The work of maintaining the respiration and circulation evidently involves a continual expenditure of energy. It is clear too that deep and rapid breathing or vigorous heart action must involve an increased activity of the muscles concerned. But it is not always clear to what extent increased respiratory and heart action are a cause and to what extent they are an effect of increased energy metabolism. Thus Murlin and Greerl emphasize the close relationship of the heart to the requirements of the tissues for energy in that the energy metabolism is immediately dependent upon oxygen supply. Since but little available oxygen can be stored in the living substance, "the response of the heart to variations in the (energy) requirement must be immediate and, within very narrow limits of time, proportional to this requirement." And Benedict states that:

If subject A, in a resting post-absorptive condition, has on a given day a pulse rate of 70 per minute, and on a subsequent day under exactly the same conditions has a pulse rate of 60 per minute, it may be asserted with every degree of confidence that the metabolism on the second day will be perceptibly, indeed measurably, lower than the first.

1 American Journal of Physiology, Vol. 33, page 253.

A large factor in basal metabolism is the maintenance of muscular tension or tone. That every living muscle is always in a state of tension is evident from the fact that it gapes open if cut. It is equally evident that the degree of tension (and therefore the expenditure of energy required to maintain it) varies greatly in different individuals under similar conditions and in the same individual under different conditions. The differences observed by Atwater and Benedict between the metabolism of the sleeping hours and that of the hours spent sitting up without muscular movement (65 and 100 Calories respectively) are largely due to the more complete relaxation of the muscles during sleep. Thus there is in the "resting" muscle a continual expenditure of energy which first takes the form of muscular tension, or tone, but ultimately appears as heat, so that the heat production, or energy metabolism, of the body at rest depends to a considerable extent upon the degree of tension which still persists in the muscles.

Benedict and Carpenter report the following figures in Calories per hour, for the energy metabolism during sleep (1 a.m. to 7 a.m.) following different conditions of activity and showing the after effects of work upon muscular tension during rest:

Energy Metabolism During Sleep - Calories Per Hour

Benedict also finds that even under the most quiet conditions a higher tension gradually develops during the waking hours. A fasting man metabolized when lying at complete rest 14 per cent more in the morning than when sound asleep at night, and 22 per cent more in the late afternoon than when asleep.