272 ENERGY CHANGES INVOLVED IN SECRETION 



if only passive absorption of water and its dissolved substances 

 formed the first stage in the process of secretion. Thus in the 

 secretion of hydrochloric acid in the gastric juice, the concentration 

 of the hydrogen ion has to be increased from an almost immeasurably 

 low concentration in the plasma up to almost the strength of a deci- 

 normal solution, and to do this by passive absorption an enormous 

 amount of water must enter the secreting cell and be again rejected 

 at the same side at which it entered. Again, in the secretion of urea 

 in the urine 75 kilograms would have to enter and pass through the 

 kidney cells and be reabsorbed in order to concentrate and separate 

 the daily output of urea. Also, in absorption from the intestine, 

 to take up a meal of 150 grms. of carbohydrate or fat in 1 per cent, 

 solution, which is probably in excess of the concentration at which 

 these foodstuffs are normally absorbed, it would be necessary for 

 15 kgrms. of water to be taken up by the absorbing cells, 

 and either returned by alternating back streams into the intestine 

 free from carbohydrate or fat, or else poured into the blood-stream. 

 Such an amount is much in excess of the total of the water swallowed 

 with the food and the combined digestive secretions. 



Hence we must suppose that the cell, whether absorbing or 

 secreting, does not undergo passive infiltration by the fluids in 

 contact with it, and allow these, or even the water, to stream 

 through passively, but is an active energy machine, and takes up 

 the various constituents and their solvents in definite and well- 

 regulated proportions. 



For the reason stated at the outset, the amounts of energy 

 involved in the formation by the cell of the new organic con- 

 stituents of the secretion not present in the plasma cannot at 

 present be estimated, and so we pass to a consideration of the 

 extrinsic mechanisms of secretion. 



THE EXTRINSIC MECHANISMS OF SECRETION. 



Alterations in the Blood-Supply to the Secreting Gland. Accom- 

 panying the increased amount of physiological work which the 

 secreting cells have to perform, there is always during secretion 

 an increase in the amount of blood supplied. This increase was 

 estimated by Chauveau and Kaufmann in the case of the sub- 

 maxillary salivary gland as amounting to three times the blood- 

 supply in the resting condition of that gland, but according to 



