544 THE URINARY APPARATUS. [CH. xxxvi. 



solutions depend merely on the number of molecules of any sub- 

 stance present ; the nature of the substance makes no difference. 

 The osmotic pressure is, in fact, equal to that which the dissolved 

 substance would exert if it occupied the same space in the form 

 of a gas. The osmotic pressure, however, of substances like sodium 

 chloride which are electrolytes, is double what one would expect ; 

 this is because each molecule in solution is split into its constituent 

 ions, sodium and chlorine. 



Hence, if the osmotic pressures of blood plasma and urine are 

 determined, the work done by the kidney cells in order to separate 

 from the blood plasma a fluid with the osmotic pressure of the 

 urine, can be estimated. 



We may take some examples from Dreser's work. He took 

 the case in which 200 c.c. of urine were excreted during a night; 

 the blood plasma in this case had an osmotic pressure = 0*92 

 per cent, solution ; while that of the urine was = 4'o per cent, 

 solution of sodium chloride. In this case the kidney had per- 

 formed 37 kilogramme-metres of work. In another case of more 

 concentrated urine obtained from a cat previously deprived of 

 water for three days, the numbers were respectively i'i and 8 - o. 

 The difference was equal to a pressure of 498 metres of water; 

 so that the kidney had separated urine from the blood against 

 a pressure of 49,800 grammes per square centimetre, a force 

 about six times greater than the maximum force of muscle. 



The actual method of estimating osmotic pressure is not by means of a 

 manometer as in the diagram, but by certain indirect methods. The one 

 usually employed depends on the fact that the freezing point of a solution 

 of any substance in water is lower than that of water ; the lowering of the 

 freezing point is proportional to the molecular concentration of the dissolved 

 substance, and that as we have seen is proportional to the osmotic pressure. 

 The gramme-molecule is the number of grammes corresponding to the 

 molecular weight. Thus the gramme-molecular weight of sodium chloride 

 is 5S'37 grammes (Na = 23 ; Cl = 35'37). and of grape sugar (C 6 H 12 6 ) 179*58 

 grammes. When a gramme-molecule of any substance is dissolved in a litre 

 of water, the freezing point is lowered by r87 C., and the osmotic pressure 

 is 16,963 mm. of mercury. From this we can calculate the osmotic pressure 

 of any solution if we know the lowering of its freezing point. 



lowering of freezing point 

 Osmotic pressure = ^ x 16.963. 



For example, a i per cent, solution of sugar would freeze at o - o52 C. 



052 x 16.963 

 Its osmotic pressure is therefore ^ = 472 mm. of mercury. 



The osmotic pressure of solutions may also be compared by observing their 

 effect on red blood corpuscles, or on. vegetable cells such as those in Trades- 

 cantia. If the solution is hypertonic, i.e. has a greater osmotic pressure than 

 the cell contents, the protoplasm shrinks, and loses water, or if red corpuscles 

 are used, they become crenated ; if the solution is tiyputoiiie, i.e. has a less 

 osmotic pressure than the material within the cell-wall, no shrinking of the 



