GENERAL FEATURES OF THE CIRCULATION 871 



are conditioned by their structure. In Fig. 373 is represented the extensi- 

 bility, i.e. the increase in capacity of an artery and a vein under gradually 

 increasing internal pressure. It will be seen that an artery which has a 

 certain capacity at zero pressure gradually distends with increasing pressure. 

 The increase in capacity is small at first, and becomes most rapid between 

 90 and 100 mm. Hg. After this point every increment of pressure brings 

 about a gradually diminishing increment of capacity. Thus a change of 

 internal pressure causes the greatest change in capacity when the pressure 

 in the artery corresponds, as we shall see, to the average arterial pressure 

 in the normal animal. In the vein, on the other hand, the capacity, which 

 is nothing at zero pressure, becomes considerable on raising the pressure 

 to 1 mm. Hg. A further rise of pressure to 10 mm. Hg causes a consider- 



Capacity in c.c. 



90 



ZO 30 40 50 60 70 80 90 100 



120 130 I4Q ISO 160 



mm. Hg, 



FIG. 373. Curves of distensibility of an artery (thick line) and of a vein (thin line). The 

 figures at the left side of the diagram represent the capacity of a section of the vessel 

 when distended under a certain pressure, expressed by the figures on the base line in 



mm. Hg. (Constructed from figures given 



pressed t 

 by ROY.) 



able increase in volume, but from this point the increments of volume 

 with rising pressure rapidly diminish. Whereas the artery is most dis- 

 tensible at about 100 mm. Hg, the vein has its limits of optimum distensi- 

 bility between and 10 mm. Hg. 



As the arteries branch, although each branch is smaller than the parent 

 vessel, the total area of the two branches into which the vessel divides is 

 greater. Thus there is a continual increase in the cross area of the bed 

 of the blood- stream as we pass from the heart towards the periphery. 

 This increase is especially marked at the junction between the capillaries 

 and the arterioles at one side and the venules on the other, so that the 

 total area of the bed in the region of the capillaries can be taken as about 

 800 times that of the area of the aorta where the blood leaves the heart. 



On cutting through an artery, blood escapes from the central end, i.e. 

 that nearest the heart, with great force and in a series of jerks, each of 

 which corresponds to a contraction of the ventricles. This manner of 



