STRUCTURE OF THE KIDNEY 2OI 



urine preexist in the blood and are simply taken out of the 

 circulation in the kidney, it may be stated that, for the most 

 part, the water and salts are extracted by the cells of the Mal- 

 pighian bodies, while the urea and related nitrogenous solids are 

 removed by the cells of the convoluted tubes ; so that the specific 

 gravity of the fluid is raised in passing down the tubes. While 

 the histology of the kidney, and especially the arrangement 

 of the glomeruli, is most favorable for the exercise of simple 

 osmosis, and while this process is doubtless mainly responsible 

 for the phenomena which occur, it seems highly probable that 

 the cells themselves modify osmotic action by taking an active 

 part in the secretion of urine. They undoubtedly exercise a 

 sel&ctive affinity accounting for the different materials handled 

 by the glomeruli and the tubes. Moreover, morphological 

 changes in the tubal cells during activity have been microscopic- 

 ally demonstrated. Vesicles are described as forming in the 

 body of the cell, approaching the lumen, bursting and discharg- 

 ing their contents which are supposed to include the urea and 

 such other materials as may be here extracted from the blood. 



As regards the elimination of water and salts by the glomerular 

 epithelium, it must also be admitted that the cells take some 

 obscure but active part. Were this only an osmotic process 

 the amount eliminated would vary exactly as the pressure. 

 While usually a rise in renal blood-pressure is accompanied 

 by an increased flow of urine and a fall by a correspondingly 

 decreased flow, the rule does not always hold good. For in- 

 stance, compression of the renal vein raises the pressure but 

 does not increase the amount of urine. 



Another fact, which seems almost if not quite as invariable as 

 the effect of blood-pressure, is that the amount of urine varies 

 directly as the amount of blood passing through the kidney, inde- 

 pendently of the pressure; and these two facts constitute about 

 all that is definitely known concerning the local conditions 

 affecting the amount of urine. Whether diuretics increase the 



