198 RESPIRATION, BLOOD, AND EXCRETION 



Figure 211. — Micro- 

 photograph of Blood 

 of Frog. 



ball). When a few of these corpuscles are examined 



through a microscope, they appear yellowish instead of 



red ; but when a large number of 

 them are seen in a mass, the red 

 color is apparent. When the red 

 cells are first formed, they have a 

 nucleus which gradually disappears. 

 As a result, the mature red corpus- 

 cles, unlike all the other cells we 

 have studied thus far, have no 

 nucleus. Red corpuscles are about 

 3<>Vo °^ an i ncn i R diameter and 

 T2T0 o °^ an ^ ncn thick. 

 The red corpuscles carry oxygen from the lungs to 



the cells of the body. This oxygen unites with the 



haemoglobin. By osmosis the oxygen passes from the 



blood to the body cells which are deficient in oxygen. 



These cells take the oxygen and use it in the process of 



oxidation, which goes on 



continuously in every 



living cell. A good 



supply of red blood 



corpuscles is, therefore, 



necessary, if the cells of 



the body are to have a Figure 212. 



sufficient supply of OXy- As the blood flows through the capil- 



2"en. The feeding" of ^ aries which are found in all voluntary 

 . ,, . , . muscles, for example, oxygen and other 



the cells with oxygen is food products are given off to the muscle 



One part of respiration. cells, and carbon dioxide and other 



At the same time that waste substances Pass off from these 



same muscle cells into the capillaries 



oxygen is received from on the way int0 the ve ins. 



the blood by the body 



cells, carbon dioxide is given off. Again osmosis explains 



the method of this transfer. Most of the carbon dioxide is 



