Physiologically, heat is generated
in the muscles by metabolic chemical reactions, mainly in the liver. Some
heat is lost through the lungs, although 90-95% is lost through the skin.
Heat is transferred from the core to the skin by blood passing through
peripheral blood vessels.
The rate of heat loss is determined by the extend to which the peripheral
blood vessels dilate; fully dilated they will allow blood to travel 100
times faster than when constricted, thus losing body heat faster. Heat
loss rates are also greatly increased by sweating, especially in dry environments.
Mouse over the image to see the effect of warm blood flowing
into cold hands.
How the body controls heat loss
The body controls heat loss by tightening the blood vessels under the skin,
restricting the flow of blood - to the peripheral blood vessels ('Vasoconstriction').
The development of peripheral vasoconstriction allows a cooler, outer 'shell'
to form an insulating barrier that slows heat loss from the body's core. Hands
and feet have fewer large blood vessels, and when the flow of blood is restricted
it is harder for the blood to keep flowing to these areas which quickly become
cold.
How heat transfers from the skin to the surrounding environment
Heat loss is due to one or more of the following - convection, conduction
, evaporation or radiation. In comfortable environments, about 65%
is lost through radiation, with most of the rest through evaporation. In cold
environment, most heat lost is via convection and conduction.
Convection happens when air or water with a lower temperature
than the body comes into contact with the skin and then moves away. An example
of convection is blowing on hot food to cool it down. The amount of heat loss
depends on the temperature difference between the body and environment plus
the speed with which air or water is moving.
Conduction is the transfer of heat to objects or substances
the body comes into direct contact with. Metal and stones are good heat conductors,
which is why they feel cold to the touch, even at room temperature. Air conducts
heat poorly, which is why still air is an excellent insulator. Water conductivity
is 240 times greater than that of dry air.
Evaporation is responsible for 20 - 30% of heat loss in temperate
conditions. About 2/3rds of evaporative heat loss occurs from the skin in thermo-neutral
conditions. The remainder happens in the lungs and airways. In cold conditions,
airway evaporative heat loss increases as the incoming air is humidifed and
warmed.
Even in hot conditions hands can become very cold if the person has been
even mildly sweating a cool breeze can easily make the hands feel very cold.
