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basal metabolism:see metabolismmetabolism,
sum of all biochemical processes involved in life. Two subcategories of metabolism are anabolism, the building up of complex organic molecules from simpler precursors, and catabolism, the breakdown of complex substances into simpler molecules, often accompanied by
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the series of metabolic and energetic processes in animals and man as measured when the subject is in a fasting state and awake and at rest in a comfortable ambient temperature. The amount of energy expended for the maintenance of heart function, blood circulation, respiration, and thermoregulation is called the basal metabolic rate (BMR). The BMR is dependent on many factors, including the subject’s weight, surface area, size, age, and sex; species, diet, and habitat also affect it. The BMR is usually expressed in kilocalories per hour or day per kilogram of body mass or square meter of surface area. Basal metabolism in warm-blooded animals is more dependent on the surface area of the body than on the weight; this phenomenon is called the law of surface areas (see Table 1).
|Table 1. Daily heat production in various animals|
|Animal||weight (kg)||Heat production (kcal*)|
|per 1 kg||per 1 m2|
|* 1 kcal = 4.19 kJ|
In man, basal metabolism is measured in the morning 12–16 hr after the subject has eaten and at an ambient temperature of 20°–22°C. In animals, it is measured at various intervals after eating, depending on the duration of digestion (from 3 hr in mice to several days in ruminants). The BMR is determined by direct and indirect calorimetry. In direct calorimetry, the heat emitted by the organism per unit time is measured in a special chamber called a calorimeter. In indirect calorimetry, the expenditure of energy is determined by direct measurement of the amount of 02 consumed per unit time (seeGAS EXCHANGE, RESPIRATORY QUOTIENT).
The BMR calculated per kilogram of body mass decreases with age. The contrast between the BMR of children and that of elderly persons is especially striking. The BMR is 10–15 percent lower in females than in males (see Table 2). The basal metabolism in a healthy adult human maintains a fairly constant level of about 6.7–7.1 megajoules, or 1,600–1,700 kcal, per day. In man and animals, the relative constancy of basal metabolism fluctuates within certain limits and is subject to both circadian and seasonal biological rhythms. Basal metabolism is highest during the active hours—daytime in diurnal animals and nighttime in nocturnal animals. In man, the BMR increases in spring and early summer and decreases in late fall and in winter.
|Table 2. Basal metabolism in men and women of different ages|
|Age(in years)||Kcal per 1 m2 of body surface per 1 hr|
The nervous system and endocrine glands play an important role in regulating the BMR. Thus, factors that increase energy expenditure, including the condition that usually accompanies physical exertion, may increase basal metabolism by a conditioned reflex. In medical practice, the BMR serves as an aid in the diagnosis of certain diseases, for example, thyroid disorders. The patient’s BMR is compared with standard values, and the results are used to determine the theoretical normal limits of the patient’s basal metabolism; allowances are made for the individual’s weight, height, sex, and age. Basal metabolism is also determined in conjunction with studies on animal ecology and as an aid in establishing feeding rates for livestock.
REFERENCESSlonim, A. D. Zhivotnaia teplota i ee reguliatsiia v organizme mlekopitaiushchikh. Moscow-Leningrad, 1952.
Ol’nianskaia, R. P. Ocherki po reguliatsii obmena veshchestv. Moscow-Leningrad, 1964.