With regard to the influence exerted by the nervous system on the renal secretion, we have but little satisfactory information, although there can be no doubt that here, as in other glands, the process is under the control of the nerves. Many of the circumstances which cause greater activity of secretion, such as taking large quantities of water, etc., have no effect on the general blood pressure, so that, if the increased flow be brought about by the vasomotor mechanisms, it must be by means of nervous channels altering the blood flow in the special arteries of the glands. Further, some emotional conditions exist, such as hysteria, in which an unaccountably great quantity of urine of very low specific gravity is evacuated.
With regard to the effects of the vasomotor nerves, we know that section of all the nervous twigs going to the kidneys causes great congestion and an immense increase in the secretion, which commonly contains albumin. This no doubt depends on the sudden rise in pressure in the glomeruli, owing to the dilatation of the arterioles. If the splanchnics, in which the renal vasomotor nerves run, be cut, a great quantity of urine is produced from the same cause - vasomotor paralysis - but, on account of the large area of vessels injured, the general blood pressure falls, and therefore the effect is not so much marked. If the peripheral end of the cut nerves be stimulated, the secretion is diminished, and, owing to spasm of the renal arterioles and fall of blood pressure in the glomerular capillaries, may be brought to a standstill. Section of the spinal cord at the 7th cervical vertebra stops the flow, because it so reduces the general blood pressure that the pressure in the renal vessels falls below that necessary for the filtration of the urine.
The introduction of various substances into the blood causes a marked change in the blood supply of the kidney and the amount of urine secreted. These changes do not correspond with the changes in general blood pressure occurring during the experiments.