266 THE CIRCULATION OF THE BLOOD 



end being connected by tubing with a piston recorder, from the movement of which 

 the volume of blood flowing into the receiver can readily be computed. To measure 

 the flow of blood, a clip on the tube of the receiver is removed at the same moment 

 that the thread around the vena cava above the diaphragm is tightened, and when the 

 receiver has filled with blood, this thread is again loosened and the receiver tilted up 

 so that the blood flows at low pressure back into the circulation. The receiver being 

 of known capacity, the length of time it takes the blood to fill it as determined by the 

 piston recorder, furnishes us with the necessary data from which to calculate the rate 

 of flow. The receiver is chosen of such a size that it takes only a few seconds to 

 fill, the diversion of blood into it not causing any material fall in arterial pressure. 

 The observations are repeated frequently. 



The Magnitude of the Flow 



By the use of these methods it has been found that the total mass 

 movement of blood to the liver of the dog varies between 1.46 and 

 2.40 c.c. per second for 100 grams of liver. Considerable changes may 

 occur in the arterial pressure without affecting the liver flow. When 

 the hepatic artery is occluded, the flow diminishes by about 30 per 

 cent, or conversely, when the portal vein is obstructed but the hepatic 

 artery left intact, by about 60 per cent, indicating that about one-third 

 of the total bloodflow through the liver is contributed by the hepatic 

 artery and two-thirds by the portal vein. Some blood, however, gains 

 the liver through anastomotic channels between it and the diaphrag- 

 matic veins. 



The relative supply by the two vessels is subject to various condi- 

 tions. That through the hepatic artery, for example, may be very con- 

 siderably altered on account of vasoconstriction in this vessel, for its 

 walls can easily be shown to be liberally supplied with vasoconstrictor 

 fibers carried by the hepatic plexus. This can be demonstrated by 

 the rise in blood pressure which occurs in a branch of the hepatic artery 

 during stimulation of the plexus. On the other hand, alterations in the 

 bloodflow in the portal vein can not be brought about by active con- 

 striction or dilatation of the intrahepatic branches of this vessel, no 

 active vasomotor fibers having been demonstrated by stimulation of 

 the hepatic nerves, although, as in the case of the vessels of the brain 

 and lung, a certain amount of constriction may occur under the influence 

 of epinephrine. 



The bloodflow through the portal vein is dependent on changes oc- 

 curring at either end of the distribution of the vessel, that is, changes 

 occurring in the liver itself or in the intestine. Of these factors the lat- 

 ter is no doubt the more important, an increase not only in portal blood 

 pressure but also in portal bloodflow being readily produced by dila- 

 tation of the splanchnic blood vessels; for example, as the result of sec- 

 tion of the splanchnic nerve. Alterations in portal bloodflow brought 



