THE RENAL CIRCULATION 



H75 



Rinden- 

 Morkzone 



cmm/mg Tmc/tengemcM 

 \Verbrautfi 



10 30 50 70 



Zeif in Min uteri 



fig. 20. Oxygen utilization (mm 3 / 

 mg tissue dry weight) in various zones 

 of the guinea pig kidney. [After Grupp 

 & Hierholzer (115).] 



sodium reabsorption in the collecting tubules has 

 been recently adduced by microcatheterization (140). 

 This process probably involves an ion-exchange 

 mechanism with K and NH 4 akin to that operating 

 in the distal convoluted tubule (315). 



Heat Production 



Grupp et al. (116, 11 7) have calculated that a dog 

 kidney of average weight of" 40 g and an average 

 surface of 6.0 cm 2 produces ca. 8 cal per min (0.18 

 cal/g/min). Fregler (95) found an average of 0.52 

 cal per g per min (0.29-0.78). By insertion of small 

 thermistor probes to varying depths into the dog 

 kidney, Ochwadt & Schmier (232) and Grupp & 

 Janssen (117) were able to compare the temperature 

 difference at varying depths from the surface of the 

 kidney to the renal arterial blood. The distribution of 

 the temperature gradient appears in figure 21. Note 

 the highest values in the cortex, a drop at the cortico- 

 medullary junction, and a secondary increase in the 

 medulla. Since differential flow effects could compli- 

 cate the picture, heat production was measured by 

 Janssen & Grupp (151) in kidneys in which the circu- 

 lation was stopped for 1 to 3 min. In these, the cortex 

 averaged 0.162 C above arterial blood, and in the 

 medulla, 0.113 C, confirming the above trend. 



The higher heat production in the cortex is related 

 to its higher metabolism. The dip in the gradient 

 at the corticomedullary junction could be explained 

 by the more direct influence of the large vessels lying 

 in this zone, reflecting systemic temperature. The 

 temperature gradient correlates only roughly with that 

 of oxygen utilization. If flow in the medulla is indeed 

 as slow as the measurement of Kramer et al. (166) 

 would indicate, heat storage could be a factor in the 

 relatively higher temperature in the medulla. 



Grupp & Janssen (117) have related the heat 



turnover of the kidney to the blood flow by the for- 

 mula: turnover (per sec) = K-i/F[(0 R — d A )/(6 v — 

 9a)], where F is the flow (ml min); K is a constant 

 based on the timing and the specific heat of kidney 

 tissue and blood; 8 R is the temperature within the 

 renal tissue; A is the temperature in the renal artery; 

 and 8 V is the temperature in the renal vein. 



The mean trend is shown in figure 22. Above a 

 flow of 2 ml per g per min, the turnover is directly 

 related to flow. Below this value, turnover slows 

 noticeably and becomes quite independent of flow. 

 Environmental factors (conduction, etc) must now 

 have a greater effect than flow on heat removal. 



PRESSURE GRADIENTS IN THE 

 RENAL VASCULAR CIRCUIT 



Pressure Gradient 



Ideal circumstances for analysis of pressure gradi- 

 ents in the renal vascular circuit would require direct 

 micropuncture of representative segments of the ves- 

 sels. The only mammal in which this has been done 

 for analysis of pressure is the rat (1 12, 347), and then 

 only in the peritubular capillaries of the cortical 

 tubules. Coupled with this were measurements of 

 intratubular (proximal) pressure. The results appear 

 in table 3. Wirz (347) reported data from 1 7 anes- 

 thetized male rats which were in excellent agreement: 

 in proximal tubules, 14.8 mm Hg (to 22); in 5 of 

 these animals, postglomerular capillary pressure was 

 17.4 ± 2.6 mm Hg. Using Winton's (343) estimate 

 that glomerular pressure is 65 per cent of arterial 

 pressure, the following gradient probably exists in 

 the rat: mean arterial pressure, 100 mm Hg; glomeru- 

 lar capillary, 65 mm Hg; peritubular capillary pres- 

 sure, 16 mm Hg; and renal vein pressure, 2.0 mm 



Hg (in). 



