208 A MANUAL OF PHYSIOLOGY 



apparently be derived from the vitiated blood of the right ventricle, 

 but is obtained directly from the aortic system by the bronchial 

 arteries. These are distributed with the bronchi, which they supply 

 as well as the connective-tissue of the interlobular septa running 

 through the substance of the lung, the pleura lining it and the walls 

 of the large bloodvessels. Most of the blood from the bronchial 

 arteries is returned by the bronchial veins into the systemic venous 

 system, but some of it finds its way by anastomoses into the pul- 

 monary veins. 



The branches of the pulmonary artery are also distributed with 

 the bronchi, and break up into a dense capillary network around the 

 alveoli. From the capillaries veins arise which, gradually uniting, form 

 the large pulmonary veins that pour their blood into the left auricle. 



The same quantity of blood must, on the whole, pass per unit of 

 time through the lesser as through the greater circulation, otherwise 

 equilibrium could not exist, and blood would accumulate either in 

 the lungs or in the systemic vessels. But it does not follow that at 

 each heart-beat the output of the two ventricles is exactly equal. If, 

 indeed, the capacity of the lesser circulation were constant, the 

 quantity driven out at one systole by the right ventricle would be 

 the same as that ejected at the next by the left ventricle. But it is 

 known that the capacity of the pulmonary vessels is altered by the 

 movements of respiration and probably in other ways, so that it is 

 only on the average of a number of beats that the output of the two 

 ventricles can be supposed equal. 



The time required by a given small portion of blood, e.g., by a 

 single corpuscle, to complete the round of the lesser circulation, is, 

 as we have seen (p. 125), much less than the average time needed to 

 complete the systemic circulation. In the rabbit the ratio is prob- 

 ably about 1:5. Since all the blood in a vascular tract must pass 

 out of it in a period equal to the circulation time, the average quantity 

 of blood in the lungs and right heart of a rabbit must be about one- 

 fifth of that in the systemic vessels. On the assumption that the 

 same proportion holds for a man, not less than 700 grm. out of the 

 4^ kilos* of blood in a 70 kilo man must be contained in the 

 lesser circulation, and about 3! kilos in the greater. This corre- 

 sponds sufficiently well with calculations from other data. 



For example, the average weight of the lungs in three persons, 

 executed by beheading, was 457 grm. (Gluge). The average weight 

 of the lungs in a great number of persons who had died a natural 

 death was 1024 grm. (Juncker). The weight of the pulmonary 

 tissue alone in the first set of cases must be less than 457 grm., for 

 the lungs of a person who has bled to death are never bloodless. 

 In a dog killed by bleeding from the carotid, one-quarter of the 

 weight of the lungs consisted of blood. Assuming the same propor- 

 tion for the decapitated individuals, we get 343 grm. as the net 

 weight of the blood-free lungs. Deducting this from 1024 grm., 

 we arrive at 68 1 grm. as the average quantity of blood in the lungs. 

 Adding to this the quantity in the right side of the heart (p. 128), 

 we get, in round numbers, 750 grm. as the amount in the lesser cir- 

 culation. It is true that in the living body the conditions are not 

 the same as after death ; but it is probable that in a large number 

 of cases taken at random the differences would be approximately 

 equalized. 



It has been further calculated that the total area of the alveolar 



* See footnote on p. 127. 



