EXCRETION. 479 



MECHANISM OF URINE SECRETION. 



The secretion of urine is a complex process and susceptible of several 

 interpretations. It was originally inferred by Bowman that, as the kidney 

 presents anatomically an apparatus for filtration, the capsule with its enclosed 

 glomerulus, and an apparatus for secretion, the epithelium of the urinary 

 tubules, therefore the elimination of the urinary constituents from the blood 

 is accomplished by the two processes of filtration and secretion; that the 

 water and highly diffusible inorganic salts simply pass by filtration, 

 under pressure, through the walls of the glomerular capillaries, while the 

 organic constituents are removed by the epithelium lining the tubules. 



Influenced largely by the facts of blood-pressure Ludwig advanced the 

 view that the factors concerned in the secretion of urine were purely physical; 

 that in consequence of the high pressure in the vessels of the glomeruli, 

 due to the high pressure in the renal artery on the one hand and to the resis- 

 tance offered by the smaller efferent vessel on the other hand all the urinary 

 constituents were filtered off in a state of extreme dilution. In order to ac- 

 count for the higher percentage of the organic constituents in the urine, it 

 was assumed that as the dilute urine passed through the tubules the water 

 and possibly other substances as well were partly reabsorbed, passing by 

 diffusion into the lymph and blood until the urine acquired its normal 

 characteristics and degree of concentration. In support of this view, a large 

 number of facts relating to the influence of an increase and a decrease of 

 pressure in the blood-vessels of the glomeruli, the velocity of the blood-stream, 

 etc., in determining the rate of urinary flow were adduced, all of which 

 apparently indicated that the former stood to the latter in the relation of 

 cause and effect, and that the formation of urine was accomplished ent rely 

 by physical forces. 



The progress of physiologic investigation, however, has thrown some 

 doubt on the validity of this physical interpretation, and has rather served 

 to support the view of Bowman that the organic constituents at least are 

 removed from the blood by a process of selection on the part of the epithelium 

 of the convoluted urinary tubules; in other words, that the secretion 

 of urine is physiologic rather than physical. Heidenhain has brought 

 forward a series of facts which support this view. As evidence that the cells 

 possess a selective power, he presented the following experiment: The 

 spinal cord of an animal is divided in the neck for the purpose of lowering the 

 blood-pressure in the kidney below the pressure at which the urine is secreted. 

 Five to twenty c.c. of a saturated solution of indigo-carmine is injected into 

 the blood-vessels; after intervals varying from ten minutes to one hour the 

 animal is killed, the blood-vessels washed out with alcohol for the purpose of 

 precipitating the indigo-carmine in situ. Section of the kidney shows a uni- 

 form blue stain of the cortex alone. (Fig. 219.) Microscopic 'examination 

 reveals the fact that the blue stain is due to the deposition of the pigment in 

 the lumen and in the lumen border of the cells of the convoluted tubules 

 (Fig. 220) and the ascending limb of Henle's loop; while the epithelium of 

 Bowman's capsule as well as the glomerular epithelium present no evidence 

 of pigmentation. The physiologic action of the cells of the convoluted 

 tubules in elimination of indigo-carmine, is supposed to indicate their action 



