140 



PHYSIOLOGY. 



the same point the renal vein returns the blood from the kidneys, and 

 pours it into the postcaval vein. Through the kidneys is pouring a 

 continuous stream of blood, varying in amount at different times and 

 in different conditions. The kidney receives a very large amount of 

 blood for its size, as compared with other organs. The flow to it is 

 made easy by the fact that the renal arteries are relatively wide and 

 short, and take the blood directly from the main current of the aorta. 

 The blood leaving the kidney, especially when in full activity, is still 

 bright red ; it is probably the purest blood in the body. 



Urine. From the kidney, through the ureter, urine is continually 

 passing to the bladder. Urine is mostly water containing urea, salt, and 

 various other substances in small amounts. Urea is a waste matter 

 brought in the blood. If the kidneys are stopped in their action, urea 

 accumulates in the blood, and death soon results ; to just the degree that 

 the kidneys fail in performing their duty, just so far must the body suffer. 



Microscopic Structure of the Kidney. If microscopic sections of 

 the kidney are at hand they should be examined ; but the kidney is so 



complicated in structure that a 



Urinary Cone diagram is needed in connection 



with the sections and the de- 

 scriptions. The unit of struc- 

 ture in the kidney is a tube which 

 takes material from adjacent 

 blood capillaries. The relation 

 of the capillaries to the tube is 

 peculiar. The inner end of the 

 tube is enlarged into a ball ; this 

 ball is deeply depressed opposite 

 the point where the tube leaves 

 it. Into this depres- 

 sion is fitted a globu- 

 lar tuft of capillaries. 

 The arrangement may 

 be illustrated by the 

 common toy known as the " cup 

 and ball." The handle of the 

 cup should be hollow to repre- 

 sent the tube ; the cup should be double walled, the space between 

 the inner and outer layers continuous with the hollow of the handle. 



Cavity of 

 Kidney 



Renal 

 Vein 



Fig. 52. 



U rete r 

 Cross Section of Kidney. 



