84 Physiology of the Kidney 



is regulated entirely by the autonomous or local activity of 

 the renal arterioles."' 



We turn then to the demonstration that the renal arter- 

 ioles are functionally connected with the nervous system. 

 Elementary physiology teaches us that Homo sapiens pays a 

 substantial price for his upright posture; every time he 

 stands erect his blood tends to accumulate in the subcardial 

 regions, particularly in the capillary and venous channels; he 

 normally resists the venous failure which this stagnation 

 favors by walking to and fro, the restless contractions of his 

 leg muscles, aided by the venous valves, serving to promote 

 the return of blood to the right heart. If, however, he stands 

 still progressive venous stagnation leads to progressive de- 

 crease in cardiac output, which in turn tends to reduce ar- 

 terial pressure; through the vaso-sensitive zones (aortic arch 

 and carotid sinus) the first lowering of mean arterial pressure 

 elicits reflex vasoconstriction throughout the body. In this 

 response to posture we have an excellent means of evoking 

 reflex excitation of the vasoconstrictor paths. 



In the experiment shown in Figure 5 the subject, after 

 three control periods in the horizontal position, stood up- 

 right, leaning against the wall until syncope occurred. This 

 is not a very protracted ordeal, as most of those who have 

 tried it can testify. As soon as he assumed the upright posture 

 the renal plasma flow decreased; in view of the fact that the 

 mean arterial pressure did not decrease, but was in fact 

 slightly increased, this renal ischemia must be attributed to 

 constriction of the renal arterioles. The fall in filtration frac- 

 tion indicates that here constriction of the afferent arterioles 



"This conclusion acquires special interest in view of recent anatomical studies of the 

 glomerular apparatus which indicate the presence in both the afferent and efiferent arter- 

 ioles of a rather elaborate system of specialized myo-epithelioid cells the local responses of 

 which may determine the caliber of these vessels (See 7). 



