METABOLISM 



247 



conditions in which no mere passive filter could do the work. Sometimes, 

 for example, the glomerulus continues its excretion when the blood's 

 pressure is below that in the tubules, while the osmotic pressure of the 

 secreted urine is several times that of the blood, the current of urine 

 none the less passing in the normal direction against this great balance 

 of resistances. There is nothing present but the epithelium to supply this 

 large amount of energy. In what way the varieties of epithelium up 

 and down the tubules correspond, if at all, to the various constituents of 

 the urine, we do not at all know. It is likely that the "salts" and much 

 of the water enter the apparatus through the glomerulus, and that the 

 organic constituents are added from the blood under much lower press- 

 ure by the protoplasm farther down. The mechanical phases of the 

 process (and they may be of considerable importance) proceed undoubt- 

 edly in the Malpighian body. 



FIG. 133 



Bowman'-s. 



Heidenhairis, 



Bfoocf- 



Theories of urinary secretion. 



Which of the cells supply the kidney's supposed internal secretion 

 (see page 228) is not known. It may serve not only to direct in some way 

 whatever proteid katabolism goes on in the renal epithelium, but also 

 to adjust the blood-pressure of the organ in the directions local needs 

 require, acting in this perhaps in connection with the vasomotor centers. 



The amount of urine excreted varies with the amount of blood passing 

 through the renal capillaries as well as on the pressure of this blood. 

 Heidenhain showed this by ligating the renal vein. This stopped 

 altogether the secretion of urine, although the glomerular blood-pressure 

 was much increased. Diuretics act in various ways: Digitalis increases 

 the heart's work and so forces more blood through the renal vessels. 

 Exposure of the skin to cold acts partly in the same way as digitalis, the 

 increase of the general blood-pressure, owing to peripheral vaso-constric- 



