1
First stage
physics
Lec-4
9/12/2015
.د
تيماء
Heat losses from the Body
Constant body temperatures permit metabolic processes to proceed at constant rates
.Because the body at a constant temperature it contains stored heat energy that is
essentially constant as long as we are alive .
The normal body (core) temperature is often given as 37 °C , only small percentage of people
have exactly that temperature .If we measured the temperature of a large number of healthy
people ,we would find a distribution of temperature with 0.5± °C of normal temperature .
The temperature depends upon the time of the day (lower in the morning );the temperature
of the environment ; and the amount of recent physical activity , the amount of clothing ,and
the health of individual.
The heat is generated in the organs and tissues of the body ; most of it is removed by
several processes that take place on the skins surface .
The main heat loss mechanisms are :
1 . Radiation.
2 .Convection.
3 . Evaporation.
Some cooling of the body takes place in the lungs where the
inspired air heated and vaporized water is added to expired air . Eating hot or cold food
may also heat or cool the body .For the body to hold its temperature close to its normal
value it must have a thermostat analogous to a home thermostat that maintains the
temperature of the rooms nearly constant . The hypothalamus of the brain contains the
body’ s thermostat .
If the core temperature rises , the hypothalamus initiates sweating vasodilatation which
increases the skin temperature . Both of these reactions increase the heat loss to the
environment .The rate of heat production of the body for a 2400 Kcal / hr diet is about 1.7
Kcal /min or 120 J / sec (120 W) .
If the body is to maintain a constant temperature it must lose heat at the same rate .The
actual heat lost by radiation , convection , evaporation of sweat and respiration
dependents on a number of factors :
2
1.The temperature of the surrounding.
2 . Temperature .
3 . Humidity .
4 . Motion of the air .
5 . The physical activity of the body .
6 . The amount of the body exposed .
7 . The amount of insulation on the body ( cloth and fat ) .
All subjects regardless of their temperature emits electromagnetic radiation .
The amount of energy emitted by the body is proportional to absolute temperature raised
to the fourth power . The body also receives radiant energy from the surrounding objects .
The approximate difference between the energy radiated by the body and the energy
absorbed from the equation :
The heat loss due to radiation(Hr) is
H
r
= K
r
A
r
e (T
s
– T
w
)
Where
H
r
is the rate of energy loss ( or gain ) due to radiation .
A
r
is the effective body surface area emitting radiation .
e
is the emissivity of the surface
T
s
is the skin temperature ( c ° )
T
w
is the temperature of the surrounding walls .
K
r
is the constant that depends upon various physical parameters and is about5 Kcal / m² .
Hr . C° .
e
in the infrared region is independent of the color of the skin and is very nearly equal to
one , indicating that the skin at this wavelength is almost a perfect absorber and emitter of
radiation .
3
The heat loss due to convection (Hc) is
Hc = Kc Ac (Ts – Tα )
Where Kc is constant that depend upon the movement of the air .
A
c
is the effective surface area .
T
s
is the temperature of the skin .
T
α
is the temperature of the air .
When the body is resting and there is no apparent wind , K is about
2.3 Kcal /m² hr c°.
When the air is air is moving , the constant K increases according to equation
Kc = 10.45
– v + 10 √ v
Where the wind speed v is in meter per second
This equation is valid for speeds between 2.23 m/sec and 20 m /sec .The equivalent
temperature due to moving air is called wind chill factor and is determined by the actual
temperature and wind speed .
3. Evaporation:
The method of heat loss that of us familiar with is the evaporation of sweat .
Under normal temperature condition and in the absence of hard work or exercise , this
method is rather unimportant compared to the radative and convective cooling Under
extreme conditions of heat and exercise ,a man may sweat more than 1 liter of liquid per
hour .Since each gram of water that evaporates carries with it the heat of vaporization 580
calories , the evaporation of 1 liter carries with it 580 Kcal . The sweat must evaporate from
the skin in order to give the cooling effect .The amount evaporated depends upon the air
movement and the relative humidity .
Insulation of clothing:
The unit of clothing is clo .This unit corresponds to the insulating value of clothing needed
to maintain a subjects sitting at rest in comfort in a room at 21 c with air movement of 0.1
m/sec and air humidity of less than 50 % .
One clo of insulation is equal to a light weight business suit .
2 clo of clothing would enable a man to withstand a colder temperature