Passive Hyperemia is an excess of venous blood in a part. It is the result of a distention of a vein on account of some obstruction to the outflow of the blood. This can be caused by obstruction within the veins or capillaries, as by thickening of their walls, by thrombi, or by pressure from without, as from a tumor. A common cause for general passive hyperemia is a lesion of the heart-valves. The circulation will continue slowly unless the venous pressure becomes as great as the arterial, when it will stop, a condition known as stasis.

A part that is the seat of passive hyperemia becomes cyanotic, swollen, edematous, cooler than normal, and its function less. The rate of blood-flow is lessened. The edema is due to the escape of fluid from the blood. If severe, red corpuscles may escape.

Following long-continued passive hyperemia the tissues will undergo a fatty degeneration on account of the decreased nutrition, or even necrosis and gangrene may result. There may also be some increase in the amount of connective tissue. Pigmentation from escaped hemoglobin is not uncommon - brown atrophy.

When stasis occurs the blood-corpuscles slowly collect in the smaller vessels, the plasma is exuded, and the cells become packed closely together. Finally, the outline of the cells cannot be seen and the vessels appear to be filled with coagulated blood. Such is not the case, as when the circulation is re-established the corpuscles separate and move along as usual.

Local anemia or ischemia is the condition in which the part contains less than its normal amount of blood. It is most commonly due to obstruction by pressure of the flow of arterial blood into a part. This may be due to tight bandaging, pressure from a tumor, or to thrombi or emboli, or to changes in the wall of the vessel.

Disturbances of the vasomotor system may bring about marked lesions. If there is a good collateral circulation the area to which the obstructed vessel goes may show very slight change. If such is not the case, infarction may follow. An anemic area is pale in color, temperature lower, and functional activity decreased.