552 THE EXCRETION OF URINE 



from tubule to blood, or one of secretion from blood to tubule is problem- 

 atical. As G-. N. Stewart points out, the reabsorption hypothesis is no 

 simpler to comprehend than the older hypothesis of Bowman and Heiden- 

 hain, for according to both views the renal cells are assumed to exercise 

 discriminative powers. 



DIURETICS 



As already mentioned, Barcroft and Straub 10 have shown that the 

 diuresis which results from the injection of saline into the blood is not 

 accompanied by any increase in the oxygen consumption of the kidney. 

 This observation, coupled with the fact that the total amount of chloride, 

 urea, and sulphate which is excreted during saline diuresis is greater than 

 under normal conditions, indicates that the excretion of these salts is 

 not due to any active secretory process in the kidney, but rather to a 

 greater filtration because of increased heart action. 



The diuresis which is caused by adding urea or sodium sulphate to the 

 blood, on the other hand, is accompanied by an increase in the oxygen 

 consumption of the kidney. Since there is no increase in oxygen con- 

 sumption accompanying the increased excretion of practically the same 

 salts during saline diuresis, the greater oxygen consumption must be 

 due to more work being done in separating the water from the sulphate 

 etc., to return it to the blood. 



The diuresis resulting from certain doses of adrenalin and from pituitrin 

 is apparently due to constriction of the vasa efferentia for, in the isolated 

 kidney perfused outside the body, the addition of these drugs causes 

 the volume of theor^a^jtoJnjsrease, the perfusion pressure to rise and 

 diuresis to occur? 15 "" 



According to the modern theory the polyuria in diabetes is caused 

 by the excessive amount of water taken and by the inability of the 

 kidney to concentrate the urine (by reabsorption) against the osmotic pres- 

 sure offered by the concentrated sugar solution in the tubules. The diuresis 

 following the injection of sugar is therefore of the same type as that pro- 

 duced by sulphate and urea. The diuretic action of the digitalis group is 

 dependent upon its influence on the circulatory system. If the circulation 

 is already sufficient, digitalis does not cause diuresis. The cause of the 

 diuresis produced by pituitary extract is not definitely known. It may be 

 owing in part to its action on the circulation and in part to a direct 

 action on the kidney (see page 811). 



ALBUMINURIA 



The plasma proteins ordinarily do not obtain entrance into the tubules 

 of the kidney. In disease such as acute nephritis and cardiac fail- 

 ure, the plasma colloids are filtered off through the capsule, proba- 



