All animals exchange energy with the environment. Some energy is exchanged as mechanical work, but most is exchanged as heat (Fig. 29.4). Heat is exchanged by conduction, convection, and radiation and as latent heat through evaporation or (rarely) condensation of water. If the sum of energy production and energy gain from the environment does not equal energy loss, the extra heat is "stored" in, or lost from, the body. This relationship is summarized in the heat balance equation:
Exchange of energy with the environment.
This hiker gains heat from the sun by radiation and loses heat by conduction to the ground through the soles of his feet, convection into the air, radiation to the ground and sky, and evaporation of water from his skin and respiratory passages. In addition, some of the energy released by his metabolic processes is converted into mechanical work, rather than heat, since he is walking uphill.
where M is metabolic rate,- E is rate of heat loss by evaporation,- R and C are rates of heat loss by radiation and convection, respectively,- K is the rate of heat loss by conduction,- W is rate of energy loss as mechanical work,- and S is rate of heat storage in the body, manifested as changes in tissue temperatures.
M is always positive, but the terms on the right side of equation 1 represent energy exchange with the environment and storage and may be either positive or negative. E, R, C, K, and W are positive if they represent energy losses from the body and negative if they represent energy gains. When S = 0, the body is in heat balance and body temperature neither rises nor falls. When the body is not in heat balance, its mean tissue temperature increases if S is positive and decreases if S is negative. This situation commonly lasts only until the body's responses to the temperature changes are sufficient to restore balance. However, if the thermal stress is too great for the thermoregu-latory system to restore balance, the body will continue to gain or lose heat until either the stress diminishes sufficiently or the animal dies.
The traditional units for measuring heat are a potential source of confusion, because the word calorie refers to two units differing by a 1,000-fold. The calorie used in chemistry and physics is the quantity of heat that will raise the temperature of 1 g of pure water by 1°C,- it is also called the small calorie or gram calorie. The Calorie (capital C) used in physiology and nutrition is the quantity of heat that will raise the temperature of 1 kg of pure water by 1°C,- it is also called the large calorie, kilogram calorie, or (the usual practice in thermal physiology) the kilocalorie (kcal). Because heat is a form of energy, it is now often measured in joules, the unit of work (1 kcal = 4,186 J), and rate of heat production or heat flow in watts, the unit of power (1 W = 1 J/sec). This practice avoids confusing calories and Calories. However, kilocalories are still used widely enough that it is necessary to be familiar with them, and there is a certain advantage to a unit based on water because the body itself is mostly water.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.