THE RENAL CIRCULATION 1 469 



not absolutely positive. Knoche, on the other hand 

 (using Bielschowsky-Gros stain), described a promi- 

 nent terminal reticulum surrounding the glomerular 

 capillaries. Since there are presumably no contractile 

 elements of the smooth muscle type in the glomerular 

 zone, the role of these nerve fibers becomes highly 

 problematical. He also found nerve filaments around 

 the efferent arterioles. Harman & Davies (133) saw 

 nerve endings in the perivascular tissue of the glomeru- 

 lus, and hypothesized an afferent function for them. 



Knoche also described a prominent reticulum 

 surrounding the complex made up of the macula 

 densa, the polkissen, Goormaghtigh's "cell aggre- 

 gate" and paravascular and paraportal cell clumps, 

 and suggested that together they formed a receptor- 

 effector zone to adjust glomerular filtration to the 

 variations in blood pressure. 



medullary nerve supply. Comparatively few nerve 

 fibers pass into the medulla as compared with the 

 cortex, and most of these are located in the boundary 

 zones. They reach the medulla largely through 

 offshoots from the nerve bundles alongside the arcuate 

 arteries, and by accompanying the arteriolae rectae 

 spuriae. A point of control of the vasa recta system 

 could reside here. 



renal afferents. Beyond the possibility of pres- 

 soreceptors in the field of the juxtaglomerular cell 

 groups, the renal tissue is generally nonsensitive to 

 afferent stimulation. Swelling gives renal pain via 

 stretch of the capsule, which has pain afferents. It is 

 said that some afferent sensory function is localized in 

 the papillary tissue and in the pelvis. 



distribution of osmotic constitutents in the 

 kidney; the countercurrent hypothesis 



The osmotic constituents of the kidney are arranged 

 so that they are isotonic with blood in the cortex, 

 then rise to three to four times this concentration at 

 the tip of the papilla. VVirz et al. (345) have demon- 

 strated by a cryoscopic method (disappearance of ice 

 crystals as observed by a polarizing microscope) that 

 points of equal osmotic pressure form shells concentric 

 to the tip of the papilla (fig. 14), and are parallel to 

 the interzonal boundary. The important osmotic 

 constituents are sodium, chloride, and urea, as 

 revealed by the analysis of Ullrich & Jarausch (312) 

 (figs. 15 and 16), and supported by the findings of 

 Schmidt-Nielsen & OTJell (267). 



A uniform distribution of osmotic constituents 



max. 700 



IsotoniscfiO 



fkj. 14. Distribution of osmotic constituents in the kidney 

 (hamster). A.Z.: outer zone of medulla: I.Z.: inner zone of 

 medulla. [After Wirz et al. (345).] 



between the loops of Henle, vasa recta, and collecting 

 tubules has been proved by micropuncture (346, 

 348) in the hamster and rat (fig. 17) [Gottschalk & 

 Mylle (1 13)-] The "hairpin" loop arrangement of the 

 loops of Henle and vasa recta has been the basis for 

 the formulation of the countercurrent multiplier 

 system concept for urinary concentration and dilution 

 (132). (For discussion of this principle as applied to 

 kidney function, see 131, 169, 171, 259, 289, 314.) 



Briefly, the principle of the countercurrent system 

 as it applies to the medullary loop of Henle system is 

 as follows: sodium, by an active process, and chloride, 

 as the result of an electrochemical gradient thus 

 established, are believed to be transported out of the 

 relatively water-impermeable ascending limb of the 

 loop of Henle into the interstitium of the medulla 

 until a gradient of ca. 200 mOsm per kg. H 2 has 

 been established (fig. 17). This single effect is multi- 

 plied as the fluid of the thin part of the descending 

 limb comes into osmotic equilibrium with the inter- 

 stitial fluid by diffusion of water out (and probably 

 by the diffusion of some NaCl into the descending 

 limb), thus raising the osmolarity of the fluid rounding 

 the hairpin loop into the ascending limb. The in- 



