2 2 4 RESPIRA TION 



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 pulmonary 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. 137), much less than the average time needed to complete 

 the systemic circulation. In man the ratio is probably about i : 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 man would thus be about one-fifth of that in 

 the systemic vessels. That is to say, not less than 700 grm. out of the 

 4^ kilos* of blood in a yo-kilo man would be contained in the lesser cir- 

 culation, and about 3^ kilos in the greater. This corresponds sufficiently 

 well with calculations from other data. 



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

 cuted 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 1,024 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 con- 

 sisted of blood. Assuming the same proportion for the decapitated 

 individuals, we get 343 grm. as the net weight of the blood-free lungs. 

 Deducting this from 1,024 gmi., we arrive at 681 grm. as the average 

 quantity of blood in the lungs. Adding to this the quantity in the 

 right side of the heart (p. 140), we get, in round numbers, 750 grm. 

 as the amount in the lesser circulation. 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 

 surface of the lungs of a man is about 100 square metres (sixty times 

 greater than the area of the skin), of which, perhaps, 75 square metres 

 are occupied by capillaries. The average thickness of this immense 

 sheet of blood has been reckoned to be equal to the diameter of a red 

 blood-corpuscle, or, say, 8 u. This would give 600 c.c. (630 grm.) as 

 the quantity of blood in tne lunge, which is probably somewhat too 

 low an estimate. 



* See footnote on p, 139 



