432 PHYSIOLOGY CHAP. 



case the whole of the chemical components of the blood plasma 

 filter through, in the capillary glomeruli (which are more resistant, 

 and are strengthened by the internal layer of the flattened cells of 

 Bowman's capsule) only the crystalloid substances, which contain 

 all the essential constituents of urine, normally filter through with 

 the water. If the concept of a specific secretory activity of the 

 glomerular walls be excluded, we must logically exclude the idea 

 that anything more than water and the salts of the blood can be 

 excreted from the glomerulus, and must assume with Ludwig that 

 the normal glomerular filtrate is only a very dilute urine, containing 

 all the crystalloid components of the lymph, less its colloidal 

 constituents. 



IV. When we reflect that the main function of the kidney is 

 to purge the blood from the products of the katabolic processes, 

 it is natural to conclude that this excretory activity is in intimate 

 relation with the composition of the blood, and that it undergoes 

 important modifications with the variations of the latter. Many 

 experimental data show this logical deduction to be correct. 



Each time the water content of the blood increases or diminishes 

 there is an increase or diminution of the amount of water eliminated 

 in the urine. After copious draughts of water, the volume of 

 excreted urine reaches its maximum after 2-3 hours, and then 

 diminishes and becomes normal again after 5-6 hours (Falck). 

 The excess of water ingested is mainly eliminated by the kidneys, 

 a little being lost by cutaneous secretion (Ferber). After heavy 

 loss of water by profuse sweating, or severe diarrhoea, there is a 

 marked fall or temporary suspension in the excretion of urine, as 

 shown by a number of clinical observations. 



These effects, which show the kidneys to be the chief regulators 

 of the water content of the blood, do not depend on changes in 

 the volume of the blood nor on the altered pressure at which it 

 circulates in the kidneys, for it has been shown, on the one hand, 

 that intravenous injection into a dog of the blood serum of the 

 same animal (Ponfick), and also of large quantities of defibrinated 

 blood (Albertoni), produces no marked increase in the excretion 

 of urine, as is the case with injection of water. On the other 

 hand, it is known that after free absorption of water by the 

 alimentary canal there is no rise, but rather a fall of arterial 

 pressure (J. Pawlow). It therefore seems legitimate to conclude 

 that these effects depend upon the altered concentration of the 

 blood, since we know that filtration through permeable membranes 

 (and therefore through the glomeruli) varies inversely with the 

 degree of concentration of the filtering fluid. 



The urinary constituents normally contained in blood are even 

 more effective in promoting a secretion of urine. The injection of 

 urea, uric acid, and the inorganic salts of urine in sufficient 

 amount to increase the concentration of the blood, excite urinary 



