696 THE SECRETION OF URINE 



which they exist in the blood, it is necessary to suppose that the 

 cells of the tubule can be influenced somehow to absorb selectively 

 substances which it is not necessary to excrete, and so adapt the 

 degree of absorption in the tubule to the needs of the organism. 



General Relation of the Renal Circulation to the Quantity of Urine. 

 Both views admit an intimate relation between the formation of 

 urine and the renal circulation. At first sight there would appear 

 to be an obvious difference between the two ; for, whereas on 

 Lud wig's view the main relation is between quantity of urine 

 and pressure in the glomerular capillaries, the influence of velocity 

 of blood-flow on filtration being a comparatively minor point, on 

 Heidenhain's view the relation is between quantity of urine and 

 rate of blood-flow through the kidney. With one exception, 

 however, any measure which raises the capillary blood pressure 

 in the kidney will at the same time increase the velocity of blood- 

 flow through the organ. Such measures are (1) A rise in the 

 general arterial blood pressure when caused either by increasing 

 the force or rate of the heart beat by such a drug as digitalis, 

 or by vaso-constriction in areas other than the kidney. This 

 latter may be brought about by stimulating the lower end of the 

 cut spinal cord or splanchnics after section of the renal nerves, 

 which in the dog leave the central nervous system in the anterior 

 roots of the llth, 12th, and 13th dorsal nerves. Unless the renal 

 nerves are cut the kidney shares in the general vaso-constriction, 

 its volume shrinks, and in spite of the greatly raised arterial blood 

 pressure no secretion of urine takes place. The same is true of 

 the marked rise in arterial pressure seen during asphyxia. 

 (2) Dilatation of the renal arterioles, the general arterial blood 

 pressure remaining unaltered. This condition may be brought 

 about by section of the vaso-constrictor fibres contained in the 

 renal nerves, or by stimulation with slow rhythmical shocks of 

 the vaso-dilator fibres for the kidney contained in the anterior 

 roots of the llth, 12th, and 13th dorsal nerves in the dog. In 

 hydrsemic plethora also, while the general arterial blood pressure 

 is scarcely altered, there is a marked dilatation of the arterioles 

 in the kidney, as well as in other abdominal viscera. Conversely 

 both the pressure and rate of flow in the kidney vessels may be 

 decreased in the following ways (1) A fall in the general arterial 

 blood pressure. This may be brought about by cardiac inhibition 

 due to stimulation of the vagus in the neck. If the heart is 



