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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



fig. 7. Electron microscope reconstruction 

 of the glomerular filtering membranes. [After 

 Hall (125).] 



INTRACAPILLARY SPACE 

 (LUMEN) 



LAMINA FENESTRATA 

 (LINING NETWORK). 



PODOCYTE 

 (COVERING CELL) 



EXTRACAPILLARY SPACE 

 (CAPSULAR SPACE) 



This may facilitate the filtration process by slowing 

 flow and reducing turbulence. 



Figure 7 shows a sectional diagram of the struc- 

 tures forming the filtration apparatus as developed 

 by electron microscopy (125). The pores in the capil- 

 lary endothelium (.05 fi thick) (lamina fenestrata) 

 are too large (0.1 /x) to restrain the plasma constitu- 

 ents. Rather, they expose the ultrafiltration mem- 

 brane, the lamina densa, to the free flow of plasma 

 by removing the endothelial cytoplasmic barrier. 



Although the lamina densa (glomerular basement 

 membrane), 0.1 n thickness, exhibits differences in 

 stratification (244), it is probable that it is a homoge- 

 nous layer; pores that have been noted are probably 

 artifactual. It appears to be the limiting membrane 

 for restraint of plasma proteins and cells. 



The podocytes (foot cells) of the visceral layer of 

 Bowman's capsule rest on the lamina densa with 

 thousands of foot processes (pedicels). Hall has sug- 

 gested that they may play an important part in the 

 regulation of filtration. The space between the pedicels 

 may be narrow enough ( 1 00 A) to be a limiting dimen- 

 sion in restriction of plasma proteins ("slit pore"). 

 Hall suggested that the foot processes may be nar- 

 rowed or widened thereby exposing a greater or lesser 

 area of basement membrane, although a mechanism 

 by which such changes could be brought about has 

 not so far been proposed. However, it is conceivable 

 that changes in caliber of the capillaries as a function 

 of internal pressure (vis a tergo) may alter the spacing 

 of the pedicels. Using as a basis the observations on 

 the frog glomerulus, Elias et al. (82) described another 

 possible method of regulation. They observed that 



the position of the glomerular blood channels is not 

 constant and undergoes changes (e.g., transverse 

 displacement) in relation to the foot processes. Thus, 

 a group of pedicels may be active while a blood 

 channel is located under them, and later at rest 

 (when that blood channel has shifted to a new loca- 

 tion). 



The permeability of the filtering membrane of the 

 kidney has been repeatedly studied by determining 

 the renal plasma clearance of molecules of varying 

 sizes. Wallenius (326), for example, by fractional 

 hydrolysis of dextran, produced and separated sub- 

 stances with a wide range of molecular sizes and 

 shapes and examined the facility with which they 

 passed into the urine (fig. 8). He calculated that the 

 pore radius in the dog glomerular membrane may 

 range from 18 A to 50 A. These findings are in accord 

 with the anatomical evidence. The findings of Gie- 

 bisch et al. (100) are in essential agreement. The 

 ratio of dextran clearance to circulation clearance 

 fell markedly at a molecular weight of ca. 50,000. 



Juxtaglomerular Complex 



Two structural entities at the vascular pole of the 

 glomerulus, the juxtaglomerular apparatus and 

 macula densa, have been thought to be related in 

 some way to the control of blood pressure or salt 

 balance and thus to be concerned with renal hyper- 

 tension (310). One of these, the juxtaglomerular 

 apparatus (JGA), is a thickening of the media of the 

 afferent glomerular arterioles (polkissen) (fig. 9). 

 The cells of the JGA become swollen, afibrillar in 



