1464 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



related to extractable renin or its precursor. It is now 

 conceived that renin is an inhibitor of an antihyper- 

 tensive function of the kidney ( 163). An explanation of 

 hypertension caused by renal ischemia would be as 

 follows: if the renal artery is constricted, the JGA 

 cells, being stretched less, would increase the secretion 

 of renin. This would then inhibit the cells subserving 

 the antihypertension function of the kidney, and 

 systemic blood pressure would rise. 



The cells are also influenced by electrolytes. A diet 

 low in sodium increases granulation in dogs, cats, and 

 rats (both intact and hypophysectomized). Increase 

 in salt intake decreases granularity (310). Although a 

 complex interrelationship with the adrenal cortex 

 is probable, a simpler explanation offered by Tobian 

 is that a low salt intake favors a decrease in blood 

 volume, reduced blood pressure, and decreased 

 stretch of the JGA cells (increased secretion). A high 

 salt diet would have the opposite effect. On the basis 

 of the above scheme, a decreased sodium diet should 

 ultimately result in increased blood pressure, based 

 upon the increased granularity of the JGA. This may 

 indeed be a compensatory mechanism to maintain 

 blood pressure in the face of lowered blood pressure 

 resulting from decreased plasma and extracellular 

 volumes caused by low sodium intake. 



The other structure of importance is a portion of 

 the distal convoluted tubule near the vascular pole, 

 the macula densa. McManus (194-196) has suggested 

 that the JGA, macula densa, and associated struc- 

 tures be together called the juxtaglomerular "com- 

 plex." This epithelial plaque appears to have a 

 reversed polarity from the rest of the tubule, in the 

 sense that the Golgi apparatus is between the nucleus 

 and the attached pole of the cell, (contiguous to the 

 vasculature), rather than between the nucleus and the 

 lumen. The suggestion made by McManus (196) and 

 supported by Garber el al. (98) was that these cells 

 abstracted from the contents of the lumen and trans- 

 mitted this material to the cells of the arterioles. It is 

 relevant to point out that a site of active sodium re- 

 absorption is found in the vicinity of the distal con- 

 voluted tubule. It has been suggested that the JGA 

 and macula densa form a regulatory system capable 

 of responding to osmotic pressure changes (and 

 possibly hydrostatic pressures), in turn modifying 

 glomerular filtration in a self-regulatory manner. 

 This interesting hypothesis needs experimental 

 verification, particularly in view of the contention 

 by de la Pefia & de Castro (70) that structures re- 

 sembling the macula densa were found in apposition 

 to efferent arterioles in the human kidney. It is worthy 



of note that afibrillar cells containing granules, similar 

 to those in the afferent arterioles, have been noted in 

 efferent arterioles (194). 



Blood Supply to the Medullary Zones 



Edwards (79) has described two types of efferent 

 arterioles which exist in the juxtamedullary zone of 

 the human kidney. One out of four to five glomeruli 

 has a "corticomedullary" efferent arteriole to capil- 

 laries of the juxtamedullary parenchyma (fig. 10). 

 The others (about 180,000 per human kidney) 

 have long, descending arterioles (arteriolae rectae 

 spuriae). These go on to the capillaries. One type 

 forms networks around the tubules, the other goes 

 on to the vasa recta system (fig. 1 1). The venae rectae 

 return to the arcuate veins. 



Note in table 1 the greater total muscle volumes 

 in the medullary efferent arterioles as compared with 

 the cortical. The total volume of muscle in the wall 

 of the afferent and efferent arterioles was 0.124 ml 

 and in the medulla 0.169 ml (79). Christensen (54) 

 found the diameters of the juxtamedullary vasa 

 efferentia of the dog kidney about the same as those 

 in the cortical vasa efferentia, contrary to the findings 

 of Trueta et al. (311) who state that the caliber of the 

 juxtamedullary efferent arterioles greatly exceeds the 



Interlobular ... s. 



\ 

 k 



Corhcal 

 Eff. arteriole 



Corhco- medullary 

 Eff. arteriole 



^8i 



s 



5> 



Medullary 

 Eff. arteriole 



fig. 10. The blood supply of the juxtamedullary zone. [After 

 Edwards (79).] 



