[480 



HANDBOOK O I- 1'HYSIOLOGY 



CIRCULATION II 



the kidney. This is verified by examination of the 

 concentration in the renal vein (I,). Such can be 

 related to the renal arterial plasma concentration 

 1 1) as the extraction ratio E: E = (A c — V c )/A c . 

 Hence, if V c = o, E will equal 1 .0. E is less than unity 

 to the extent that the material is not removed by 

 urinary excretion. It is clear that if the clearance is 

 divided by the extraction ratio (C/E), the resultant 

 quotient will yield the total renal plasma flow (RPF). 



UV 



RPF 



A c- V c 



This formula is an expression of the Fick principle, 

 for if P be taken as A c (systemic venous plasma con- 

 centration equal to renal arterial plasma concentra- 

 tion), we have: 



UV 



RPF' 



Ac 



UV 



A c~ v c 



% 



In order to obtain total renal blood flow, the hemato- 

 crit measurement of the blood is introduced : 



RBF= 



E ( l-hemot ) 



or 



UV 



(A c -V c ) < l-hemat) 



Several substances are so efficiently removed by 

 combined processes of glomerular filtration and active 

 tubular (proximal) transport (secretion) at low plasma 

 concentrations, that the renal vein concentration is 

 very low (i.e., extraction nearly complete, and E 

 close to unity). These include Diodrast (D) and/>-ami- 

 nohippurate (PAH) (287, 288). Then C D or C PA h is 

 nearly equivalent to plasma flow. The fact that ex- 

 traction is not complete is interpreted as indicating 

 that a small fraction of blood does not perfuse ex- 

 cretory tissue: this would include capsule and inert 

 supportive tissue, medullary tissue (loops of Henle, 

 collecting ducts), calycine mucosa, and pelvis. On 

 this basis, Smith has referred to this as the "effective" 

 plasma or blood flow. 



A considerable amount of study has been made of 

 the extraction ratios of Diodrast and PAH. A repre- 

 sentative group of findings is shown in table 5. 



Although E D and £p AH seem comparable in the 

 dog, £ PAH is more efficient in man than E D . This was 

 particularly the experience of Bergstrom et al. (16) 

 who made simultaneous comparisons (E PAB = 0.90; 

 E ly = 0.74). Differences in kinetics of erythrocyte to 

 plasma shift for PAH ma\ lie involved, and this is 



probably less important for man than the dog. A 

 factor to be considered is that the animal work has 

 done largely under anesthesia, while the human sub- 

 jects were unanesthetized. 



The Fick principle can be employed with any sub- 

 stance cleared by the kidney which shows a measura- 

 ble A-V difference. Obviously, the smaller the A-V 

 difference, the more prone to error the calculation 

 will be. Thus, phenol red, urea, mannitol, and inulin 

 have been employed, but have considerably smaller 

 A-V differences than Diodrast and PAH. 



the nitrous oxide method. This is an adaptation of 

 the method employed for the measurement of cerebral 

 blood flow and involves inhalation by the subject of 

 nitrous oxide, and uptake from the blood by the 

 kidney. The Fick principle is employed (58, 68). 



N 2 



100V, ,-. 

 f' (A * 



VJdt 



RBF S „ is the blood flow per 100 g kidney tissue 

 per minute as measured by N 2 uptake; V cV -S is 

 the kidney uptake of N 2 per g tissue during the time 

 from O to /' (time of blood-tissue equilibrium); and 

 A c and V c are arterial and renal venous concentra- 

 tions, respectively, which finally become equal at 

 time I'. S is the partition coefficient between blood 

 and tissue (assumed to be unity in this instance). 



