338 PRINCIPLES OF GENERAL PHYSIOLOGY 



capillarv tube enters into Poiseuille's formula, which was used, so that a slight difference iu 

 this value would make a large one in the result, and JJrodie's measurements are made on a 

 hardened kidnev. Apart from this, it is very difficult to believe that a pressure of S.S nun. 

 of mercury could exist in the glomerulus end of the tubule without causing great dilatation. 

 If this distension \\ere prevented by the resistance of the capsule of the kidnev, it would surely 

 ivMilt in obstruction of the capillaries and veins. There is no evidence of a strong muscular 

 coat to the tubules, such as there is in the arterioles of the glomerulus. The conclusion drawn 

 l>\ Krodie from his calculation is that the glomerulus must lie an actively secreting organ for 

 water, and that the water is driven on in some way by the aid of blood pressure. But if we 

 grant that the cells act as secreting organs, like those of the salivary glands, why ia it in-i-eary 

 t" assume any further driving pressure? 



THE WORK DONE IN SECRETION AND THE CONSUMPTION 



OF OXYGEN 



Although the glomerular filtrate of the kidney has no higher osmotic pressure 

 than that of the blood plasma, it is well known that the urine, as it leaves the 

 kidney, has a much higher molar concentration. Work must therefore be done 

 in the total process. This work can be calculated in the way indicated in a 

 previous chapter (page 33), and will be described presently. 



Chemical Work. In the production of the special constituents of any secretion, 

 chemical work must be done by the gland cells. Moreover, the source of the 

 energy required for their osmotic work has also its origin in chemieal reactions 

 in the cell systems. We have no direct way of measuring this work, but the 

 amount of oxygen consumed, or the carbon dioxide given off, by the gland in 

 different states of activity, gives us valuable indirect information. The energy 

 at the disposal of the cells is derived from the oxidation of substances of high 

 chemical potential to substances of low chemical potential, such as carbon dioxide 

 and water. In the process, a part of the free energy is degraded to heat and 

 carried off by the blood current, so that it is impossible in practice, at all events 

 as yet, to obtain an absolute measurement of the amount of work required to 

 form a given amount of secretion. 



Osmotic Work. It is advisable to give some further details of the method 

 of calculating this, in addition to those already given in a general foim when 

 discussing the formula for the isothermal compression of a gas. 



We must recollect that we cannot look upon the urine as merely a concen- 

 trated glomerular filtrate, as was done by Dreser (1892) in the first approximate 

 calculation of the renal work. The relative proportion of the constituents is 

 not the same. For example, the ratio of sodium chloride to urea in the blood 

 (or glomerular filtrate) is about 10 to 1, whereas in the urine it is reversed, and 

 becomes 1 to 2. Thus, while the osmotic pressure of the sodium chloride has 

 only to be raised from that of a 0'18 molar solution to that of a 0'36 molar one, 

 or about doubled, that of the urea has to be raised from a O'Ol molar strength to 

 that of a 0*4 molar strength, or increased forty times. 



It will be best, however, to obtain first the work done, as Dreser did, on 

 the supposition that we are dealing only with an increase of concentration, leaving 

 for the present the fact that the various constituents are unequally affected. 



At the outset it is well to call attention to the fact that the results obtained 

 by such methods of calculation are valid, whatever be the exact mechanism by 

 which the process is brought about in the organism. 



On page 33 we saw that the expression which gives us the work done in 

 compressing a gas isothermally from a volume v l to a volume v. 2 is 



' V 2 



or = 2-303.RT log 



R, of course, can be expressed in any convenient units, and is 



0821 litre atmospheres, 

 T991 gram calories, 

 or 0'848 kilogram metres ; 



