Efficiency of the human BodyEfficiency of the human Body

We can consider the human body We can consider the human body as a machine in doing external as a machine in doing external workwork..

The efficiency of the human body as a The efficiency of the human body as a machine can be obtain from the machine can be obtain from the usual definition of the efficiency usual definition of the efficiency (( εε ))::

Efficiency Efficiency (( εε ) ) = = Work doneWork done Energy consumed Energy consumed

Efficiency Efficiency ((εε)) is lowest at low power is lowest at low power, , but can increase to 20 % for trained but can increase to 20 % for trained individuals in activities such as individuals in activities such as

cycling and rowingcycling and rowing . . Table 1 shows the efficiency of man for Table 1 shows the efficiency of man for several activities along with the several activities along with the efficiency of several mechanical efficiency of several mechanical

enginesengines . .

Table 1 Shows the efficiency of Table 1 Shows the efficiency of man for several activitiesman for several activities

•Task or MachineTask or Machine EfficiencEfficiency %y %

CyclingCycling ~~2020

Swimming(on surface)Swimming(on surface) ( ( under waterunder water ) )

<<22 ~4~4

ShovelingShoveling ~~33

Steam engineSteam engine 1717

The maximum work capacity of the body The maximum work capacity of the body is variableis variable. .

For short periods of time the body can For short periods of time the body can perform at very high power levelsperform at very high power levels , ,

but for long but for long –– term efforts it is more term efforts it is more limited. Experimentally it has been limited. Experimentally it has been found thatfound that

long -term power is proportional to the long -term power is proportional to the maximum rate of oxygen consumption maximum rate of oxygen consumption in the working musclesin the working muscles . .

The body supplies instantaneous energy The body supplies instantaneous energy for short – term power needs by splitting for short – term power needs by splitting energy – rich phosphates and glycogen, energy – rich phosphates and glycogen, leaving an oxygen deficit in the body . leaving an oxygen deficit in the body . This process can only last about a minute This process can only last about a minute and is called the anaerobic (without and is called the anaerobic (without

oxygen ) phase of workoxygen ) phase of work ; ; Long –term activity requires oxygen Long –term activity requires oxygen 11(aerobic work ) as shown in figure(aerobic work ) as shown in figure

The maximum work capacity of the The maximum work capacity of the body is variablebody is variable

Figure1:phases of work

Heat losses from the BodyHeat losses from the Body

Constant body temperatures permit Constant body temperatures permit metabolic processes to proceed at metabolic processes to proceed at constant ratesconstant rates. .

Because the body at a constant Because the body at a constant temperature it contains stored heat temperature it contains stored heat energy that is essentially constant as energy that is essentially constant as

long as we are alivelong as we are alive . .

•The normal body (core) temperature is The normal body (core) temperature is often given as 37 °C , only small often given as 37 °C , only small percentage of people have exactly that percentage of people have exactly that temperature .If we measured the temperature .If we measured the temperature of a large number of temperature of a large number of healthy people ,we would find a healthy people ,we would find a distribution of temperature with 0.5± distribution of temperature with 0.5± °C of normal temperature°C of normal temperature

The temperature depends uponThe temperature depends upon the time of the day (lower in the the time of the day (lower in the morning );the temperature of the morning );the temperature of the environment ; and the amount of environment ; and the amount of recent physical activity , the amount recent physical activity , the amount of clothing ,and the health of of clothing ,and the health of individualindividual

The heat is generated in the The heat is generated in the organs and tissues of the organs and tissues of the body ; most of it is removed body ; most of it is removed by several processes that by several processes that take place on the skins take place on the skins surfacesurface. .

The main heat loss mechanisms areThe main heat loss mechanisms are: :

11 . . RadiationRadiation22. . ConvectionConvection

33 . . EvaporationEvaporation

some cooling of the body takes some cooling of the body takes place in the lungs where theplace in the lungs where the inspired air heated and inspired air heated and vaporized water is added to vaporized water is added to expired air . Eating hot or expired air . Eating hot or cold food may also heat or cold food may also heat or cool the bodycool the body. .

For the body to hold its temperature For the body to hold its temperature close to its normal value it must have close to its normal value it must have a thermostat analogous to a home a thermostat analogous to a home thermostat that maintains the thermostat that maintains the temperature of the rooms nearly temperature of the rooms nearly constant . The hypothalamus of the constant . The hypothalamus of the brain contains the bodybrain contains the body’’ s thermostat s thermostat . .

If the core temperature rises , the If the core temperature rises , the hypothalamus initiates sweating hypothalamus initiates sweating vasodilatation which increases the vasodilatation which increases the skin temperature . Both of these skin temperature . Both of these reactions increase the heat loss to reactions increase the heat loss to the environmentthe environment. .

The rate of heat production of the body The rate of heat production of the body for a 2400 Kcal / hr diet is about 1.7 for a 2400 Kcal / hr diet is about 1.7 Kcal /min or 120 J / sec (120 W)Kcal /min or 120 J / sec (120 W). .

If the body is to maintain a constant If the body is to maintain a constant temperature it must lose heat at the temperature it must lose heat at the same ratesame rate. .

The actual heat lost by radiation , The actual heat lost by radiation , convection , evaporation of sweat convection , evaporation of sweat and respiration dependents on aand respiration dependents on a number of factorsnumber of factors: :

11..The temperature of the surroundingThe temperature of the surrounding . .22 . . TemperatureTemperature. .

33 . . HumidityHumidity. . 44 . . Motion of the airMotion of the air. .

55 . . The physical activity of the bodyThe physical activity of the body. . 66 . . The amount of the body exposedThe amount of the body exposed . .

77 . . The amount of insulation on theThe amount of insulation on the body ( cloth and fat )body ( cloth and fat ) . .

All subjects regardless of their All subjects regardless of their temperature emits electromagnetic temperature emits electromagnetic radiationradiation. .

The amount of energy emitted by the The amount of energy emitted by the body is proportional to absolute body is proportional to absolute temperature raised to the fourth powertemperature raised to the fourth power. .

The body also receives radiant energy The body also receives radiant energy from the surrounding objectsfrom the surrounding objects. .

The approximate difference between the The approximate difference between the energy radiated by the body and the energy radiated by the body and the energy absorbed from the equationenergy absorbed from the equation: :

Hr = Kr Ar e (Ts Hr = Kr Ar e (Ts –– Tw ) Tw )

Where Hr is the rate of energy loss Where Hr is the rate of energy loss ( or gain ) due to radiation( or gain ) due to radiation. .

..

Ar is the effective body surface area Ar is the effective body surface area emitting radiationemitting radiation. . e is the emissivity of the surfacee is the emissivity of the surface Ts is the skin temperature ( c Ts is the skin temperature ( c °° ) )Tw is the temperature of the Tw is the temperature of the surrounding wallssurrounding walls. .

Kr is the constant that depends upon Kr is the constant that depends upon various physical parameters and is various physical parameters and is aboutabout

55 Kcal / mKcal / m²² . Hr . C . Hr . C°°. .

e e in the infrared region is in the infrared region is

independent of the color of the skin independent of the color of the skin and is very nearly equal to one , and is very nearly equal to one ,

indicating that the skin at this indicating that the skin at this wavelength is almost a perfect wavelength is almost a perfect

absorber and emitter of radiationabsorber and emitter of radiation. .

The heat loss due to convectionThe heat loss due to convection (Hc) is(Hc) is

Hc = Kc Ac (Ts Hc = Kc Ac (Ts –– T Tαα ) )Where Kc is constant that depend upon Where Kc is constant that depend upon the movement of the airthe movement of the air. .

Ac is the effective surface areaAc is the effective surface area. . Ts is the temperature of the skinTs is the temperature of the skin. .

T T αα is the temperature of the air is the temperature of the air. .

When the body is resting and When the body is resting and there is no apparent wind , K is there is no apparent wind , K is

about 2.3 Kcal /mabout 2.3 Kcal /m²² hr c hr c°°..When the air is moving , the When the air is moving , the constant K increases according constant K increases according to equationto equation

Kc = 10.45 Kc = 10.45 –– v v + 10 √ + 10 √ vvWhere the wind speed Where the wind speed vv is in is in meter per secondmeter per second. .

This equation is valid for speeds This equation is valid for speeds between 2.23 m/sec and between 2.23 m/sec and

20 m /sec .20 m /sec .The equivalent temperature due to The equivalent temperature due to

moving air is called wind chill moving air is called wind chill factor and is determined by the factor and is determined by the actual temperature and wind speed actual temperature and wind speed ..

33 . .EvaporationEvaporationThe method of heat loss that of us The method of heat loss that of us familiar with is the evaporation of familiar with is the evaporation of sweatsweat. .

Under exterme conditions of heat and Under exterme conditions of heat and exercise ,a man may sweat more exercise ,a man may sweat more than 1 liter of liquid per hourthan 1 liter of liquid per hour. .

Since each gram of water that Since each gram of water that evaporates carries with it the heat of evaporates carries with it the heat of vaporization 580 calories , the vaporization 580 calories , the evaporation of 1 liter carries with it evaporation of 1 liter carries with it

580 Kcal580 Kcal . .

The sweat must evaporate from the The sweat must evaporate from the skin in order to give the cooling skin in order to give the cooling effecteffect. .

The amount evaporated depends upon The amount evaporated depends upon the air movement and the relative the air movement and the relative humidityhumidity. .

Insulation of clothingInsulation of clothing

The unit of clothing is cloThe unit of clothing is cloThis unit corresponds to the insulating This unit corresponds to the insulating value of clothing needed to maintain a value of clothing needed to maintain a subjects sitting at rest in comfort in a subjects sitting at rest in comfort in a room at 21 c with air movement of 0.1 room at 21 c with air movement of 0.1

m/sec and air humidity of less than 50m/sec and air humidity of less than 50 . % . % One clo of insulation is equal to a light One clo of insulation is equal to a light weight business suitweight business suit. .

22 clo of clothing would enable a man to clo of clothing would enable a man to withstand a colder temperature than 1 withstand a colder temperature than 1

cloclo . .