THE URINARY SYSTEM. 3QI 



the glomeruli, its amount should be increased by raising the capillary 

 pressure in the glomeruli and decreased by lowering that pressure, 

 since the pressure in the ureter is nil, and its composition should be 

 that of blood plasma minus proteins. 



Experiment shows that such is the case. The capillary pressure in 

 the kidney is increased by dilatation of its arterioles, so long as the 

 general blood pressure remains constant, or by a rise in the general 

 blood pressure, if this is not accompanied by active constriction of the 

 renal arterioles. In either case the amount of blood flowing through 

 the kidney is increased, and more blood is present in it at any moment. 

 Conversely, the capillary pressure is diminished either by a fall in 

 the general arterial pressure, or by constriction of the arterioles, the 

 general arterial pressure remaining unchanged. 



The changes in capillary pressure cannot be observed directly, but 

 may be measured indirectly by recording either the alterations in 

 volume of the kidney, or the rate of blood flow through it by one or 

 other of the methods already described (p. 225) ; an increase in the 

 volume of the kidney indicates a rise of pressure in the capillaries of 

 the glomeruli. 



The capillary pressure in the glomeruli is high, partly because the 

 renal arteries arise directly from the aorta, and partly because the 

 efferent vessels of the glomeruli are smaller than the afferent vessels ; 

 it is probably only about 20 to 30 mm. Hg below that in the renal artery. 

 The kidneys are amply supplied with vaso-constrictor nerves from 

 the sympathetic system, and the calibre of the arterioles can be altered 

 by section or stimulation of these nerves. On section of the renal 

 vaso-constrictor nerves the kidney dilates, the rate of blood flow 

 through it is increased, and more urine is formed ; stimulation of the 

 nerves causes shrinking of the kidney, and the flow of urine diminishes 

 or ceases altogether. Division of the spinal cord in the cervical 

 region leads to dilatation of all the arterioles, including those in the 

 kidneys ; but the general arterial pressure falls so low that, although 

 the renal arterioles are dilated, the rate of blood flow through the 

 kidney is much diminished and the flow of urine ceases altogether. 

 Stimulation of the spinal cord in the neck leads to constriction of 

 arterioles and a large rise of blood pressure; and the renal arterioles 

 become so constricted that, in spite of the rise in blood pressure, the 

 volume of the kidney is lessened, and the flow of urine is small or 

 absent. The injection of adrenalin has the same effect as stimulation 

 of the spinal cord. These and other experiments make it clear that, 

 as seen in the following table, the amount of urine formed by the 

 kidney varies directly with the volume of the kidney, that is to say, 



