846 PHYSIOLOGY OF DIGESTION AND SECRETION. 



Dreser has shown that the acidity of the urine is due to an action 

 of the epithelium of the tubules. If an acid indicator, such as 

 acid fuchsin, is injected into the dorsal lymph-sac of a frog, and an 

 hour or so later the kidneys are examined, it will be found that the 

 convoluted tubules are colored red, while the capsular end is 

 colorless, indicating that the secretion at the latter point has an 

 alkaline reaction. The experiment shows that the acid substances 

 in the urine are produced in the convoluted tubules. The sim- 

 plest explanation is that they are formed by a secretory activity 

 of the epithelial cells. (7) Studies of the gaseous exchanges in the 

 kidney during diuresis* and during the glycosuria caused by 

 phlorhizin f tend to support the secretion hypothesis to the extent 

 that they prove an increased metabolism during functional 

 activity. (8) The action of diuretics (see below). On the whole, 

 it must be admitted that the weight of evidence is in favor of 

 the Bowman-Heidenhain theory of secretion. 



Action of Diuretics. An important side of the theories of 

 secretion of urine is their application to the action of diuretics. 

 Water; various soluble substances, such as salts, urea, and dextrose; 

 and certain special drugs, such as caffein or digitalis, exert a diuretic 

 action on the kidneys. Much experimental work has been done 

 to ascertain whether the action of these substances can be explained 

 mechanically by their influence on the blood-flow or the blood- 

 pressure in the kidney capillaries, or whether it is necessary to fall 

 back upon a specific stimulating effect exerted by them, upon the 

 epithelial cells of the tubules. Adherents of the original Ludwig 

 theory are forced to explain their action by the effect they pro- 

 duce upon the pressure in the kidney capillaries, and, indeed, it 

 has been shown with reference to the saline diuretics that their 

 effect upon the secretion is in proportion to the osmotic pressure 

 they exert. It has been suggested, therefore, that the action of 

 these diuretics lies in the fact that they attract water from the tis- 

 tues into the blood and thus cause a condition of hydremic plethora. 

 But whether the elimination of this excess of water is due to filtra- 

 tion or to an active secretion by the glomerular epithelium is a 

 question that revives the discussion that has been presented briefly 

 above. Most observers find that the vascular changes in the kid- 

 ney, particularly after the administration of caffein and digitalis, 

 do not explain satisfactorily the phenomenon of diuresis. It has 

 been shown also that some diuretics cause an increased flow of 

 urine without affecting the amount of oxygen absorbed from the 

 blood by the kidney. Others, however, cause a distinct increase 

 in the oxygen consumption. Since the oxygen consumption may 



* Barcroft and Brodie, "Journal of Physiology," 33, 52, 1906. 

 t Pavy, Brodie, and Siam, ibid., 29, 467, 1903. 



