CH. XXXVin.] FUNCTIONS OF THE KIDNEY 573 



certain percentage is removed from the blood ; sodium chloride is an 

 instance of these. 



Although the glandular epithelium of the convoluted tubes is 

 par excellence the secreting mechanism of the kidney, much difference 

 of opinion exists as to the part played by each of the several units in 

 the histological complex we have already described, and this is 

 especially the case in relation to that unique structure, the glomerulus. 

 We have seen that the efferent vessel of each glomerulus has a 

 smaller calibre than the afferent vessel, and this produces high 

 pressure in the glomerular capillaries. The efferent vessel, more- 

 over, resembles an arteriole in its abundance of muscular tissue, and 

 this maintains the high in tra- glomerular blood - pressure. This 

 arrangement led Ludwig to the conception that the glomerulus is a 

 filter, and the filter theory has formed the basis of much subsequent 

 work, and numerous theories. 



It is impossible in a question upon which physiologists are so 

 divided, to make a complete statement of the case which will meet 

 with universal acceptance, and still more difficult is it to build any 

 sure system of medical treatment upon so insecure a foundation; 

 and before we attempt to consider the function of this or that type 

 of cell in the kidney, let us see what can be laid down in relation to 

 the physiology of the kidney as a whole. 



We may sum up the process of urinary secretion as follows : 

 One fluid, the arterial blood, enters the kidney ; two fluids, the 

 venous blood and the urine, leave it. Both of these fluids are 

 different in composition from the arterial blood. The following 

 table gives the approximate values of the principal constituents in 

 the plasma of the arterial blood, and in the urine : 



Arterial Blood-plasma. Urine. 



Total solids . . 10 per cent. 4 per cent. 



Proteins . . 7 '5 to 8 



Sodium chloride . 0'8 ,, ,, 1'2 



Urea . . . 0'03 2*0 



Sugar . . 0-15 



Uric acid . . . traces '05 



Hippuric acid 



0'07 



We know that it is not possible to convert any fluid into two 

 others, each of different composition from itself, without an ex- 

 penditure of energy which must come from somewhere outside the 

 fluids themselves. In the case of the kidney, as in other secreting 

 glands, this energy comes from the cells of the organ. 



The secretion of urine is therefore the result of work done by the 

 kidney. The quantity of this work may be measured within certain 

 limits, and the energy transformed by the kidney may be estimated in 

 more than one way. The urine is much more concentrated, as regards 



