Using mathematical calculations, the processes of heat radiation of the blood moving along skin vessels of different diameter, were studied. The temperature of the blood in capillaries, small arteries and venules was shown to be equal to that of surrounding tissue. The data show that, under normal physiological conditions, a situation occurs in the skin vascular bed when the vessels on the diameter lesser than 100 mcm cease to be able to be heat-exchanging means in the skin of the living organism.