48 SECRETION 



these nerves causes a shrinkage of the organ and a diminution 

 of the secretion. When the fibers are cut, the arteries dilate, 

 the organ enlarges, more blood passes through the kidney, 

 and the secretion is augmented. The vasodilator fibers to 

 the kidney emerge from the cord through the anterior roots 

 of the eleventh, twelfth, and thirteenth spinal nerves. Normally 

 the m fibers i. e., constrictors and dilators are brought into 

 activity reflexly and regulate the formation of /the secretion. 

 Any factor that increases the difference in pressure in the 

 renal artery and the renal vein will cause increased secretion 

 of urine. Vascular dilatation of the vessels of the kidney, 

 unless counterbalanced by a general fall of blood pressure, 

 will give an increased secretion. The following table is useful 

 for reference : 



TABLE OF THE RELATION OF THE SECRETION OF URINE TO ARTERIAL 

 PRESSURE. (KIRKE.) 



A . Secretion of urine may be increased 



(a) By increasing the general blood pressure by 



1. Increase of the force or frequency of the heart beats. 



2. Constriction of the small arteries of areas other than that of 



the kidney. 



(6) By increasing the local blood pressure by relaxation of the renal artery, 

 without compensating relaxation elsewhere by 



1. Division of the renal nerves (causing polyuria). 



2. Division of the renal nerves and stimulation of the cord 



below the medulla (causing greater polyuria) . 



3. Division of the splanchnic nerves; but the polyuria is less 



than in 1 or 2, as these nerves are distributed to a wider 

 area, and the dilatation of the renal artery is accompanied 

 by dilatation of other vessels, and, therefore, by a some- 

 what general increase of blood supply. 



4. Puncture of the 'floor of the fourth ventricle or mechanical 



irritation of the superior cervical ganglion of the sympa- 

 thetic, possibly from the production of dilatation of the 

 renal arteries. 



B. Secretion of the urine may be diminished 



(a) By diminishing the general blood pressure by 



1. Diminution of the force or frequency of the heart beats. 



2. Dilatation of capillary areas other than that of the kidney. 



3. Division of the spinal cord below the medulla, which causes 



a dilatation of the general abdominal area, and urine gen- 

 erally ceases being secreted. 



(b} By increasing the blood pressure by stimulation of the spinal cord 

 below the medulla, the constriction of the renal artery which 

 follows not being compensated for by the increase of general 

 blood pressure. 



(c) By constriction of the renal artery by stimulating the renal or 

 splanchnic nerves or the spinal cord. 



