This is not frequently met with except as an accompaniment of inflammation or other affection of the lung. It may be produced by the inhalation of irritating vapours. A Collateral hyperaemia occurs when, in consequence of disease which has rapidly developed, a considerable portion of the pulmonary circulation is obstructed. In cases of pneumonia where the exudation in the lung alveoli exercises pressure on the vessels, the unaffected parts of the lung are commonly hyperaemic. Embolism of the pulmonary artery will also produce a collateral hyperaemia.

A more equivocal collateral hyperaemia is sometimes produced by the application of cold to the surface, as in bathing while over-heated, or the imbibition of large quantities of cold liquids. The effect of this is to cause great contraction of the arteries in the skin on the one hand, or the stomach and neighbouring parts on the other. The anaemia thus produced causes, when extreme, a rise of blood-pressure in the pulmonary circulation, an acutely developed hyperaemia, which, however, partakes of the nature of passive more than of active hyperemia. There is in some such cases acute oedema, which may be accompanied by haemorrhage. (See case by Hertz).