11.9 Resistance to Cold

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.