The discoloration and swelling of the skin following a blow is due to rupture of vessels and escape of blood and fluid, and further injury may result from the interruption of the circulation.
By the application of heat the tissue may be charred and the albumen of the blood and tissue fluids coagulated.

Living cells are very susceptible to the action of heat, a temperature of 130 degrees being the thermal death point, and even lower temperatures are fatal when their action is prolonged.

The action of the heat may produce definite coagulation of the fluid within the cells in the same way that the white of an egg is coagulated.

Certain of the albumens of the body coagulate at a much lower temperature than the white of the egg (as the myosin, one of the albumens of the muscle which coagulates at 115 deg.
F., egg white coagulating at 158 deg.

F.), and in addition to such coagulation or without it the ferments within the cell and to the action of which cellular activity is due may be destroyed.
In diseases due to parasites, the parasite produces a change in the tissue in its immediate vicinity often so great as to result in the death of the cells.