748 PHYSIOLOGY OF DIGESTION AND SECRETION. 



tion of urine, finds that under these conditions injection of urea into 

 the circulation is not followed by a secretion. (5) 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 in- 

 jected 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 tu- 

 bules 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 simplest explanation is that they are 

 formed by a secretory activity of the epithelial cells, although one 

 may adopt the less probable view that the cells produce the acid 

 phosphates by a selective absorption of alkaline salts. On the 

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

 of the Bowman-Heidenhain theory of secretion, and it remains for 

 future investigations to explain more definitely what is meant by 

 the obscure term secretory activity. 



Under pathological conditions it has been shown satisfactorily 

 that the albumin and sugar which may be present in the urine are 

 secreted or eliminated at the glomerular end of the tubule. 



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, and al- 

 though it is necessary to admit that the diuretics, or some of them, 

 act in part by the changes which they cause in the circulation in 



