478 A TEXTBOOK OF PHYSIOLOGY 



causing diuresis; sodium chloride still continues to do so. Also it is 

 found, under these circumstances, that if the blood be much diluted 

 with Ringer's solution, the chlorine content of the urine closely 

 approximates to that of the blood. 



From these experiments it would seem that the excretion of such 

 bodies as urea and sodium sulphate call forth the cell activity of the 

 kidney, while chlorides do not. Such experiments, however, do not 

 negative the view that work may also be spent in actively concen- 

 trating the urine in the tubules. 



The theories of Bowman and Ludwig have been variously modified 

 as the result of the ever-extending researches into the nature of the 

 renal functions. The following are the chief modifications: 



1. A modification of the original view of Ludwig. A dilute urine 

 containing all the urinary constituents is filtered through the glomeru- 

 lus, and becomes concentrated by cell activity during its passage 

 through the urinary tubules. 



2. Modifications of the original view of Bowman, (a) Water and 

 salts are filtered through the glomerulus, and the organic constituents 

 are actively added by the tubules. (6) The water and salts are 

 actively secreted by the glomerular cells, and the organic substances 

 by the cells of the tubules. 



3. A combination of the above views namely, that water and 

 salts are either filtered or secreted from the blood in the glomeruli, 

 and that the tubules have a double function (a) to add the organic 

 constituents of the urine by cell activity in one part of their course; 

 and (6) to concentrate the urine by a similar agency in another part. 



It will be seen at once that research resolves itself into an inquiry 

 into 



1. The function of the glomerulus and its mode of action. 



2. The functions of the tubules. 



The Function of the Glomerulus. Three points have to be settled: 

 (1) Is a dilute urine filtered through the glomerulus ? or (2) are only 

 the water and salts of the urine isolated in the glomerulus ? and 

 (3) if so, is it by a simple physical process, such as filtration, or by an 

 active process of cell secretion ? 



Those who uphold the filtration hypothesis base it, in the first 

 place, upon the arrangement of the bloodvessels of the glomerulus. 

 As measured in histological preparations of injected kidney, the 

 afferent artery appears to be of greater bore than the efferent vein 

 It is suggested that this develops a high filtration pressure in the 

 capillaries of the glomerulu*s. It is claimed that evidence of this is 

 afforded by the results of various experiments which are directed 

 towards the increase of the arterial pressure within the kidney, either 

 by producing a general rise of arterial pressure or a local rise by causing 

 vaso-dilatation within the organ. Whenever the pressure is thus 

 raised as, for example, by stimulation of the spinal cord after section 

 of the renal nerves, or during the injection of large amounts of fluid, 

 which temporarily leads to a condition of plethora there is an in- 

 creased flow of urine. When, however, the arterial pressure is lowered, 



