402 THE MECHANICS OF THE CIRCULATION, HEMODYNAMICS 



nitrous Oxid instead of nitrogen . A similar procedure has been followed by Boothby ^ 

 who has determined the minute-volume of the pulmonary blood stream of man dur- 

 ing rest and muscular exercise. These experiments have shown that the total blood 

 flow through the lungs amounts to more than 3 liters in a minute, and hence, about 

 60 c.c. of blood must be discharged by each systole of the heart. But this figure 

 may be varied somewhat by changes in posture, muscular work, and a more thor- 

 ough ventilation of the lungs and consumption of oxygen. 



The Velocity of the Blood Flow. — We have seen that the main pur- 

 pose of the circulation is to supply the different colonies of cells with 

 nutritive material and to remove from them all those substances which 

 are of no further use to them. This interchange occurs in the capil- 

 laries, where the blood and the body-fluid are separated from one 

 another by only a very thin layer of cells. These tubules, therefore, 

 are of much greater metabolic value than the arteries and veins. The 

 latter merely play the part of supply channels. 



The systemic and pulmonary circuits arise from single tubes, the 

 repeated division and subdivision of which eventually gives rise to an 



intricate network of the finest pos- 

 sible tubules, the capillaries (Fig. 

 216). The gradual reunion of 

 these in turn leads to the forma- 

 tion of large collecting channels 

 which are finally united in a com- 

 mon reservoir, the auricles. It 

 should be remembered, however, 

 that the total cross- section of the 

 vascular system increases con- 

 j ^ ^ stantly in the direction of the capil- 



Fio. 216.-Diagr1m to' Illustrate Varies, but diminishes again distally 



THE Changes in the Cross-section of to these, and the more SO the closer 



THE Vascular System. ^^ approach the heart. The 



^' ^'''''''±^:VL^^^'^'''' ""' s^^llest blood-beds, therefore, are 



found at the aorta and at the venae 

 cavae. The latter, however, is somewhat larger than the former. 

 Their peripheral ramifications put together represent a blood-bed 

 which is very much larger than that of either the arteries or veins. 

 As has just been stated, the blood-bed again decreases in size on the 

 other side of the capillaries, because while the sectional areas of the 

 different single veins increase constantly as they unite into larger 

 channels, their combined area becomes less. Consequently, the size 

 of the vascular system at the venae cavae is almost as small as that at 

 the aorta. It is also of interest to note that the blood-bed of the 

 aorta is somewhat larger than that of all the arteries combined, which 

 fact again tends to show that the aorta serves as the elastic reser- 

 voir of the circulatory system. 



As far as the velocity of the blood flow is concerned, the preceding 

 statements must show immediately that the speed of flow is greatest 



1 Am. Jour, of Physiol., xxxvii, 1915, 383. 



