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11.9 Resistance to Cold
Even in a very hot, sunny environment, a person can cope with the heat generated by mod erate activity with the help of evaporative cooling.
The act of walking consumes about 7 m2 of energy.
The heat is delivered to the skin.
The skin is heated by both the environment and the sun.
The radiative heating by the sun is about 215 Cal/hr.
The only way to cool the body is to sweat.
The total amount of heat that needs to be removed is.
The sweat will provide 1.5 liters of cooling.
The heat load is reduced if the person is protected by light clothing.
The human body is well-equipped to deal with heat.
For a period of time that was enough to cook a steak, people have survived a temperature of 125*C.
The body temperature can be maintained at a proper level when the environment cools.
An animal's ability to survive cold is measured by this temperature.
Humans are like tropical animals.
They are better able to handle heat than cold.
The critical temperature for a heavily furred fox is -40*C.
The increased rate of heat outflow from the skin is what causes the cold.
The rate is determined by the temperature, wind speed and humidity.
At 20*C, air moving with a speed of 30 cm/s removes more heat than still air.
The body defends itself against cold by decreasing the heat outflow and increasing the production of heat.
The blood flow to the skin is reduced when the temperature of the body drops.
When the ambient temperature drops to 19*C, this mechanism is fully utilized in a naked person.
The metabolism increases the amount of heat needed to maintain the body temperature.
It is possible to achieve this with a one-in-a-million response.
The metabolism is raised to about 250 Cal/m2hr.
If these defenses fail and the temperature of the skin and underlying tissue falls below 5*C, frostbite and more serious freezing can occur.
Thick fur, feath ers, or appropriate clothing provide the most effective protection against cold.
The heat loss is mostly convective and radiative at -40*C. With a thick layer of fur or similar insulation the skin is protected from the elements and the heat is transferred to the environment.
The heat transfer from the skin at 30*C to the ambient environment at -40*C through 1 cm of insulation is from Eq.
The rate for most animals is below this.
Our calculation shows that well-insulated animals, including clothed people, can survive in cold environments.
The amount of heat removed by breathing at a normal rate is small at moderate temperatures.
The heat is removed by this channel at very cold temperatures.
As the ambient air temperature drops, the amount of heat required to warm the inspired air to body temperature increases.
This heat loss limits the animal's ability to survive in cold weather.
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