390 THE EXCRETIONS OF THE BODY 



general blood pressure be low (Paneth). It is evident that the same effect 

 can be produced with the renal circulation unobstructed. 



To the diuretic substances belong: urea, common salt, sodium nitrate, caf- 

 fein, grape sugar, peptone, albumoses, etc. Their effect undoubtedly depends 

 in part upon the accompanying dilatation of the renal vessels; but it is con- 

 nected also with a rise in the osmotic pressure of the blood occasioned by these 

 substances, and the consequent abstraction of water from the tissue spaces into 

 the blood vessels. In this way the blood is diluted, and the vessels are more 

 tensely filled, the result being a more copious flow of blood through the kidneys. 

 Here we have almost the same process as when water is slowly transfused into a 

 vein after a certain quantity has been injected; the excretion of urine increases 

 up to a certain point, beyond which the transfusion and excretion keep pace 

 with each other. 



The effects of diuretic substances cannot, however, be explained from this 

 point of view alone. For there are various experimental facts which indicate 

 that the ingested substance stimulates the kidneys to increased activity, quite 

 independently of changes in the diameter of the blood vessels, etc., and that, 

 therefore, these substances are specific stimuli for the kidney cells. 



Finally, the general condition of the body plays a part in the secretion of 

 the urine which is not to be neglected. When certain diuretics are given to 

 a body poor in sodium chloride, there is no increase in the excretion of NaCl. 

 Notwithstanding the diuresis, the body holds on to its Nad very energetically, 

 giving it up only in the smallest possible quantities. But in cases where the 

 body has plenty of NaCl, whenever there is a strong secretion of urine, there 

 is also an abundant output of this salt. 



It has long been known that one kidney is sufficient for all purposes of metab- 

 olism. One can even remove as much as two-thirds of the kidney substance and 

 still leave an efficient excretory apparatus. Here moreover we meet the remark- 

 able fact that the renal secretion increases considerably and permanently. At 

 the same time the elimination of urea is increased and animals die within two 

 to six weeks in spite of a fairly good appetite (Bradford). How this phenomenon 

 is to be explained or what theoretical weight it has, we are not able to say at 

 present. 



Nothing is known which would indicate the presence of secretory nerves 

 to the kidneys. It is true that by various operations on the central nervous 

 system or on peripheral nerves, changes in the secretion of urine may be 

 obtained; but all these admit of an explanation as vasomotor effects. The 

 secretion continues also, though somewhat diminished, after division of the 

 nerves running along the renal vessels. So far, then, as we are able to judge 

 at present, the secretion of urine is accomplished by the influence of the urine- 

 producing substance in the blood, and is regulated by variations in the quan- 

 tity and the quality of these substances, as well as by alterations in the blood 

 supply to the kidney. 



3. MICTURITION 



From the pelvis of the kidney the urine flows through the ureters into the 

 bladder, remains there for a time, and is finally expelled at varying intervals. 



