80 THE INDIVIDUAL ORGANISM 



bladder from the kidneys is stored in the bladder (which may become 

 markedly distended), until a voluntary impulse causes the muscular 

 valve to relax. Contraction of the muscular wall of the bladder then expels 

 the urine from the body. 



Although not ordinarily considered a part of the excretory system, 

 the liver also plays a role in the excretion of nitrogenous wastes. It re- 

 moves a considerable variety of waste nitrogenous compounds from the 

 blood, transforms them into urea, and returns the urea to the blood, to 

 be carried to the kidneys. It also takes from the blood certain waste 

 products formed by the breakdown of hemoglobin and excretes them in 

 the bile in the form of bile pigments. The skin has a minor excretory 

 function in its elimination of part of the excess body water and certain 

 salts through the sweat glands. 



