NORMA], KIDNEY TRACING 196 



rise and fall of blood pressure during respiration are represented by 

 an increase and decrease of volume of the kidney. This is exactly 

 the result we should expect to obtain if the vessels follow passively 

 any variations of blood pressure, and this is the result during ordinary 

 conditions when the kidney is at rest. Comparing fig. 156 with fig. 

 157, both of which were yielded by the same animal, we see how 

 closely volume changes follow blood pressure changes. Fig. 156 was 

 obtained at the commencement of the experiment, and is therefore to 

 be taken as especially typical of the volume changes. Fig. 157 was 

 obtained later, and with a less magnification. The respiratory move- 

 ments altered in character as this tracing was being recorded. There 

 occurred an increase in kidney volume and a fall in blood pressure, 

 so that the fall in blood pressure is chiefly to be explained as due to 

 dilatation of blood vessels in which the kidney took part. The altera- 

 tion in volume closely follows that of the blood pressure. In watch- 

 ing the two being recorded it is very obvious how the rise of the 

 oncometer lever precedes by a quite appreciable interval the rise of 

 the manometer float. This difference is purely instrumental in origin, 

 the kidney rise being recorded by apparatus having very little inertia, 

 whereas the inertia of the mercury manometer is great. 



Changes in volume of the kidney may be brought about in either 

 of two ways : — 



(i.) Passively, in which case the increase and decrease of volume 

 follow proportionately a rise and fall of blood pressure. This is the case, 

 as we have seen, with the ordinary kidney trace, where the rapid undu- 

 lations in the blood pressure, due to heart beats, and the slower, due 

 to respiratory effects, are exactly reproduced in the kidney oncogram. 



(ii.) Actively, in which case changes in volume of the kidney are 

 brought about independently of changes of blood pressure or force of 

 heart beat, and may even work against these. Thus an active con- 

 striction of the kidney vessels leads to a diminution of kidney volume 

 and a rise in blood pressure. These changes in kidney volume are \ 

 best studied by experiments which locally influence the kidney i 

 vessels. Of these changes some of the best known are those pro- 

 duced by drugs which are known to influence the secretion of urine. 

 They are also of greatest interest because they teach us at the same 

 time something of the way in which the kidney works when it is called 

 upon to secrete more actively. 



Action of Caffeine. — In fig. 158 is reproduced the effect of an 

 injection of 2 c.c. of a 4 per cent, solution of caffeine citrate in 

 normal saline directly into the external jugular vein. After a period 

 of delay it is seen that the blood pressure falls, the variations in prefl 

 sure with each heart beat become less, then increase, and the blood 



