Metabolic Rates For Typical Tasks

The rate at which we generate heat (the metabolic rate) depends mostly upon our level of muscular activity, partly upon what we eat and drink (and when), and partly on where we are in our normal daily cycle. Our heat production is measured in metabolic (met) units (Table 4.1). One met is defined as 50 kcal/h m2 (equal to 18.4 Btu/h ft2 or 58.2 W/m2). One met is the energy produced per unit of surface area by a seated person at rest. Under these conditions, the total heat produced by a normal adult is about 360 Btu/h (106 W). The more active we are, the more heat we produce.

There are a number of interactions between our skin and the rest of our body. These include the sense of touch, the circulation of blood, and the exchange of water vapor. The sensations of touch include pressure and pain as well as heat and cold. The experiences of heat and cold are produced by contact with building or object surfaces and by immersion in air, as well as by radiation. These sensations frequently signal impending shifts in bodily heat regulation, a process that is controlled by the portion of the brain called the hypothalamus.

• ^aFor average adult with a body surface area of 19.6 ft2 (1.8 m2). For whole‐body average heat production.
• ^bOne met = 18.4 Btu/h ft2 = 58.2 W/m2

The hypothalamus triggers changes in our blood circulation patterns in response to signals from our skin and changes in our core body temperature. If the body temperature is dropping (we are cold), the rate of heat loss from the body needs to be reduced. This is accomplished through a decrease in the flow of blood from the core toward the surface of the skin. This decrease in blood flow toward the surface is called vasoconstriction, and is triggered in part by temperature (cold) signals from our skin. Blood carries heat around the body, and reduced flow to the extremities under cold conditions reduces heat loss. Under this condition, our sweat glands also force less water to the skin surface, which reduces evaporation and thus heat loss.

Note the implications of this zoning arrangement. We strive to maintain, at all costs, a nearly constant core temperature for our vital organs. This protected zone takes thermal precedence over the less vital extremities zone, including the arms and legs and then the fingers and toes. The farther from our central body mass (fingers and toes) and the greater the surface area (ears), the more and faster the temperature will drop in cold conditions. The most variable thermal zone is our skin surface.

When cold conditions worsen, we get goosebumps, symptoms of our skin’s unsuccessful attempt to create insulation by fluffing up our body hair. Because we cannot add insulation this way, we soon increase our metabolic rate, or burn more fuel, by shivering, muscular tension, or increased muscular activity. At the point where shivering incapacitates us, we may reach 6 met. Before this point, we seek help from our second and then our third skins of clothing and buildings.

The opposite occurs when we are too hot: first, blood flow toward the skin surface increases (vasodilation), triggered primarily by warm signals from our core. The sweat glands greatly increase their secretion of water and salt to the skin surfaces. This increases heat loss by evaporation (although salt accumulations impede evaporation by lowering the vapor pressure of water).