400 RELATIONS TO BLOOD-PRESSURE. [BOOK u 



depend on the constriction or dilation of the renal arteries and their 

 ramifications, for distension due to venous obstruction will only 

 occur in special cases. Hence variations in the volume of the 

 kidney may be taken as a measure of variations in its vascular 

 supply, increase of volume indicating dilated renal vessels, and 

 decrease of volume indicating constriction of the renal vessels. 



When by means of the instrument just described a tracing is 

 taken of the volume of a kidney in what may be considered 

 a normal condition, some such result as that shewn in Fig. 65 is 

 obtained. 



BLOOD PRESSURE. 



KIDNEY CURVE. 



FIG. 65. BLOOD-PBESSUBE TBACING, AND CUBVE FBOM KENAL ONCOMETEB. Natural 

 size. The blood-pressure abscissa line has been raised 2*75 cm. (the actual medium 

 blood-pressure having been 115 mm. Hg.). The time-curve gives interruptions re- 

 curring every three seconds. 



The volume of the kidney is seen to be so delicately responsive 

 to changes in the mean arterial pressure that the curve reproduces 

 almost exactly a blood-pressure curve, shewing not only the respira- 

 tory undulations, but even the rise and fall due to the individual 

 heart beats. With each rise of mean arterial pressure more blood 

 is driven into the renal vessels and the kidney swells: with each fall 

 of pressure less blood enters and the kidney shrinks. On other 

 tracings taken in the same way may often be seen the wider 

 variations corresponding to the Traube-Hering curves; but it will 

 be observed that in these the kidney shrinks with the rise of 

 pressure and swells with the fall. For as we have seen (p. 373) the 

 rise in the Traube-Hering undulation is due to an augmentation of 

 peripheral resistance caused by the constriction of minute arteries ; 

 and this constriction occurs in the kidney as elsewhere ; the renal 

 arterioles take their share in producing the result, and in consequence 

 of their constriction the kidney shrinks. Similarly the relaxation 

 of the renal vessels contributes to bring about the sequent fall. 



Other variations in the volume of the kidney are seen to arise 

 from various influences. When respiration is stopped the in- 

 creasingly venous blood, acting on the medullary or spinal vaso- 

 motor centres, leads to constriction of the renal as well as of other 



