KIDNEY AND SKIN AS EXCRETORY ORGANS. 769 



functional activity. (7) 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, 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 

 the kidney, it is not possible to demonstrate that all the phenomena 

 under this head can be thus explained. The bulk of the work 

 published indicates that some at least of the known diuretics act as 

 stimulants to the secreting cells. In the case of the inorganic salts 

 it may be said (Magnus) that there is for each salt a "secretion 

 threshold." An increase in concentration above this level leads 

 to the elimination of the excess of salt and an increased secretion 

 of water. 



The Blood-flow through the Kidneys. It will be inferred 

 from the discussion above that, other conditions remaining the same, 

 the secretion of the kidney varies with the quantity of blood flowing 

 through it. It is, therefore, important to refer briefly to the nature 



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