Circulation, Assimilation and Excretion 417 



The plasma is about ninety per cent water and holds in solution 

 organic and mineral matter which form most of the other ten 

 per cent. The organic and the mineral matter are the nutrients 

 of the cells. Waste matter from cells is constantly thrown into 

 the plasma and is carried away by it. 



Red Corpuscles. The red corpuscles are flat circular plates 

 with rounded edges and a depression in the center of each. They 

 are cells without a nucleus, microscopic in size and countless in 

 number. When seen in mass they have a bright red color. They 

 make up nearly one-half the weight of the blood and are com- 

 posed of an albuminous substance, mainly hemoglobin. They 

 carry oxygen. The red blood corpuscles are formed in the mar- 

 row of the bones and in a small organ called the spleen. 



White Corpuscles. The white corpuscles are irregularly 

 shaped, colorless cells having a nucleus. They are microscopic in 

 size and are about one five-hundredth as numerous as the red cor- 

 puscles. They are able to change their shape and to wander among 

 the cells of the body, even passing through the walls of the capil- 

 laries. They destroy disease germs and aid in the healing of 

 wounds. 



Changes in the Composition of the Blood. The chemical sub- 

 stances of the blood are constantly changing their proportions 

 as it passes through the body. In the lungs the blood takes up 

 oxygen and excretes carbon dioxide. In the muscles and other 

 tissues it gives off" food and oxygen and takes up carbon dioxide, 

 urea and other wastes. In the walls of the alimentary canal 

 digested food is taken up and wastes are given off. As it cir- 

 culates, the blood gives out the material needed by the cells for 

 growth and repair and for making of digestive fluids. In the 

 kidneys and skin, the blood gives off oxygen and nutritive mate- 

 rial and gets rid of water and a nitrogenous waste called urea. 



Clotting of Blood. When blood flows from the body, as from 

 a cut, it forms a jelly-like substance and is said to coagulate, or 

 clot, separating into two parts. The liquid part of clotted blood 

 is called serum, which consists of water with organic and mineral 

 matter in solution. The solid portion, called fibrin, holds the cor- 

 puscles in its meshes. 



