200 



ESSENTIALS OF PHYSIOLOGY. 



resistance of the arterioles. The cannula in the artery is also attached 

 to a mercurial manometer F, which records the arterial pressure. The 

 small cylinder E contains air, which forms an air cushion and to some 

 extent replaces the elasticity of the arterial wall. After traversing the 

 tube A, the blood enters a cylinder B in which it is warmed and from 

 which it passes into the superior vena cava, and then through the lungs 

 to the left side of the heart. The circulation is thus confined to the 

 heart and lungs, artificial respiration being maintained by a pump. 



FIG. 71. Arrangement of apparatus in the heart-lung preparation. 

 (Knowlton and Starling. ) Description in the text. 



The output of the left ventricle in a given time, e.g. five seconds, can be 

 measured by clamping the tube G, opening the clip on H, and allowing 

 the blood to flow into a graduated vessel instead of into the cylinder ; 

 and if the rate of the heart is observed, its output at each beat can be 

 calculated. 



In man, the output of the heart has been indirectly determined in 

 the following manner. The individual takes a deep breath of air 

 containing a certain amount of nitrous oxide, which is very soluble 

 in blood. After a few seconds he expires deeply, a sample of the 

 expired alveolar air being collected in the mariner described on p. 251. 



