Our own bodies, like all living organisms, are energy conversion machines. Conservation of energy implies that the chemical energy stored in food is

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2 Humans: Work, Energy, and power: Our own bodies, like all living organisms, are energy conversion machines. Conservation of energy implies that the chemical energy stored in food is converted into work, thermal energy, or stored as chemical energy in fatty tissue, as shown in. Energy consumed by humans is converted to work, thermal energy, and stored fat. By far the largest fraction goes to thermal energy, although the fraction varies depending on the type of physical activity. The fraction going into each form depends both on how much we eat and on our level of physical activity. If we eat more than is needed to do work and stay warm, the remainder goes into body fat.

3 All activities of the body including thinking, involve energy changes. The conversion of the energy into work occurs continuously in the body.

4 For example under resting (Basal) conditions skeletal muscles and the heart using 25% of the body's energy. another 19%is being used by the brain. 10%is being used by the kidneys. and 27% is being used by the liver and the spleen.

5 The body uses the food energy to:- 1. Operate its various organs. 2. Maintain a constant body temperature. 3. Do external work e.g. lifting. A small percentage (~5%) of the food 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 appears as body heat.

6 Conservation of energy in the body:- Change in stored energy Heat loss from body Work done This is similar to the first law of thermodynamic:- ΔQ= Δu + Δw

7 This is similar to the first law of thermodynamic:- ΔQ= Δu + Δw Where ΔQ is the change of quantity of heat of the system. Δu is the change in the internal or stored energy. Δw is the work done. This can be written as Δu= ΔQ - Δw

8 A body doing no work (Δw=0) at constant temp. It continues to lose heat to its surroundings, and ΔQ is negative. Therefore, Δu is also negative, indicating a decrease in stored energy. The rate of change of their variables is just taken per unit time ( by dividing on Δt). Δu / Δt = ΔQ/Δt - Δw/Δt

9 Energy change in the body Kilocalories (kcal) for food energy. Kcal per minute for the rate of heat production. In physics the unit of energy is Newton-meter or joules. Power is given in joules per second =watts. 1cal= 4.184J or 1Kcal=4184J

10 The body's basic source of energy is the food energy; it must be chemically changed by the body to make molecules that can combine with oxygen in the body's cells.

11 The rate at which the body uses food energy to sustain life and to do different activities is called the metabolic rate, and the corresponding rate when at rest is called the basal metabolic rate (BMR). The energy included in the basal metabolic rate is divided among various systems in the body, with the largest fraction going to the liver and spleen, and the brain coming next. About 75% of food calories are used to sustain basic body functions included in the basal metabolic rate.

12 Work done by a person is sometimes called useful work, which is work done on the outside world, such as lifting weights. The energy consumption of people during various activities can be determined by measuring their oxygen use, because the digestive process is basically one of oxidizing food.

13 Example: Suppose you wish to lose 4.54kg either through physical activity or by dieting. 1-How long would you have to work at an activity of 15Kcal/min to lose 4.54kg of fat if you know maximum loss of fat = 9.3 kcal/g if you worked for T minutes? (T min.)(energy loss/min)=(energy of 4.45 kg of fat) (T min)(15 kcal/min)=(4.54x10^3 g)(9.3kcal/hr) T=2810 min =47 hr

14 2-It is much easier to lose weight by reducing your food intake. If you normally use 2500kcal/day, how long must you diet at 2000kcal/day to lose 4.54kg of fat? T= (energy of 4.54kg fat/energy deficit per day) 4.2X104kcal / 5x10^2kcal/day 84 days

15 The rate at which the body uses food energy to sustain life and to do different activities is called the metabolic rate(mr). The total energy conversion rate of a person at rest is called the basal metabolic rate (BMR) and is divided among various systems in the body. The largest fraction goes to the liver and spleen, with the brain coming next. Of course, during vigorous exercise, the energy consumption of the skeletal muscles and heart increase markedly. About 75% of the calories burned in a day go into these basic functions. The BMR is a function of age, gender, total body weight, and amount of muscle mass (which burns more calories than body fat). Athletes have a greater BMR due to this last factor.