THE SECRETION OF URINE 



1139 



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 nitration 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 

 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 through the kidney-vessels. The flow through the vessels 

 can be determined directly either by a cannula in the inferior vena cava, all 

 veins other than the renal being clamped, or by Brodie's method already 

 described (p. 994). 



The results of the experiments carried out by these methods can be 

 represented in the following tabular form : 



