THE SECRETION OF THE URINE 497 



portion. The diminished viscosity of the blood (p. 23) produced 

 by the excess of serum will aid the flow through the kidney and 

 therefore increase the diuresis, while in the case of the plethora 

 produced by injection of blood the elimination of liquid will at once 

 increase the viscosity, diminish the velocity of the renal flow, and 

 tend to lessen diuresis. 



There is, then, little more reason to assume that the copious flow 

 of urine which follows the absorption of a large quantity of water 

 is due to a mere process of nitration than there is to believe that 

 filtration, and not selective secretion, is the cause of the gush of 

 saliva which precedes vomiting, or the sudden outburst of sweat on 

 sudden and severe exertion. In addition, there are the positive 

 proofs already mentioned that the ' rodded ' epithelium of the 

 tubules, which no one supposes to be abandoned more to mere 

 physical influences than the epithelium of the salivary glands, plays 

 a part in the secretion of some of the urinary constituents. 



As to the nature of the mechanism set in motion, and the series 

 of events that take place as the constituents of the urine journey 

 from the interior of the bloodvessels to the lumen of the tubules, 

 we know no more than in the case of other glands. This alone is 

 clear, that the separation of the urine from the blood implies the 

 performance of a large amount of work by 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 correspond- 

 ing to -0-6 C. is 5,662 millimetres of mercury (p. 422), 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 concentrated 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 kilo- 

 gramme-metres. It is evident that the excess of the blood-pressure 

 in the glomeruli over the pressure 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 perma- 

 nently maintaining a difference of fifteen atmospheres in the osmotic 



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