217. Increased Respiration And Blood Flow

For these reasons, there occurs more frequent respiration and a more rapid passage of the blood through the lungs where it comes in contact with the respired air. Still further, the blood is more fully thrown to the surface of the body where it may cool more rapidly, and perspiration also occurs in order that its evaporation may aid in ridding the body of the excess of heat (see p. 168). All this means more food, somewhat in proportion to the work done, the energy of which is expended not only to carry on external work, but also in part to support the work attending the increase of breathing and blood flow. More external work causes more physiological or internal work.

1 Loc. cit., p. 252.

Additional data may be cited to support the above statements. In the case of two men, it was found that in climbing up a steep incline the inspired air increased not less than five times in volume over the use when resting. When a person is walking rapidly or cycling, the number of respirations per minute is at least doubled, and the depth of respiration is increased several times, so that the volume of each breath becomes greater than under rest conditions. Even the work of dressing and undressing, with the attendant influence of a period of nakedness, caused in twenty-one observations an average increase in oxygen use of 34 per cent and an increase in heat radiation of 18 per cent.1

218. Fuel Efficiency With Man

Measurements of the oxygen consumption under various conditions show that one foot-pound increase of mechanical labor costs in extra food energy approximately the equivalent of three foot-pounds of food energy, that is, the factor of efficiency of human food as fuel is about 33 per cent.2 This shows that the living human machine is relatively a most efficient one. Practically the same factor holds for work animals.

219. How Fuel Efficiency Is Modified

Several conditions materially modify this factor of efficiency. When a person takes up mechanical operations with which he is not familiar, or enters upon work that exercises a new set of muscles, a unit of work accomplished costs more in food energy than is the case with operatives whose muscles are trained to do a particular thing. Trained workmen will do a given amount of la))or on less food than the untrained. Very strenuous exercise, like athletic contests, is wasteful of food energy. The general rule is that the energy cost of a unit of work increases with the rate of work above what would be the natural movement. The figures of the table on the following page show this.

1 " Metabolism and Energy Transformations of Healthy Man during Rest," Benedict and Carpenter, p. 247.

2 Benedict and Carpenter calculate the factor of efficiency to be 20.9 per cent, that is, that proportion of excess food energy above maintenance is realized in labor performed.

Unnaturally slow movements also are expensive of energy. After a continuance of the same labor for hours, there is an increase in the energy expenditure per unit of work performed, and fatigue, whether it comes after a shorter or longer time, has a similar effect.

Economy in the use of the energy that the food supplies to the body, which is equivalent to economy in the use of the body itself, is most fully secured when the movements in labor are at the natural rate, neither hurried nor restrained, and when periods of intense effort do not occur, and when labor is not too long continued and is not carried to the point of extreme fatigue. In considering the general nature of the diet for sustaining work, it should be remembered that the non-nitrogenous constituents of the food, the carbohydrates and fats, furnish the main supply of energy (see pp. 156, 171).