234 PHYSIOLOGY FOB DENTAL STUDENTS. 



To explain these facts, Ludwig, a famous physiologist of the 

 nineteenth century, formulated what is known as the mechanical 

 theory of urine excretion. Impressed by the peculiar relation- 

 ship of Bowman 's capsule and the glomerular capillaries, he con- 

 cluded that the Malpighian corpuscle is a filtering apparatus 

 which separates, in dilute solution, a portion of all the diffusible 

 substances of the blood. The absence of such diffusible sub- 

 stances as sugar in normal urine and its presence in the blood in 

 a relatively large amount, he believed to be due to the ability of 

 the epithelium of the tubules to reabsorb these substances from 

 the dilute urine. Likewise, the high concentration of salts and 

 nitrogenous bodies, such as urea, he explained by reabsorption 

 of water through the tubules into the blood. In support of this 

 theory Ludwig demonstrated that the urine excretion varied 

 directly with the blood flow and the blood pressure of the kid- 

 ney. In other words, the greater the supply of blood and the 

 greater its pressure, the more rapidly will the watery solution 

 of the urine be filtered from the blood. He was not able, how- 

 ever to bring any satisfactory proof of the reabsorption of water 

 or other substances by the epithelium of the urinary tubules. 

 Indeed, most experiments show that this does not occur. 



It is impossible to explain all the facts of urinary excretion 

 by simple physical laws. For example, urea and dextrose are 

 both found in the blood and both obey the same physico-chemical 

 laws; nevertheless the one is excreted in the urine and the other 

 is retained in the blood. Furthermore, when certain pigments 

 are injected into the blood, they are excreted by the kidney cells, 

 but do not appear in those of other parts of the body. 



That an increase in the pressure of blood in the renal vessels 

 has a very marked accelerating effect on the excretion of urine, 

 is not necessarily evidence that the increased blood supply is the 

 cause of the excretion. That other factors are concerned is demon- 

 strated by the action of drugs which cause an increase in renal ex- 

 cretion. For example, digitalis, a drug stimulating the circulatory 

 apparatus, causes a marked diuresis in cases of a weak heart 

 where the pressure has been totally inadequate to maintain a 

 urine excretion, but has little or no action on the normal kidney. 



