574 THE URINAKY APPARATUS [CH. XXXVIII. 



crystalline constituents, than the plasma from which it was produced. 

 Thus, urine contains about 2 per cent, of urea on an average, plasma 

 0'03 per cent., and ths same is true in different degrees for other sub- 

 stances. It follows that if urine were placed inside an osmometer and 

 an unlimited supply of plasma outside, water would be sucked into 

 the osmometer until a column of fluid of great height had been 

 established and much work had been performed in raising it. In a 

 specific instance, the blood-plasma had an osmotic pressure equivalent 

 to a 0'92 per cent, solution, and the urine to a 4 per cent, solution, of 

 sodium chloride. From these data, and from the amount of urine 

 secreted, it is possible to make a calculation of the work performed 

 by the kidney. In other words, the energy used by the kidney in 

 secreting the urine cannot be less than what is given by this purely 

 physical consideration. 



The maximum energy used up by the kidney may be calculated 

 in quite another way. Estimations have been made of the amount 

 of oxygen used by the kidney in secreting urines of known concen- 

 tration ; this oxygen may be taken as a measure of the amount of 

 kidney material used up. If the amount of metabolism be thus 

 determined, we can arrive at the amount of energy used up by a 

 knowledge of the heat produced by the decomposition of this amount 

 of kidney material. 



The kidney cannot be doing more work than its metabolism accounts for. If 

 we suppose the kidney living on protein (and the figures would not differ greatly if 

 we supposed it to be living on carbohydrate), we may start with the following 

 constants: 1 c.c. of oxygen oxidises 1 milligramme of protein, and forms water, 

 carbon dioxide, urea, etc. In doing so, it gives out 4000 small calories (see 

 Chapter XLIL), and this is equivalent to 170,000 gramme-centimetres of work. In 

 a typical experiment during diuresis, the kidney used 4 c.c. of oxygen per minute ; 

 this was, therefore, equivalent to 680,000 gramme-centimetres of work, and the 

 energy transformed from potential to kinetic energy by the kidney cannot have 

 been less than this. Let us consider what evidence there is of mechanical work 

 which the organ does as an offset against this; one way in which the work 

 manifests itself is in the concentration of the urine ; this fluid is many times more 

 concentrated than the blood-plasma. The degree of concentration can be calcu- 

 lated from a knowledge of the freezing-points of the blood and urine ; the greater 

 the concentration of a solution of a crystalline substance, the lower is its freezing- 

 point (see p. 326). In this way, it was calculated that 14,700 gramme-centimetres of 

 work was done in the case just referred to. If the calculation is made for each 

 salt separately, a much higher figure than this would, however, be obtained. 



The practical importance of these considerations to the physician 

 lies in the fact that the expenditure of energy involves combustion, 

 and combustion demands oxygen. For this reason, if for no other, 

 an efficient supply of oxygen, that is, an efficient circulation of blood, 

 is the first condition necessary to a healthy kidney. Eenal trouble 

 is often secondary to cardiac trouble, and may be the result of 

 accumulation of blood in the great veins ; in such cases it is obvious 

 that the renal trouble cannot be overcome by treating the kidney, 



