THE RENAL CIRCULATION 



I 46 I 



located in the renal vein at its entrance to the vena 

 cava, and in the main branches of the renal vein (the 

 latter not as a rule in man). They are found also at 

 the orifices of the interlobar veins, arcuate veins, and 

 occasionally just before the opening of the capsular 

 (stellate) veins into the interlobular veins. 



Koester el al. (162) have found, in both human and 

 dog kidneys, structures in the veins which might act 

 as effluent constrictions, which they described in 

 terms of "stenoses" and '"sinusoidal cushions." 

 Stenoses are common at the ostia of smaller tributaries 

 entering interlobar veins in the human kidney. They 

 seem to be composed of a dense collagenous framework 

 lined with endothelium; usually muscle is present as 

 a proliferation of the media of the vessel. In the dog, 

 stenoses appear occasionally along the course of the 

 interlobars and primary tributaries to the renal vein; 

 however, they are present primarily at the con- 

 fluence of arcuates with interlobars and of the 

 interlobular with arcuates. 



At the confluence of arcuates with one another to 

 form an interlobar vein in the human kidney, the 

 sinusoidal cushions usually appear, sometimes at 

 the confluence of the interlobulars with arcuates. 

 These structures characteristically contain venous 

 sinuses (which connect with interlobular and medul- 

 lary veins) in the connective tissue matrix. These 

 structures are often interlaced with smooth muscle. 

 Their appearance is said to resemble erectile tissue 

 (162). In the dog, they are less extensive and lie 

 primarily close to the arcuate-interlobar junction. 

 Smooth muscle in the cushions of this species is very 

 inconspicuous or absent. 



It is worthy of emphasis that the aforementioned 

 structures are not valves in the sense of those found in 

 systemic veins, although many of those pictured by 

 von Kiiglegen et al. (320, 321) exhibit a cusplike 

 organization. In any event, their designation as 

 "effluent constrictors" at present best describes their 

 function, although the functional significance is hard 

 to assess. The relatively high pressure found in the 

 arcuate veins of dogs (24 mm Hg) by Swan el al. 

 (302) appeared to give functional evidence of a point 

 of increased resistance at the arcuate-interlobar 

 junction. When the catheter was withdrawn into the 

 interlobar vein, pressure decreased immediately to 7 

 mm Hg. Brun et al. (41) found wedged catheter 

 pressures averaging 18 mm Hg in the human kidney; 

 the pressure in the renal vein averaged 5.6 mm Hg. 

 It was concluded that the wedged renal vein pressure 

 equalled arcuate venous pressure and hence very near 

 to the pressure in the peritubular capillaries and 



interlobular veins. According to Koester et al. (162), 

 the effluent constrictors keep the kidney "functionally 

 distended" with fluids; they state also that these 

 structures cause smaller vessels of the vascular system 

 (venous channels?) to widen in bore, reducing 

 resistance to blood flow. The logic of this can be 

 doubted since this at best would only compensate for 

 the initial resistance imposed. Nor is this supported by 

 physiological studies in which venous pressure has 

 been experimentally elevated (119, 122, 123, 233, 

 273, 281, 32g), under which circumstance over-all 

 renal resistance in fact increases, possibly by a 

 "venous-arteriolar" reflex. 



Glomerular Circulation 



The studies of Boyer (29), Elias et al. (82), Hall 

 (125, 126), Johnston (154), and Kurlz & McManus 

 ( 1 70) show that the glomerular capillaries are not 

 simple loops, but form a freely branching, anastomotic 

 network (fig. 6). More specifically, larger through 

 channels exist with an associated capillary network 

 of smaller anastomotic channels. Hall has suggested 

 that this may afford a structural basis for the skim- 

 ming of plasma relatively freed of cells into the net- 

 work of small capillaries, while the greater mass of 

 blood cells directly and rapidly flows through the 

 lobule to the efferent arterioles as an axial stream. 



fig. 6. Glomerular capillary supply, showing anastomotic 

 connections. [After Elias (82).] 



