On a cold day the body may tend to lose much more heat. The temperature of the blood begins to fall causing reactions which minimize the heat loss. The low external temperature brings about a reflex constriction of the skin arterioles (vasoconstriction). Less blood flows through the skin and less heat is lost by convection, radiation and conduction. The sweat glands become less active. So less latent heat is lost from the body. The metabolic rate of the body increases and more heat is produced.
Fur bearing animals can fluff up their hairs by a contraction of the hair muscles on a cold day. The layer of still air trapped between the hairs become thicker. As air is a bad conductor of heat, it takes a longer time for the heat to travel across a thicker layer of still air. Consequently heat is lost more slowly and can be slowed to almost nil. In a man a similar reaction also takes place but this is an unsuccessful attempt. It merely produces the characteristic "goose pimples".
Sometimes the above reactions are not sufficient to prevent a drop in body temperature and a reflex contraction of the body muscles occurs which we recognize as "shivering". This spasmodic contraction of the muscles increases heat production and helps to raise the body temperature to normal.
Fur bearing animals can fluff up their hairs by a contraction of the hair muscles on a cold day. The layer of still air trapped between the hairs become thicker. As air is a bad conductor of heat, it takes a longer time for the heat to travel across a thicker layer of still air. Consequently heat is lost more slowly and can be slowed to almost nil. In a man a similar reaction also takes place but this is an unsuccessful attempt. It merely produces the characteristic "goose pimples".
Sometimes the above reactions are not sufficient to prevent a drop in body temperature and a reflex contraction of the body muscles occurs which we recognize as "shivering". This spasmodic contraction of the muscles increases heat production and helps to raise the body temperature to normal.