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 spleen  Basal 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-  W Heat Loss/gained from/by Body  Q Work done  W Change in stored energy in the body  U Another 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 =10 7 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/ m 2 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 O 2 used= 686/(22.4x6) =5.1 Liters of O 2 used /gram of fuel = (22.4x6) /180 =0.75 Liters of CO 2 produced /gram of fuel = (22.4x6) /180 =0.75 Respiratory Quotient =

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 10  C change in temperature, rate changes by a factor of 2 Normal Core body temperature:  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