330 ANATOMY FOR NURSES [Chap. XVII 



system. Many of these nerves are vasomotor nerves, and by regu- 

 lating the contraction and relaxation of the blood-vessels, they in- 

 fluence the blood pressure in the kidney. They are also well sup- 

 plied with lymphatics. 



Function of the kidneys. — The function of the kidneys is to 

 separate waste matters (urine) from the blood, and thus help to 

 maintain the normal composition of the blood. The waste matters 

 are those resulting from metabolism, particularly of proteins, 

 water, salts, and foreign matters such as toxins, whether formed 

 in the body, or taken into the body from outside. 



The secretion of urine. — The exact way in which the kidneys 

 separate the urine from the blood is not known, but it is thought 

 to be a double process, being partially accomplished by transuda- 

 tion, and partially by the selective action of the secreting cells 

 lining the tubules. 



(1) Into each hollow capsule which forms the beginning of a 

 uriniferous tubule an afferent artery enters. This artery breaks 

 up into capillaries which form a bunch of looped and twisted 

 blood-vessels called a glomerulus. The walls of the capsule being 

 double, the glomerulus pushes back the inner wall or visceral 

 layer, until the capsule is entirely filled, leaving only a small space 

 between it and the outer wall or parietal layer. The blood in the 

 glomerulus is only separated from the interior of the tubule by 

 the thin walls of the capillaries and the inverted wall of the 

 capsule. The artery (afferent) which enters the capsule is larger 

 than the issuing (efferent) vessel, and during its passage through 

 the glomerulus, the blood is subjected to considerable pressure. 

 As a result of this, a transudation of the watery constituents of 

 the blood, with some dissolved salts, takes place through the 

 walls of the blood-vessels and the walls of the capsule into the 

 capsular space, then into the tubule. 



(2) After leaving the capsule, the efferent vessel communicates 

 with other similar vessels, which together form a meshwork or 

 plexus of capillaries closely surrounding the tubules, so that the 

 blood is again brought into close communication with the in- 

 terior of the tubules. The tubules are lined with secreting cells, 

 and these cells appear to have the power of selecting from the 

 blood the more solid waste matters (especially the urea), which 

 fail to filter through the flat cells forming the wall of the capsule. 



