RENAL SECRETION. 



645 



renal secretion is diminished, it may almost invariably be concluded that 

 the process of filtration has been interfered with by some means or 

 other; and, conversely and as a consequence, the so-called diuretics are, 

 as a rule, substances which directly increase blood pressure and thus 

 facilitate transudation. So, anything -which reduces blood pressure in 

 the kidneys will reduce the secretion. Section of the spinal cord acts in 

 this way. By the universal vascular relaxation produced, the blood 

 pressure is reduced in the kidney, as elsewhere, and filtration rendered 

 almost impossible. 



Stimulation of the spinal cord acts in the opposite direction by con- 

 stricting the general vascular areas, together with the renal artery, and 

 yet produces the same result ; for the increase of general pressure does 

 not compensate for the increased resistance in the renal artery. As a 



Fig. 269.— The Secreting Portions of the Kidney. (Lcmdois.) 



n, Bowman's capsule and glomerulus ; a, vas afferens ; fi, vas efferens ; r, capillary network of the 

 cortex; h, endothelium of the capsule; A, origin of a convoluted tubule. Ill, "rodded" cells from a 

 convoluted tubule ; 2, seen from the side, with g, inner granular zone ; 1, from the surface. IV, cells 

 lining Henle's loop. V, cells of a collecting tube. VI, section of an excretory tube. 



consequence, the flow of blood in the latter is reduced and filtration pre- 

 vented. 



If the local pressure in the kidney be increased, as by section of the 

 renal nerves or by section of the splanchnics, both of which lead to the 

 dilatation of the renal arteries, and hence an increased pressure in the 

 glomeruli, an increased secretion is produced. Conversely, stimulation 

 of the splanchnic or renal nerves arrests filtration by reducing the blood 

 supply to the kidney. 



The correlation between the action of the kidneys .and skin is 

 explainable on these data : It is known that in cold weather the kidneys 

 are more active than in warm weather, while the reverse holds with the 

 skin. In cold weather the capillaries of the external integument are con- 

 stricted and the blood pressure in the internal organs is, therefore, 



