EXCRETION 471 



The pressure of blood in the renal artery and therefore in the glomeruli, 

 may be diminished by a decrease in the inflow of blood. This effect 

 may be the result: 



1. Of a contraction of the renal artery and its branches. 



2. Of a diminution in the rate and force of the heart's action, or a decrease 



in the contraction of the arterioles of large vascular areas in any or 



all parts of the body. Should either of these conditions arise the 



volume of the blood delivered to the renal artery in the unit of time 



would be diminished and hence its pressure would diminish. The 



dilatation of the cutaneous vessels in summer, the result of the high 



temperature leads to a diminished blood supply to the kidney and 



hence a decrease in the amount of urine secreted. 



The dilatation and contraction of the renal artery whereby the inflow 



of blood is increased and decreased is regulated by the nerve system which 



will be alluded to in a following paragraph. 



It has also been supposed that the pressure in the vessels of the glomeruli 

 may be varied according to the degree of contraction or relaxation of the 

 muscle coat of the afferent and efferent vessels alone. See Fig. 208 

 and the accompanying explanation. 



Coincident with the rise and fall of pressure in the glomerular capillaries 

 there is a rise and fall in the rate of urinary flow. Thus it has been found that 

 an increase in the aortic pressure from 127 to 142 mm. of mercury, from 

 ligation of the carotid, femoral, and vertebral arteries, increased the rate of 

 urinary flow from 8.7 grams in thirty minutes to 21.2 grams. On the 

 contrary, a decrease in aortic pressure below 40 mm. of mercury caused by 

 division of the spinal cord is followed by a total abolition of the urinary flow. 

 These facts serve to indicate the dependence of the secretion on blood- 

 pressure. 



The period of functional activity of the kidney is accompanied by an 

 increase in the volume of blood flowing through it as is evident from an in- 

 spection of the organ. At this time it is enlarged, swollen, and red in color. 

 The blood in the renal vein is bright red in color and contains more oxygen 

 and less carbon dioxid than venous blood generally. During the intervals 

 of activity the kidney is supplied with a less amount of blood and hence it 

 diminishes in size, becomes pale in color and the blood of the renal: vein 

 becomes dark and venous in character. These variations in the volume of 

 the kidney have also been experimentally determined and registered by 

 means of the oncometer and oncograph devised by Roy. 



The Renal Oncometer. The oncometer consists of a metallic capsule, 

 (Fig. 209) composed of halves which open and close by means of a hinge. 

 The capsule encloses two thin membranous and distensible sacs. These are 

 connected with a piston recorder by means of a tube. The kidney, with- 

 drawn from the body, is placed within the oncometer. Through an opening 

 in the side pass the artery, vein, and ureter. A thin light oil is then poured 

 through a side tube from a pressure bottle until the membranous sacs 

 are completely filled and surround the kidney on all sides. When the 

 tube from the pressure bottle is closed, the conditions are such that all^varia- 

 tions in the volume of the kidney are taken up and reproduced by the lever 

 attached to the piston recorder. A curve of the variations in the volume 

 of the kidney is shown in Fig. 210. An examination of this curve shows that 



