Thermodynamics

Chapter 4 | 193TOPIC OF SPECIAL INTEREST*Thermodynamic Aspects of Biological SystemsAn important and exciting application area of thermodynamics is biologicalsystems, which are the sites of rather complex and intriguing energy transferand transformation processes. Biological systems are not in thermodynamicequilibrium, and thus they are not easy to analyze. Despite their complexity,biological systems are primarily made up of four simple elements: hydrogen,oxygen, carbon, and nitrogen. In the human body, hydrogen accounts for63 percent, oxygen 25.5 percent, carbon 9.5 percent, and nitrogen 1.4 percent ofall the atoms. The remaining 0.6 percent of the atoms comes from 20 other elementsessential for life. By mass, about 72 percent of the human body is water.The building blocks of living organisms are cells, which resemble miniaturefactories performing functions that are vital for the survival of organisms. Abiological system can be as simple as a single cell. The human body containsabout 100 trillion cells with an average diameter of 0.01 mm. The membraneof the cell is a semipermeable wall that allows some substances to passthrough it while excluding others.In a typical cell, thousands of chemical reactions occur every second duringwhich some molecules are broken down and energy is released and some newmolecules are formed. This high level of chemical activity in the cells, whichmaintains the human body at a temperature of 37°C while performing thenecessary bodily tasks, is called metabolism. In simple terms, metabolismrefers to the burning of foods such as carbohydrates, fat, and protein. The rateof metabolism in the resting state is called the basal metabolic rate, which isthe rate of metabolism required to keep a body performing the necessaryfunctions (such as breathing and blood circulation) at zero external activitylevel. The metabolic rate can also be interpreted as the energy consumptionrate for a body. For an average male (30 years old, 70 kg, 1.8-m 2 body surfacearea), the basal metabolic rate is 84 W. That is, the body dissipates energy tothe environment at a rate of 84 W, which means that the body is convertingchemical energy of the food (or of the body fat if the person has not eaten)into thermal energy at a rate of 84 W (Fig. 4–37). The metabolic rateincreases with the level of activity, and it may exceed 10 times the basalmetabolic rate when a body is doing strenuous exercise. That is, two peopledoing heavy exercising in a room may be supplying more energy to the roomthan a 1-kW electrical resistance heater (Fig. 4–38). The fraction of sensibleheat varies from about 40 percent in the case of heavy work to about 70 percentin the case of light work. The rest of the energy is rejected from the bodyby perspiration in the form of latent heat.The basal metabolic rate varies with sex, body size, general health conditions,and so forth, and decreases considerably with age. This is one of the reasonspeople tend to put on weight in their late twenties and thirties even though theydo not increase their food intake. The brain and the liver are the major sites ofmetabolic activity. These two organs are responsible for almost 50 percent ofthe basal metabolic rate of an adult human body although they constitute onlyabout 4 percent of the body mass. In small children, it is remarkable that abouthalf of the basal metabolic activity occurs in the brain alone.*This section can be skipped without a loss in continuity.FIGURE 4–37An average person dissipates energy tothe surroundings at a rate of 84 Wwhen resting.© Vol. 124/PhotoDisc1.2 kJ/s1 kJ/sFIGURE 4–38Two fast-dancing people supply moreenergy to a room than a 1-kW electricresistance heater.