Efficiency of the human Body
We can consider the human body as a machine in doing external work.The efficiency of the human body as a machine can be obtain from the usual definition of the efficiency ( ε ): Efficiency ( ε ) = Work done Energy consumed
Efficiency (ε) is lowest at low power ,but can increase to 20 % for trained individuals in activities such as cycling and rowing . Table 1 shows the efficiency of man for several activities along with the efficiency of several mechanical engines.
Table 1 Shows the efficiency of man for several activities
Efficiency %Task or Machine
~20
Cycling
<2 ~4
Swimming(on surface) ( under water)
~3
Shoveling
17
Steam engine
The maximum work capacity of the body is variable .For short periods of time the body can perform at very high power levels , but for long – term efforts it is more limited. Experimentally it has been found that long -term power is proportional to the maximum rate of oxygen consumption in the working muscles.
The body supplies instantaneous energy for short – term power needs by splitting energy – rich phosphates and glycogen, leaving an oxygen deficit in the body . This process can only last about a minute and is called the anaerobic (without oxygen ) phase of work ; Long –term activity requires oxygen 1(aerobic work ) as shown in figure
The maximum work capacity of the body is variable
Figure 1Figure1:phases of work
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 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 :
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 :Hr = Kr Ar e (Ts – Tw ) Where Hr is the rate of energy loss ( or gain ) due to radiation ..
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 .
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 . Ac is the effective surface area . Ts 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 /mІ hr c°.When the air is moving , the constant K increases according to equation Kc = 10.45 – v + 10 √ vWhere 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 exterme 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 .