Chapter 5
and Energy ,Work , Power of the BodyEnergy ,Work ,and Power of the Body
We can consider the body to be as an energy converter All activities of the body , including thinking involve energy changes. The conversion of energy into work such as lifting a weight or riding a bicycle represents only a small fraction of total energy conversions of the body .Under resting conditions about 25% of thebody’s energy is being used by the skeletal muscles and the heart , 19 % is being used by the brain , 10 % is used by the kidneys , and 27 % is being used by the liver and spleen .
The body’s basic energy (fuel) source is food . The food is converted into molecules chemically .The body uses the food energy to operate its various organs :Maintain constant temperatureDo external force for example ,lifting.
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A small percentage (5 %) of the food energy is excreted in the feces and urine ; any energy that is left over is stored as body fat . The energy used to operate the organs eventually appears as body heat .Some of this heat is useful in maintaining the body at its normal temperature, but the rest must be disposed of .
Conservation of Energy in the Body
Conservation of energy in the body can be written as a simple equation. [Change in stored energy ] = Heat lost from the [ in the body (food energy,] body + Work [ fat, and body heat] done There are a continuous energy changes in the body both when is doing work and when it is not .The first law of thermodynamic equation is :∆ U =∆ Q + ∆ W ----------(1)Where ∆U is the change in stored energy∆Q is the heat lost or gain ∆W is the work done by the body in some interval of time .
A body doing no work(∆W = 0)and at a constant temperature to lose heat to its surroundings , and ∆Q is negative .∆U is also negative, indicating a decrease in stored energy .
The change of ∆ U , ∆ Q and ∆ W in a short interval of time ∆ t , equation (1) becomes∆U= ∆Q + ∆W ∆t ∆t ∆t where ∆U/∆t is the rate of change of stored energy ∆Q/∆t is rate of change of heat loss or gain,∆W∆t is the rate of doing work, that is mechanical work .
Energy Changes in the Body
The unit of energy in SI unit is Joule . The physiological unit of food energy is Kilocalories . The unit of heat production = Kcal/minute 1 Kcal =4184 Jmet
Metabolic rate (MR)
Metabolic rate :is define as the rate of oxidation In oxidation process within the body heat is released as energy of metabolism. Basal Metabolic Rate (BMR) : is the lowest rate of energy consumptionBasal metabolic rate (BMR)
Is defined as the amount of energy needed to perform minimal body functions ( Such as breathing and pumping the blood through the arteries ) under resting conditions .In oxidation of the glucose , heat energy is released Example : C 6 H12 O6 + 6 O 2 6 H 2 O + 6 CO 2 +686Kcal 1 (mole) + 6 (mole) 6(mole) +6 (mole)+heat energy 180gm 192gm 108gm +64 gm + 686 Kcal
Energy released per gm of glucose = 686/180 = 3.8Kcal /gm Energy released per liter of O2 used = 686 /6 x22.4 =5.1 Kcal /liters Liters of O2 used per of fuel = 22.4 x 6/ 180 =0.75liters/gm Liters of CO 2 produced per gm of fuel = 6 x22.4/180 = 0.75liters /gm
Example (metabolic)
Suppose you wish to lose 4.54 kg either through physical activity or by dieting . a . How long would you have to work at an activity of 15 kcal/ min to lose 4.54 of fat ?
from table energy release for 1 gm of fat is 9.3 kcal/g If you work for T minutes ,then (T min)( 15 kcal/min) = (4.54 x 10і g)(9.3 kcal/g ) (T min)( 15 kcal/min) = 4.2 x 10 kcal T = 28810 min T = 47 hour the time taken to lose 4.54 kg of fat
b. It is usually much easier to lose weight by reducing your food intake . If you normally use 2500 kcal/day , how long must you diet at 2000 kcal/day to lose 4.54 kg of fat
Work and power
Chemical energy stored in the body is converted into external mechanical work as well as into life –preserving functions .External work
External work is defined as a force moved through a distance Δx ΔW = F Δx where W is the work Δx is the distanceThe force and the motion x must be in the same direction .External work is done when a person is climbing a hill or walking up stairs , We can calculate the work done : Work done = persons weight x vertical distance moved W = m g h
When a man is walking or running at a constant speed on a level surface , most of the forces act in the direction perpendicular to his motion . Thus , the external work which done by him appears to be zero .
However , his muscles are doing internal work which appears as heat in the muscle and causes a rise in its temperature .this additional heat in the muscle is removed by blood flowing through the muscle , by conduction to the skin , and by sweating .
We can measure the external work done and power supplied by a subject , for example :riding a fixed bicycle we can also measure the oxygen consumed during this activity . the total food energy consumed can be calculated since 5 kcal are produced for each liter of oxygen consumed .
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 ( aerobic work ) as shown in figure 1 .
The maximum work capacity of the body is variable
Figure 1Figure1:phases of work