THE SECRETION OF URINE 1189 



present at the upper end of the urinary tubule are sufficient to account for the 

 production of a glomerular transudate free from protein, but containing the 

 same proportion of water and salts as the blood plasma circulating through 

 the capillaries. 



If the process occurring in the glomeruli is simply one of filtration, three 

 conditions must be realised : 



(1) The amount of filtrate, so long as the ureter pressure is constant, 

 must depend on the pressure and rate of flow of the blood in the glomerular 

 capillaries, and must fall or rise with the lattei. 



(2) The constitution of the fully formed urine as it appears in the ureters, 

 after modification by addition or subtraction on the part of the tubular cells, 

 must approximate more closely to the supposed glomerular transudate, 

 containing the same proportipn of salts as the blood plasma, the more rapidly 

 the formation of the glomerular transudate takes place : i. e. the quicker the 

 flow of urine the more nearly must its composition, reaction, and osmotic 

 pressure resemble those of the blood serum. 



(3) The total quantity of solids excreted in any given time must be 

 increased with any increase in the urinary flow. For, whatever the activity 

 of the tubules, the glomeruli must blindly turn out a certain proportion of 

 solids with every cubic centimetre of fluid that they form. 



We may deal first with the influence of alterations in the renal blood 

 supply on the flow of urine. Ligature of the renal vein diminishes and soon 

 stops the flow altogether. Since this procedure must cause a large rise of 

 pressure in the capillaries of the kidney, this result was regarded by Heiden- 

 hain as disproving any possibility of the glomerular process being of the 

 nature of a filtration. At any given time however, the glomeruli contain but 

 little blood. With total cessation of the renewal of this blood, their contents 

 will rapidly become so concentrated that they will be little more than a mass 

 of red corpuscles. No filtration of water and salts can take place unless there 

 is a continual renewal of the fluid on the blood side of the filter. 



On the other hand, alterations in the blood supply to the kidney, 

 determined by changes on the arterial side, have pronounced effects on the 

 amount of urine formed. The pressure in the glomerular capillaries and the 

 rate of flow through these capillaries can be increased in either of two ways : 



(a) By increase of the driving force, i. e. the general blood pressure ; 



(b) By a diminution of the resistance to the flow of blood through the 

 kidneys, as- by dilatation of the vessels of this organ. 



The blood flow through the kidney can be investigated, either by record- 

 ing the total volume of this organ, or by determining the amount of blood 

 which -leaves it through the renal vein, according to the methods described 

 in Chapter xiii. 



It is necessary at the same time to take a record of the arterial blood 

 pressure by means of a mercurial manometer. It is evident that an expan- 

 sion of the kidney may be caused by a rise of general arterial pressure or, 

 the latter remaining constant, by a dilatation of the kidney 'vessels ; and, 

 conversely, a fall of kidney volume may be due either to a fall of general 



