RESPIRATION AND EXCRETION 



341 



-Ureter 



Longitudinal section through a 

 kidney. 



to animals. In man, the skin and 



kidneys remove this waste from 



the body, hence they are called the 



organs of excretion. 



The Human Kidney. The 



human kidney is about four inches 



long, two and one half inches wide, 



and one inch in thickness. Its 



color is dark red. If the structure 



of the medulla and cortex (see 



figure above) is examined under 



the compound microscope, you will 



find these regions to be composed 



of a vast number of tiny branched 



and twisted tubules. The outer 



end of each of these tubules opens into the pelvis, the space within 



the kidney ; the inner enpl, in the cortex, forms a tiny closed sac. 



In each sac, the outer wall of the tube has grown inward and 

 carried with it a very tiny artery. This 

 artery breaks up into a mass of capillaries. 

 These capillaries, in turn, unite to form a 

 small vein as they leave the little sac. 

 Each of these sacs with its contained blood 

 vessels is called a glomerulus. 



Wastes given off by the Blood in the 

 Kidney. In the glomerulus the blood 

 loses by osmosis, through the very thin 

 walls of the capillaries, first, a consider- 

 able amount of water (amounting to 

 nearly three pints daily) ; second, a nitrog- 

 enous waste material known as urea; 

 third, salts and other waste organic sub- 

 stances, uric acid among them. 



These waste products, together with the 



lus and tubule : a, artery 

 bringing blood to part ; 

 b, capillary bringing blood 



to glomerulus; b', vessel wa t er containing them, are known as urine. 

 ^^?toW* The total amount of nitrogenous waste leaving 

 G, glomerulus.' the body each day is about twenty grams. It 



