138 Physiology. 



The Blood-supply of the Kidneys. On entering the 

 kidney the renal artery divides and subdivides, forming a 

 very complicated set of capillaries. Through the thin walls 

 of the capillaries certain waste matters pass into the cavity 

 of the kidney, from which they are conveyed by the ureter 

 to the bladder. (See Fig. 32.) 



Urea. Urea is the nitrogen-containing waste of the 

 body. There is nitrogen in muscle, brain, and in all the 

 important organs of the body. When they work, some 

 urea is formed. If the urea accumulates in the blood, it 

 acts as a poison to the tissues. 



Importance of the Work of the Kidneys. The kidneys 

 are the only organs that can remove the urea from the 

 blood ; hence their great importance. Small as they are, 

 their removal would soon cause death. Urea is a solid, and 

 could not very well be carried out of the body unless dis- 

 solved. So the urine consists mainly of water containing 

 urea, salt, and various other substances in small amounts. 



Relation between the Kidneys and the Skin. There is a 

 very close relation between the kidneys and the skin. In 

 warm weather, and when exercising actively, we sweat more 

 and the kidneys excrete less water ; on the other hand, 

 when we exercise less, and especially in a cool place, we 

 sweat less, and the amount excreted by the kidneys is in- 

 creased. For instance, when one has a cold he is 

 more or less feverish ; that is, the action of the skin is inter- 

 fered with, and there is less perspiration. At such time 

 the kidneys have more work to do, and nay be so over- 

 worked as to injure them permanently. 



Effects of Alcohol on the Excretion of Waste Matter. - 

 It has long been known that the use of alcohol diminishes 

 the amount of urea given off. This was thought to prove 



