Human thermodynamics is the study of the energy and entropy aspects of the work cycles involved in human life, namely those existent between heat, spontaneity, irreversibility and the laws defining therein. In short, human thermodynamics is the study of heat and its relation to the motion and changes in the equilibriums of human bodies. (1) Eating Now consider the effects of eating. The body metabolizes all the food we consume. Eating increases the internal energy of the body by adding chemical potential energy. In essence, metabolism uses an oxidation process in which the chemical potential energy of food is released. This implies that food input is in the form of work. Food energy is reported in a special unit, known as the Calorie. This energy is measured by burning food in a calorimeter, which is how the units are determined.(2) Internal energy Our body loses internal energy, and there are three places this internal energy can go—to heat transfer, to doing work, and to stored fat (a tiny fraction also goes to cell repair and growth). As shown in Fig 1 heat transfer and doing work take internal energy out of the body, and then food puts it back. If you eat just the right amount of food, then your average internal energy remains constant. Whatever you lose to heat transfer and doing work is replaced by food, so that, in the long run, ΔU=0. If you overeat repeatedly, then ΔU is always positive, and your body stores this extra internal energy as fat. The reverse is true if you eat too little. If ΔU is negative for a few days, then the body metabolizes its own fat to maintain body temperature and do work that takes energy from the body. This process is how dieting produces weight . (2) Metabolism Life is not always this simple, as any dieter knows. The body stores fat or metabolizes it only if energy intake changes for a period of several days. Once you have been on a major diet, the next one is less successful because your body alters the way it responds to low energy intake. Your basal metabolic rate is the rate at which food is converted into heat transfer and work done while the body is at complete rest. The body adjusts its basal metabolic rate to compensate (partially) for over-eating or under-eating. The body will decrease the metabolic rate rather than eliminate its own fat to replace lost food intake. You will become more easily chilled and feel less energetic as a result of the lower metabolic rate, and you will not lose weight as fast as before. Exercise helps with weight loss because it produces both heat transfer from your body and work, and raises your metabolic rate even when you are at rest. (2)


Cooling of the Human Body An adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism. This becomes a problem when the ambient temperature is above body temperature, because all three standard heat transfer mechanisms work against this heat loss by transferring heat into the body. Our ability to exist in such conditions comes from the efficiency of cooling by the evaporation of perspiration. At a temperature of 45 Celsius or 113 Fahrenheit the evaporation process must overcome the transfer of heat into the body and give off enough heat to accomplish a 90 watt net outward flowrate of energy. Because of the body's temperature regulation mechanisms, the skin temperature would be expected to rise to 37°C at which point perspiration is initiated and increases until the evaporation cooling is sufficient to hold the skin at 37°C if possible. With those assumptions about the temperatures, the Stefan-Boltzmann law for an area of 2 m2 and emissivity 0.97 gives a net input power of 109 watts to the body. The perspiration cooling must overcome that and produce the net outflow of 90 watts for equilibrium. (3) References 1- 2- 3-