THE SECRETION OF URINE 



1185 



of freezing-point in the two fluids. Whereas serum ordinarily freezes a't 

 - 0-56 C. to 0-59 C., the freezing-point of urine is generally lower and 

 may vary from this figure to as much as 4-5 C. For the production 

 therefore of urine from blood plasma, a certain amount of work has to be 

 done, and the seat of this work we can locate only in the cells of the kidney. 

 We may determine the minimumwoik, necessary to form a certain amount of 

 urine of a given concentration, by measuring the amount of heat that must 

 be imparted to the blood plasma in order to reduce it to the same concentra- 

 tion and volume, or we can calculate it if we know the freezing-points of the 

 two flui'ds. A depression of freezing-point A = 1 C. corresponds to an 

 osmotic pressure of 122-7 metres of water. To concentrate 100 c.c. of a 

 saline fluid, such as urine, so as to halve its bulk and double its depression of 

 freezing-point, e.g. from 1 C. to 2 C., would therefore require the 

 expenditure of work equivalent to that which would be required to compress 

 100 c.c. of a gas at a pressure of 122*7 metres of water to half its bulk. 



In this way can be determined the work necessary to change a fluid of A = 0-56 

 (such as plasma) to one of 2-3 (urine). The work done in forming 200 c.c. of urine 

 of this concentration from fluid plasma would amount to 42-9 kgm. metres. But the 

 c jncentration in the kidney does not occur in this simple fashion. If we compare the 

 c imposition of blood plasma with that of urine, we see that almost every constituent is 

 changed in different proportions. 



RELATIVE COMPOSITIONS OE BLOOD PLASMA AND NORMAL URINE IN MAN (CUSHNY) 



If we added up the work required to produce the change in concentration of each 

 constituent, we should arrive at a figure probably ten times as great as that given above. 

 The large amount of work done under some conditions by the kidneys in the formation 

 of urine is indicated by measurements of the oxygen consumption of this organ. This 

 may amount to -04 to -06 c.c. per gramme per minute, and in some forms of diuresis may 

 rise to as much as -28 c.c. per gramme per minute. It is worthy of note that this rise 

 in oxygen consumption is found when the diuresis is caused by the intravenous injection 

 of urea, sodium sulphate, or phlorhizin, but not when the diuresis is brought about by 

 the injection of water, Ringer's solution or sodium chloride. 

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