The Evolution of the Kidney 39 



ordinarily complex, but on anatomical analysis this complex- 

 ity is reducible to fairly simple terms. Each of the two kid- 

 neys, which are of about the same size, is made up of slightly 

 more than one million microscopic units, or nephrons. (See 

 Figure 1 of the preceding lecture.) These nephrons are all 

 essentially alike and consist of a filtering bed composed of a 

 capillary tuft, or glomerulus, which drains directly into a 

 long, elaborate tubule. These million-odd glomerular-tubu- 

 lar units empty into common collecting ducts which through 

 confluent union finally deliver the urine into the pelvis of 

 the kidney, whence it flows down the ureter into the bladder. 

 In the two million-odd glomeruli, i.e., in the renal filtering 

 bed where the formation of urine begins, the blood is literally 

 spread out over a great surface by being divided among the 

 innumerable capillary channels. The total surface of the 

 glomerular capillary bed in the two human kidneys exceeds 

 1.0 sq. meter. Through this bed there is filtered off in each 

 minute's time about 125 cc. of water, or about 0.01 cc. per 

 square centimeter per minute, which is a rate of filtration 

 well below that of the ordinary laboratory filters. But this 

 capillary bed is still a filter in the ordinary laboratory sense 

 for it permits everything in the plasma to pass through it ex- 

 cept the blood cells, the plasma proteins and similar large 

 molecular aggregates. To supply this 125 cc. of filtrate 1300 

 cc. of blood are perfused each minute through the capillary 

 bed of the glomeruli. 



After leaving the glomerulus the blood passes into a sec- 

 ond set of capillaries surrounding the tubule; here an oppor- 

 tunity is afforded for the tubule cells to transfer various sub- 

 stances from blood to tubular urine, or from tubular urine 

 back into the blood, and here is where all specific chemical 

 operations are carried out. For as the glomerular filtrate 



