THE SECRETION OF URINE. 



I. 



The blood supply to the kidneys is an important consider- 

 ation in the secretion of urine. At the outset, one is impressed 

 by the fact that a relatively large artery supplies a relatively 

 small gland. The blood pressure of the renal artery is nearly, 

 if not quite, as great as that of the abdominal aorta, which 

 means that about as much pressure is required to force the 

 blood through the kidneys as is required to send the blood 

 through the pedal extremities. The renal vein is also a rela- 

 tively large vessel as compared with the size of the kidney or 

 with the efferent vessels of other glands. The pressure within 

 the renal vein is very low, practically as low as that in the 

 posterior vena cava with which it connects. 



The artery, after entering the kidney, breaks up into 

 branches which pass between the pyramids. At the junction 

 of the cortex and medulla these branches form arches in the 

 substance of the kidneys and from these arches branches run 

 outward into the cortex to supply the glomeruli ; other branches 

 pass inward to supply the pyramids. Each glomerulus has 

 its afferent and efferent vessel and of these the efferent is 

 smaller. 



On issuing from the capillaries of the glomerulus, the 

 efferent vessel soon breaks up into a second set of capillaries 

 which supplies the uriniferous tubules ; so that in this arrange- 

 ment the blood goes first to the glomeruli and later supplies 

 the tubules. The blood from the second set of capillaries is 

 finally gathered up by small vessels which unite to form ulti- 

 mately the renal vein. The glomeruli and the uriniferous 

 tubules are the portions of the kidney actively concerned in 

 the production of its secretion — the urine. 



The following points are therefore worthy of special 

 notice: in the malpighian body the arterial blood in the glom- 

 erular capillaries is separated from the inside of the capsule 

 of Bowman by a thin layer of flattened epithelial cells only; 

 two sets of capillaries exist, one set forming the glomeruli and 

 the other supplying the uriniferous tubules, the blood supply- 



