THE EXCRETION OF URINE. 235 



On the other hand, sodium sulphate injected into the blood 

 causes a diuresis without marked change in rate of blood flow 

 or blood pressure by direct stimulation of the renal epithelium. 

 In almost every case, moreover, an increase in the excretion of 

 urine is followed by an increase in the amount of oxygen used 

 up by the kidney. It is a general law that every increase in cell 

 activity is accompanied by an increase in the amount of oxygen 

 used by the organ, and the increased blood flow accompanying 

 most forms of diuresis is readily explained on the basis of the 

 physiological need of the tissue for water and oxygen. If physi- 

 cal laws were sufficient to explain all the phenomena of excre- 

 tion, there would be no need for oxygen in increased amounts 

 during periods of increased urine formation. A conception of 

 the actual amount of work which the cells must do to excrete the 

 urine may be obtained by comparing the osmotic pressure of the 

 urine with that of the blood. The osmotic pressure of the blood 

 is only half that of the urine, and for each one thousand cubic 

 centimetres excreted, it is sufficient to call for the expenditure, 

 on the part of the renal cells, of a force capable of lifting a 

 pound through one thousand feet. 



We may conclude that the nature of the excretory mechanism 

 cannot be explained by the physico-chemical laws as we now 

 know them, i. e., the phenomena of osmosis, filtration, absorption, 

 etc., but rather that it must be due to a vital action on the part 

 of the renal cells. It is this vital function of the cells which 

 enables them to remove one substance from the blood and to leave 

 another which is identically the same so far as physico-chemical 

 properties are concerned. 



Micturition. The urine discharged from the collecting 

 tubules of the kidney into the pelvis, is carried to the urinary 

 bladder through the ureters (Fig. 38). The muscular coats of 

 the ureter have a movement similar to that of the digestive canal 

 and by peristaltic waves force the urine down through the ureter 

 into the bladder. The urine thus collected by the bladder is 

 retained for a time and is at intervals ejected through the urethra 

 by the act of micturition. This consists of strong contraction of 

 the bladder walls, together with the contraction of the diaphrag- 



