EXCRETION OF MATTER FROM THE BODY 405 



(a) Blood pressure in the vessels of kidney. The pressure 

 of blood in the glomerular tufts may be increased in two 

 ways (a) By dilating the arteries, and (b) by constricting or 

 occluding the veins. The former leads to an increased pres- 

 sure on the arterial side of the capillary loops and to a more 

 rapid flow of blood, the latter to a rise in pressure on the 

 venous side and a slower flow of blood (see p. 269). In both 

 cases the amount of blood in the kidney is increased, and the 

 organ expands ; but while the former condition is associated 

 with an increased flow of urine, the latter is accompanied by 

 a decreased or arrested flow. A high pressure in the glome- 

 ruli, with a rapid flow of blood, increases the flow of urine. 

 This may be produced (a) by cutting the renal nerves, which 

 maintain a tonic constricting action on the arterioles ; (b) by 

 raising the general arterial pressure and thus forcing more 

 blood into the kidneys. 



The converse condition of decreased pressure in the gloine- 

 ruli and diminished rate of blood-flow may be produced by 

 stimulating the renal nerves derived from the llth, 12th, 

 and 13th dorsal nerves, or by causing a decrease of the 

 general arterial pressure. A fall of the carotid pressure to 

 about 50 mm. Hg in the dog is generally sufficient to arrest 

 the flow of urine. The influence of such a fall of pressure is 

 often seen in heart disease, where, as a result of it, the excre- 

 tion of water by the Malpighian bodies is so impeded that 

 dropsy supervenes. 



(b) If the ureter be ligatured the production of urine stops 

 when the pressure behind the ligature rises to about 50 mm. 

 Hg. This further supports the view that the formation of 

 urine in the Malpighian bodies is due to filtration under 

 pressure. 



The urine formed in these bodies is alkaline, as has been 

 demonstrated by applying an indicator a substance the 

 colour of which is changed by alkalies to the cut surface 

 of the kidney ; and this further supports the conclusion that 

 it is simply an exudation from the blood. 



That various substances in solution pass out with the 

 water is demonstrated by the fact that when haemoglobin is 

 injected into the blood vessels it soon makes its appearance 

 in Bowman's capsules and in the urine. 



