THE SECRETION OF URINE 1273 



the tubular cells, must approximate more closely to the supposed 

 glomerular transudate, containing the same proportion of salts as the 

 blood-plasma, the more rapidly the formation of the glomerular transu- 

 date 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 giomeruli 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 Heidenhain as disproving any possibility of the glomerular 

 process being of the nature of a filtration. At any given time, how- 

 ever, the giomeruli 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 cor- 

 puscles. 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 ; 



(6) 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 

 recording 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 expansion 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 blood pressure or to a constriction 

 of the renal blood-vessels. By taking these two records it is possible 

 to tell whether a given increase of blood-flow through the organ is of 

 local or of general causation, i.e. is active or passive. Thus the volume 

 of the kidney gives us an indirect clue to the pressure in and the flow 



