Heat is produced in the interior of the body chiefly in the muscles and great glands, and the increased afflux of blood brings more heat to the surface.

A certain degree of swelling of the tissue is due to the dilatation of the vessels; but this is a negligible factor as compared with the effect of the presence of the fluid and cells of the exudate.[1] The fluid distends the tissue spaces, and it may pass from the tissue and accumulate on surfaces or in the large cavities within the body.

The greatly increased discharge from the nose in a "cold in the head" is due to the exudation formed in the acutely inflamed tissue, and which readily passes through the thin epithelial covering.

Various degrees of inflammation of the skin may be produced by the action of the sun, the injury being due not to the heat but to the actinic rays.

In a mild degree of exposure only redness and a strong sense of heat are produced, but in prolonged exposure an exudate is formed which causes the skin to swell and blisters to form, these being due to the exudate which passes through the lower layers of the cells of the epidermis and collects beneath the impervious upper layer, detaching this from its connections.