6 9 8 ANIMAL HEAT 



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. 251) 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 Httle. 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- 

 culation through the skin. This greater cutaneous circulation means 

 the permanent withdrawal of blood from the rest of the body, which 

 can be compensated by a corresponding increase in the total circulating 

 mass. The fact that the blood contained in the living skin must 

 form a substantial fraction of the total blood, and a fraction rela- 

 tively more important in the smaller than in the larger animals, 

 would of itself help to establish a relation between surface area 

 and total quantity of blood. The greater volume of the blood 

 will contribute to the increased output of the heart in the smaller 

 animals. In this connection the fact already alluded to more than 

 once may be again emphasized, that it is not the quantity of blood con- 

 tained in an organ or an organism, but the quantity passing through 

 the capillaries in a given time, which is the important thing for its 

 function, in the case of the lungs for the function of the gaseous ex- 

 change, in the case of the skin for the regulation of the heat-loss. 



