work ,energy , and power

Energy, work and power of the body

Energy is a basic concept of physics .For basal conditions there are about 25% of body s energy used by skeletal muscles and heart, 19%used by the brain, 10%used by kidneys, 27% used by liver, spleen. The body uses food energy to operate its various organs, maintain constant body temperature and do external work, small percentage 5% of food energy excreted in the feces and uric any energy left over is stored as body fat. The body basic energy source is food that chemically changed by the body to make molecule that can combine with oxygen in body s cells.

Conservation of energy in the human body

A thermodynamic system exchange energy with its surroundings by mean of heat and work. When heat is added to the system and work is done by it ,both Q, W are positive, when heat is transferred out of the system and work is done on it Q,W are both negative.
The energy of human body is controlled by the first law of thermodynamics for a closed system.



?U: changed in stored energy
?Q: heat lost or gain
?W: work done by the body.
Its important to determine the rates for the energy changes in a small time ?t:


Where ?U/?t : catabolic rate
?Q/?t : heat loss or absorption rate
?W/?t : mechanical power


In this lecture we will use kilocalories and joules for energy units where 1 kcal=4184 J

In the oxidation process within the body heat is released as energy of metabolism
Basal Metabolic Rate (BMR): The amount of energy needed to perform minimal body function (breathing, pumping blood through arteries under resting condition). BMR depends on thyroid function. A person of an overactive thyroid has a higher BMR than with normal thyroid function.
Since the energy used for metabolism becomes heat and dissipated from the skin so it related to surface area or the mass of the body.



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This figure shows a plot of BMR for various animals of different weights. BMR depends on the temperature of the body, if a patient has temp. 40° C or 3 above normal, BMR is about 30% greater than normal. The patient temp. is lowered during heart surgery.


Work and power:

Chemical energy stored in the body is converted into external mechanical work as well as into life-preserving functions.




The rate of doing work is the Power P=?W/?t = F?x/?t = Fv

When a man is climbing a hill or walking up stair we calculate the work done by multiplying person weight (m g) by vertical distance (h). when a man walking at constant speed on level surface the force act in the direction perpendicular to his motion, thus external work done by him appears to be zero. However his muscles are done as internal work which appear as heat and causes a rise in muscle temp.
Heat losses from the body
The human body core temperature is constant around 37.1 ± 1.0 oC
( degree Fahrenheit = degree Celsius - 1.8 + 32 )












The hypothalamus of brain contains body thermostat to keep temp. close to normal value. If the core temp. rises, the hypothalamus initiates sweating and vasodilatation which increases skin temp. if the skin temp. drops, the thermo- receptors on the skin inform hypothalamus and it initiate shivering, which causes increase in the core temp. the difference between energy radiated by the body and the energy absorbed from surrounding can be calculated by :



Hr energy loss or gain
Ar surface area of emitting radiation
e emissivity of surface
TS skin temp.
Tw surrounding temp.
Kr constant= 5kcal/m2hr.°C


The heat loss due to convection Hc is:


KC const. depend on the movement of air
AC surface area
Ta air temp.

When the wind const. =2.3 Kcal/m3 hr.C, Ta=25 °C, TS=34°C, AC=1.2m2
The nude body loses about 25 Kcal /hr by convection or when the air is moving, the constant Kc increase according to the equation



Where V is wind speed in m/sec.



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10


2 3 10 15 20



Homework :
calculate the convective heat loss per hour for a nude standing in a 5 m/sec wind. Assume TS=33°, Ta=10°, AC=1.2 m2. if wind speed 2.23 m/sec find the still air temp. that would produce the same heat loss. Hent kc=27.8