672 ANIMAL HEAT 



Oxygen must be absorbed by the lungs in proportion to the heat 

 produced, and where more oxygen is to be absorbed, more blood passes 

 through the lungs to take it up.* It is interesting to inquire whether 

 in the smaller animals the contact of the alveolar air with the rela- 

 tively increased quantity of blood is obtained by a similar increase in 

 the area covered by capillaries (or in what is doubtless approximately 

 proportional to this area, the mass of the lung tissue), or by a shortening 

 of the pulmonary circulation time (p. 137). The answer is that the pul- 

 monary circulation time is markedly shortened as the size of the 

 animal diminishes, although not in proportion to the diminution in 

 weight, but rather in proportion (roughly speaking) to the square root 

 of the surface. This could not be the case if the area of the pulmonary 

 surface bore the same proportion to the area of the skin as the output 

 of the heart does. For in this case the vascular capacity of the lungs 

 (or the quantity of blood contained in them) would be proportional to 

 the heart output, and the time required for the blood discharged by 

 the right ventricle to displace the whole of the blood contained in the 

 lungs at any given moment that is to say, the average pulmonary 

 circulation time would be the same for animals of all sizes. But it 

 may very well be the case if the vascular capacity of the lungs (or 

 the mass of the lung tissue) decreases more rapidly than the surface 

 decreases, say in proportion to the body-weight. At first thought it 

 might appear advantageous that the area of the surface through which 

 oxygen is absorbed should be proportional to the area through which 

 the heat produced in the oxidations of the body is chiefly eliminated, 

 so that the greater the necessary heat loss, the greater should be the 

 facilities for absorption of oxygen. We have, however, already seen 

 (p. 250) that the blood, even when it passes through the pulmonary 

 capillaries at its maximum speed, has still sufficient time to practically 

 saturate itself with oxygen. Within the limits where this holds good 

 there would be no advantage in increasing the relative size of the 

 lungs rather than in increasing the linear velocity of the blood passing 

 through them that is, diminishing the pulmonary circulation time 

 and on general principles it may be assumed that a larger pulmonary 

 reservoir than is necessary for the maximum possible intake of oxygen 

 would not be provided. For if the pulmonary reservoir holds an ex- 

 cessive amount of blood, some other tissues must have too little. It 

 has been stated, indeed, that in animals of different size in the same 

 species the total quantity of blood in the body is a function not of the 

 body-weight, but of the surface, so that the smaller animals have 

 a relatively larger amount of blood (p. 56). If this be so, it is probably 

 related to the greater intensity of metabolism and the greater loss of 

 heat from the surface of small animals, which entails a greater cir- 



* For simplicity, the possibility that the coefficient of utilization of the oxy- 

 gen-carrying capacity of the blood (i.e., the quantity of oxygen absorbed by 

 a litre of blood during its passage through the lungs divided by the total 

 quantity of oxygen which it can take up) may vary, is disregarded. This 

 possibility would imply that the average oxygen content of the venous blood 

 coming to the right side of the heart varied in animals of different size, more 

 oxygen being abstracted, for example, from the blood in passing through the 

 tissues of small animals than of large. The greater the utilization of the 

 oxygen the smaller would the quantity of blood passing through the lungs 

 require to be. There is no evidence, however, that such differences exist be- 

 tween animals of the same species of different size, although it has been sug- 

 gested that a higher coefficient of utilization coupled with a proportionately 

 smaller heart output may be one way in which training diminishes the diffi- 

 culty and discomfort of hard muscular effort. 



