By threatening the vitality of a tissue

By threatening the vitality of a tissue

(2) Irritants. Heat and cold, chemical or mechanical. Any of these, by threatening the vitality of a tissue, induce immediately an augmented flow of blood to the part to furnish the means of repair a hot iron, frostbites, acids, or a blow.

(3) Nerve influence. This may produce congestion either by acting on the part reflexly or as the result of some central nerve disturbance affecting the branch which supplies a given organ.

Fonte: Diseases of the Horse