THE RENAL CIRCULATION 



1479 



showed that IRP was related to mean arterial pres- 

 sure (P A ) as follows: IRP = g.4 + .22 P A . Thus, 

 when arterial pressure increased 1 mm Hg, IRP 

 increased 0.22 mm Hg. 



Other factors: a) location of needle tip. Gottschalk 

 compared needle pressure in the cortex versus the 

 medulla. In rats and rabbits there was no significant 

 difference. In dogs, repeated determinations were 

 more variable, but there were no consistent differ- 

 ences in deep and superficial measurements. VVinton 

 (343) has recorded a much higher IRP in the medulla 

 than in the cortex (fig. 24). This may have particular 

 significance in interpretation of the countercurrent 

 system (vide infra). 



b) Decapsulation. Miles & DeWardener (206) found 

 no difference in IRP as the result of decapsulation. 

 Nor did differences appear as IRP was increased by 

 osmotic diuresis (mannitol), elevation of venous pres- 

 sure, and increase in perfusion pressure after KCN 

 elimination of renal circulatory autonomy. VVinton 

 and Swann reported a reduction of IRP to about one- 

 half as the result of decapsulation (342). Although the 

 latter results seem most logical, the former investiga- 

 tors emphasize the necessity of care in handling to 

 avoid decreases in IRP due to vasoconstriction, and 

 the need for an adequate recovery period before 

 measurements are made. The significance of the 

 change of IRP with decapsulation will be considered 

 further below in relation to the problem of renal 

 circulatorv autonomv. 



"1 



40 



NEEDLEX^ * 

 PRESSURE 

 mm Hg 



**?£-■ 



URETER PRESSURE ram H9 



O S IO 15 20 25 30 35 



fig. 24. Needle pressures in medulla and cortex of the dog 

 kidney related to ureter pressure. The straight diagonal line 

 represents perfect correlation. [After VVinton (343).] 



Intrarenal pressure and the countercurrent system. Reduc- 

 tion in urine volume and electrolyte output of the 

 kidney as the result of increase in venous and ureteral 

 pressure are well-known phenomena (127, 273, 277, 

 279). Increases in intrarenal pressure could favor 

 reduction in urine volume in several ways: pressure 

 on the collecting ducts would slow movement of urine 

 in this segment and favor removal of water molecules 

 to the zone of hyperosmolarity, operationally domi- 

 nated by ADH. Secondly, compression of the vasa 

 recta would slow the flow of blood and favor uptake 

 of water into the hyperoncotic zone. Finally, a high 

 interstitial pressure could favor inward flow. If the 

 observation of Winton (fig. 24) that IRP is higher 

 in the medulla than in the cortex can be confirmed, 

 particularly specific and effective mechanism would 

 exist. 



The mechanism underlying decreased excretion of 

 electrolytes is more complex, involving among other 

 factors alterations in load offered to the tubules re- 

 sulting from changes in filtration rate brought about 

 by venous and ureteral pressure elevation (127, 273, 

 277. 2 79)- 



MEASUREMENT OF RENAL BLOOD FLOW 



Methods 



Both direct and indirect methods have been em- 

 ployed. Direct methods employ flowmeters of various 

 types which record either arterial inflow or venous 

 outflow. These include the following: thermostroh- 

 muhr or direct-recording strohmuhr (Ludwig type); 

 rotameter; bubble flowmeter; electromagnetic flow- 

 meter; direct venous outflow (cylinder and stop- 

 watch). The indirect methods are an application of 

 the renal clearance principle. This is based on the 

 clearance (C) ratio: C = 11' P, in which U is the 

 urinary concentration in mg per ml., V is the minute 

 urine volume, and P is the plasma concentration 

 (usually systemic vein) in mg per ml. [For method- 

 ology, including the use of clearance techniques, see 

 (272, 288).] 



The plasma clearance of inulin (C lu ) has been 

 universally proved to be solely by glomerular filtra- 

 tion. In dog and other mammals (except anthropoids) 

 the clearance of creatinine (C Cr ) is also a measure of 

 filtration rate. 



The requisite for the clearance of a substance to 

 measure plasma flow is that it be entirely removed 

 (or nearly so) from the plasma in one transit through 



