Renal Function in Man 5 



immediately adjoins the glomerulus and has the largest di- 

 ameter, and which is made up of irregular epithelial cells with 

 brush-like striations at their internal border; an ''interme- 

 diate" segment of smaller diameter, made up of quite flat or 

 squamous cells; and a "distal" segment with fairly regular 

 columnar epithelium having striations in the basal aspect of 

 the cell, but lacking the brush formation in the internal 

 border. The urine formed in this nephron is conducted to 

 the renal pelvis by way of an arborized system of collecting 

 tubules which appear to have no function other than that of 

 conduits. According to Moritz and Hayman^* there are 

 an average of 1,283,000 such nephrons in each of the human 

 kidneys. 



The blood supply to this nephron consists of an afferent 

 arteriole which breaks up immediately within the glomerulus 

 to form an elaborate tuft of parallel capillaries; these capil- 

 laries converge into the efferent arteriole, which in turn sub- 

 divides again to form a secondary plexus of capillaries closely 

 applied to the external or basement membrane of the tubule 

 cells. Until recent years it was believed that all the blood 

 reaching the tubules must first pass through the glomeruli, 

 but the studies of MacNider,"*^ OHver^" and Spanner^*^ indi- 

 cate that in the diseased kidney and possibly in the normal 

 kidney as well, circulation in the peritubular capillaries may 

 be established independently of the glomeruli by direct anas- 

 tomoses between the arteriolar tree and either the capillary 

 or venous channels. (See also 8). 



Every student of renal physiology recalls Cushny's theory 

 of urine formation. It was the cardinal premise of this theory 

 that the formation of urine begins with the separation in the 

 glomeruli of an ultrafiltrate identical in composition with 

 the plasma except for the absence of the plasma proteins, to 



