1 8 PHYSICAL CHEMISTRY IN MEDICINE. 



concentration, either through a determination of the 

 osmotic pressure the attraction between dissolved par- 

 ticles and solvents or the change in the freezing-point 

 or boiling-point of the solution. 



All these values are related in a simple way and are 

 independent of the nature of the dissolved substances. 

 If we wish to increase the concentration of an aqueous 

 solution we must remove a part of its water. It is 

 immaterial whether we do this through evaporation ? 

 freezing, or expression of the water through a membrane 

 impermeable to the dissolved substances, or whether this 

 process takes place in the kidneys: in every case the 

 amount of work required must, according to the laws of 

 energy, be the same. The amount of this work is deter- 

 mined solely by the original concentration and the change 

 in concentration finally attained. With these facts in 

 mind, we will try to formulate the conception of renal 

 work more clearly. 



While the molecular concentration or the freezing- 

 point of normal blood has an almost constant value, that 

 of the urine varies within wide limits. Our kidneys are 

 able to furnish a secretion the freezing-point of which 

 may be higher or lower than that of the blood. For the 

 sake of simplicity, it may be well to consider these two 

 possibilities separately. 



If the kidneys furnish a urine having a higher freezing- 

 point than that of the blood, then their entire activity 

 consists in the mere preparation of a dilute urine, and 

 these organs do their osmotic work by expressing from the 

 more highly concentrated blood a certain amount of water. 

 During all this time the osmotic pressure of the blood is 

 Of course kept at its original height through the tissues, 



