2 DISEASES OF THE KIDNEY. 



shoots running into the cortical and medullary substance, and others 

 wliich act directly as nutrient vessels for the medullary substance. 

 Finally, the renal artery has branches upon which there are no glome- 

 ruli at all, and whose function is simply nutrition of the medullary 

 substance. 



The resistance encountered by the blood wliich passes through the 

 gloineruli is far greater than that met by the blood which merely flows 

 directly from the arteries into the capillaries. Even under normal 

 conditions the blood-pressure in the renal arteries is a very consider- 

 able one, that vessel being both short and of disproportionately large 

 calibre. When the pressure within these arteries is increased, hyper- 

 a-inia of course will first arise in that portion of the kidney where the 

 blood encounters the greatest resistance, namely, in the cortical sub- 

 stance, and, above all, in the glomeruli. Where the resistance is less, 

 as in the medullary substance, although the circulation is accelerated 

 there, the actual amount of blood which the part contains is not ma- 

 terially increased. It is very different, however, when the escape of 

 the blood from the renal veins is impeded. In such a case the quan- 

 tity of blood in the veins and capillaries is augmented, but the en- 

 gorgement cannot extend through the narrow efferent vessels into the 

 glomeruli ; and, as the contents of the arteries are abnormally reduced 

 in most of the disorders in which there is an obstruction of the renal 

 veins (as in cardiac and pulmonary diseases), the reason at once be- 

 comes apparent why, even in cases of extreme obstructive hyperaemia 

 of the kidney, the glomeruli are scantily supplied with blood, and the 

 secretion of urine is proportionately small. Perhaps, too, nervous in- 

 fluence may not be without effect in bringing about this condition, 

 since it is possible that in the kidneys, just as in other organs, the ar- 

 teries of the various vascular systems are not subject to the same kind 

 of innervation. 



The causes which induce fluxion to the kidney are : 



1. The transient plethora induced by every copious draught of 

 liquid This hyperaemia is most pronounced in the secretory portion 

 of the kidney, and the profuse transudation which takes place out of 

 the overloaded glomeruli is the principal step in the process by which 

 the general plethora of the system is relieved. 



2. There is a second cause of renal hyperaemia, which is closely 

 analogous to the first, and which occurs when the left side of the heart 

 is hypertrophied, and which also is confined to the arterial system, in- 

 cluding the glomeruli. 



3. Collateral fluxion to the kidney may result from compression of 

 the abdominal aorta, or iliac arteries, by a tumor or gravid uterus, as 

 well as from derangement of the circulation in the capillaries of the 



