EXCRETION 465 



the kidney. For the osmotic pressure of urine is several times 

 as great as that of the plasma of the blood. Blood-plasma 

 freezes at -0-55 to - 0-65 C. (on the average, say, -0-6 C.). 

 The osmotic pressure corresponding to 0-6 C. is 5,662 milli- 

 metres of mercury (p. 400), or, in round numbers, 75 metres of 

 water. Human urine has been found to freeze at 1-38 to 

 -2-11 C. (say, on the average, -1-8 C.), and for highly con- 

 centrated urines the depression of the freezing-point may be 

 considerably greater. The osmotic pressure corresponding to 

 1-8 C. is 16,986 millimetres of mercury or 225 metres of 

 water. This exceeds the osmotic pressure of the plasma by 

 150 metres of water. In separating a kilogramme of urine from 

 the blood the kidney accordingly does work approximately 

 equivalent to raising a weight of a kilogramme to the height 

 of 150 metres i.e., 150 kilogramme-metres. It is evident that 

 the excess of the blood-pressure in the glomeruli over the pres- 

 sure of the urine in the tubules, which, even if we neglect the 

 latter altogether since there is only slight resistance to the flow 

 of urine towards the bladder cannot at most be greater than 

 100 millimetres of mercury, or 1-35 metres of water, will account 

 for only an insignificant part of this work. The rest must be 

 done at the expense of the energy of the food materials taken 

 up by, and transformed in, the cells concerned with the secre- 

 tion of the urine. But we do not know in what way these cells, 

 by applying this energy, perform the remarkable feat of per- 

 manently maintaining a difference of fifteen atmospheres in the 

 osmotic pressure of the liquids in contact with their attached 

 and free surfaces. A token of the intensity of the metabolic effort 

 required is the marked increase in the absorption of oxygen 

 (as much as 0-28 c.c. per gramme) which occurs during diuresis, 

 although it is not in proportion to the amount of the diuresis. 

 In one experiment the oxygen absorbed by a dog's kidneys was 

 ii per cent, of what would have been used up by the entire 

 animal under normal conditions. There is no definite relation 

 between the oxygen taken in and the carbon dioxide given out 

 at any moment. 



What is the significance of the peculiar arrangement of the 

 glomerular bloodvessels, if the epithelium of the capsules has secre- 

 tive powers like that of ordinary glands ? It is difficult to believe 

 that these unique vascular tufts have not a near and important rela- 

 tion to the renal function ; but it is by no means clear what that 

 relation is. The secretion of water, and even its rapid secretion, is 

 not at all bound up with any set arrangement of bloodvessels. 

 Gland-cells all over the body secrete water under the most varied 

 conditions of blood-pressure, although a comparatively high pressure 

 is upon the whole favourable to a copious outflow. 



But the kidney has other functions than mere excretion (pp. 508, 

 569). And it may be that the simplest part of the latter process, 



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