ARTERIAL PRESSURE. 237 



changes in the rate of the current in the capillaries. The 

 pressure on the blood in the pulmonary artery is about J of 

 that in the aorta. 



Since the blood flows from the aorta to its branches and 

 from these to the capillaries and thence to the veins, and 

 liquids in a set of continuous tubes flow from points of 

 greater to those of less pressure, it is clear that the blood- 

 pressure must constantly diminish from the aorta to the 

 right auricle; and similarly from the pulmonary artery to 

 the left auricle. At any point in fact the pressure is pro- 

 portionate to the resistance in front, and since the farther 

 the blood has gone the less of this, due to impediments at 

 branchings and to internal friction, it has to overcome in 

 finishing its round, the pressure on the blood always di- 

 minishes as we follow it from the aorta to the venae cavae. 

 In the larger arteries the fall of pressure is gradual and 

 small, since the amount of resistance met with in the 

 flow through them is but little. In the small arteries and 

 capillaries the resistance passed -by is (on account of the 

 great internal friction due to their small calibre) very great, 

 and consequently the fall of pressure between the medium- 

 sized arteries and the veins is rapid and considerable. 



Modifications of Arterial Pressure by Changes in the 

 Rate of the Heart's Beat. A little consideration will make 

 it clear that the pressure prevailing at any time in a given 

 artery depends on two things the rate at which, the vessel 

 is filled, i.e. upon the amount of work done by the heart; and 

 the ease or difficulty with which it is emptied, that is upon, 

 the resistance in front. Returning to the system of elas- 

 tic tubes with a pump represented in Fig. 87, let us sup- 

 pose the pump to be driving as before 10,800 cub. cent. 

 (360 oz.) per minute into the tubes B and that these latter 

 are so distended that they drive out just that quantity in 

 the same time. Under such conditions the pressure at any 

 given point in B will remain constant, apart from the small 

 variations dependent upon each stroke of the pump. 

 Now, however, let the latter, while still sending in 180 cub. 

 cent. (6 oz.) at each stroke, work 80 instead of 60 times a 

 minute and so send in that time 180 X 80 = 14,400 cub. 



