive Congestion

Passive Congestion depends upon direct palsy of the nervi vasorum, wherewith is commonly associated a depressed energy in the remainder of the nervous system. The palsy may originate in the centres, or it may be peripheral. It is often determined by dyscrasis, and especially by the higher degrees of decomposition of the blood.

To this class must be referred those hyperaemiae, introductory to so-called asthenic inflammations, in organs exhausted by excess of functional activity, enfeebled by hyperaemiae and inflammation, or paralyzed; as also in those hypostatic hyperaemiae of the lungs, of the abdominal and pelvic organs, of the common integuments, which are developed under diminished impulse from the heart, in dependent parts of the body, during the progress of various adynamiae and marasmi.

3. Mechanical Hyperemia

Mechanical Hyperemia, namely, -

(a.) Hyperemia arising from palpable mechanical impediments to the return of the blood through the veins, or to the ultimate disgorging of the venous trunks into the heart. The extension of the hyperaemiae varies with the locality of the impediment. It affects single organs and sections of organs; for example, a portion of intestine strangulated, invaginated through tension and compression of its bloodvessels. Or it may have a more comprehensive range in impermeability of the liver, of the lungs, in stenosis (coarctation) of the heart's valves. The hyperaemiae occasioned by spontaneous coagulation within the capillaries, by various elements obstructive of these vessels, as pus-corpuscle, cancer-cell, injected mercury globule, are commonly referred to the same head.

(b.) Hyperemia ex vacuo, as they occur in atrophy of the brain within the unyielding skull, or in the gravid uterus after rapid delivery, often to the extent of producing hemorrhage; hyperaemiae due to the eccentrical rarefying atrophy of organs.

The hyperaemiae is either of a more or less transitory or of an abiding nature, of which latter kind mechanical hyperaemiae, from heart disease or from induration of the liver, present the most frequent and the most marked examples.

The sequelae of hyperaemiae are multiplex, varying with the duration, the repetition, the degree, of the congestion. Much likewise depends upon the character of the affected organ, the congestion being significant in proportion to the general importance of such organs, and to the vulnerability of its texture. Organs are prone to congestion proportionately to their vascularity and to the degree of their functional activity. Under particular circumstances of life, of occupation, of civilization, certain organs, such as the brain and its membranes, and the lungs, are hardly ever entirely free from congestion. Hyperemia affects morbid growths equally with normal formations.

Intense congestion suddenly developed in organs essential to life (the brain or lungs) may prove fatal directly, as so termed vascular apoplexy, or through the sudden effusion of blood serum into the textures - acute oedema.

High degrees of congestion occasion laceration of capillaries and parenchymatous hemorrhage (apoplexy with bloody extravasation) in the brain, the lungs, and other organs.

The same causes lead, by an overloading of the bloodvessels, to absolute palsy of the bloodvessels, to stasis, inflammation, and gangrene.

Moderate but habitual or repeated congestion gradually engenders oedema and the dropsy of serous cavities - genuine dropsy, increased exudation of blood plasma, preternatural nutrition of the textures - hypertrophy, augmented secretion.

In this relation, abiding mechanical hyperaemiae, from heart disease, are worthy of especially notice, with their unfailing consequences, hypertrophy of the glandular abdominal viscera; preternatural secretion of the intestinal and bronchial mucous membrane; excessive, saturated secretion of bile.

Hyperemia create and bequeath permanent dilatation and elongation with coil-like or serpentine deflection - properly termed varicosity - of the bloodvessels, as more particularly exemplified in the less resilient veins.

Hyperemia frequently occasion and obviously accompany the development of various heterologous growths. Finally, in some organs, a proportion of blood-pigment, effused with the plasma, constitutes the basis of rust-colored, slate-gray, bluish-black coloration, as in the lungs or on the intestinal mucous membrane.

Organs attacked by a high degree of hyperaemia present different shades of dark red, become swollen, loosened in texture, and consequently friable, lacerable. In organs of a porous spongy texture, the swelling seems due to a bloated condition of the tissue itself.