This section is from the book "A Manual Of Pathology", by Guthrie McConnell. Also available from Amazon: A Manual Of Pathology.
There may be an increase throughout the body of the total volume of blood. This seldom remains for any length of time, as the various excretory structures of the body get rid of it. The condition known as plethora is the result of persistent overeating and drinking. It is usually associated with a hypertrophy of the left ventricle.
Local Hyperemia is an increase in the amount of blood in a part of the body. It may depend upon either an increased supply to the part or be due to a diminished outflow - in one case a dilatation of the arteries, in the other an obstruction of the veins. The first is known as active or arterial, the second, as passive or venous, hyperemia.
Active Hyperemia is an excess of arterial blood in a part. It occurs with increased functional activity (increased metabolism). It may be brought about through the central nervous system or by direct stimulation of the peripheral nerves. Any pathologic condition that will bring about a local dilatation of the arteries will cause active hyperemia.
The spinal cord or a nerve may be pressed upon as the result of a tumor or of an injury, and a paralytic dilatation occurs. The same condition follows the use of certain drugs acting peripherally either upon the muscular coat of the artery or upon the local nervous mechanism, or both.
In active hyperemia the part affected is redder than normal and more or less swollen as the result of the increased amount of arterial blood that it contains. The temperature is higher than in the surrounding parts, but never higher than that of the internal organs. There is also an increase in the rate of the blood-flow.
This form of hyperemia if continued for some time is followed by (I) hypertrophy of the part on account of the increased nutrition, (2) parenchymatous degeneration from over-nutrition or overstimulation of the cells, and (3) a proliferation of the connective tissue around the blood-vessels.
It is found as one of the phenomena of inflammation. Postmortem, it cannot be recognized on account of the contraction of the arterial walls, which drives out the blood. It may persist in the kidneys.
Fig. 1. - Passive Hyperemia of the Lung. X 250 (Dürck).
1, Ectatic and distended blood-vessels, filled with blood; 2, engorged and tortuous capillaries; 3, lumen of alveolus; 4, increased interlobular connective tissue; 5, cells, containing blood-pigment, within the alveolar lumen; 6, free, amorphous blood-pigment.