Energy

ML 504: Class 6

Work

Power

2013: Supriya Babu

Energy

• Basic concept of physics• Even thinking requires energy!!!• Conversion of energy to work happens in

body• At rest:– 25% of body’s energy is used by skeletal

muscles and heart– 19% by brain– 10% by kidneys– 27% by liver and spleenBasal Metabolic Rate

Fuel (source of energy)

• Food• Food is not in a suitable form for direct energy

conversion Has to be Chemically modified (digestion)

• From physics point: body is energy converter Law of conservation of energy First Law of Thermodynamics

• Body uses food energy to:– Operate various organs– Maintain constant body temperature– Do external work– Store in form of fat (Insurance)

Conservation of Energy

• Change in energy storage: food energy, body fat and body heat

• First law of thermodynamics: U = Q- WHeat

Loss/gained from/

by BodyQ

Work

done

W

Change in

stored energy in the bodyUAnother form that is most commonly used:

U/ t = Q/ t - W/ t

Energy Changes in body

• Unit of energy used by physiologists: kilocalories (kcal or Cal; 1kcal = 4184 Joules)

• Physics unit of energy: Joule (J = N-m =107 ergs)

• Physics unit of power: Joules/sec or Watt (W)• In terms of rate of energy consumption by

body, the unit used is met. –met = 50kcal/ m2 of body area per hour– Indicates energy consumption during resting

conditions: Basal Metabolic Rate= 92 kcal/hr or 107 W

Oxidation of food

• The chemical energy used by body is obtained from the oxidation of food molecules

• Occurs in cells of body• In oxidation process heat is released as energy of

metabolism: metabolic rate• The glucose sugar molecule, for example, is oxidized

as follows:

• 1 mole of Glucose (180g) gives 686 kcal energy• The caloric value per unit weight is different for

various foods.• On the average,– carbohydrates (sugars and starches) and proteins

provide about 4 Cal/g,– lipids (fats) produce 9 Cal/g, – alcohol produces 7 Cal/g.

Energy Release

• Caloric Value = kCal of energy released /gram of fuel = 686/180 =3.80

• kCal of energy released /liter of O2 used= 686/(22.4x6) =5.1

• Liters of O2 used /gram of fuel = (22.4x6) /180 =0.75

• Liters of CO2 produced /gram of fuel =

• (22.4x6) /180 =0.75• Respiratory Quotient = usedMoles of O

producedOMoles of C

2

2

Basal Metabolic Rate

• Becomes heat• Depends on body temperature, with 1°C

change in temperature metabolic rate changes by 10%

• Primarily dissipated from skin• Is related to surface area and mass of body• As animals get larger their BMR increases

faster than their surface area but not as fast as their mass (volume)

• Chemical processes are temperature dependent

Work and power

• External work: defined as force (F) moved through distance (x), force and distance must be same direction– W = F x

• Rate of doing work is power,– For constant force, P = W/ t = F x / t =

Fv

• Efficiency () of human body as machine is defined as:– = work done / energy consumed

Temperature Regulation

• To maintain rates of Metabolic processes– For 10C change in temperature, rate

changes by a factor of 2

• Normal Core body temperature: 36 - 38 C

• >44 C - protein denatures• < 28 C - heart stops beating

Moderate Physical Activity

• 260 Cal/hr of energy is consumed– 208 Cal is released as heat If all heat remained in body - 3 C /hr

raise in temperature 2 hrs of such activity causes complete

collapse Excess heat must be dissipated

Schematic of Heat Loss System

Medical Physics by J. R. Cameron & J.G. Skofronick – John Wiley and Sons, 1978, Pg.- 98

Heat Exchange

Heat regulation controls

bcs.whfreeman.com/thelifewire/content/chp41/41020.html

Summary