450 
RESPIRATION AND EXCRETION 
cells of the body. As soon as the oxygen in the respired air 
enters the blood, it unites chemically with the haemoglobin 
contained within the red blood corpuscles. Here it remains 
until it reaches cells that are deficient in oxygen, when it 
passes from the blood by osmosis to such cells. 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 sufficient supply of oxygen. The 
feeding of the cells with 
oxygen is one part of 
respiration. 
At the same time that 
oxygen is received from 
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 
carried by the plasma, 
although some of it unites 
with the haemoglobin. 
White blood corpuscles are much like the amoeba in that 
they are colorless and can change their form. They move 
about in the body and often leave the blood vessels and col¬ 
lect at one place to aid the body in destroying disease germs. 
The white blood cells eat these disease germs in just the same 
manner that the amoeba or paramecium eats bacteria. 
The blood plasma is straw-colored and varies in composi¬ 
tion from day to day, and hour to hour. It contains the 
foods on their way to the cells and waste products on their 
way to the kidneys, lungs, or skin. 
The volume of blood in the average person is about six 
quarts. 
Figure 399. 
As the blood flows through the capil¬ 
laries which are found in all voluntary 
muscles, for example, oxygen and other 
food products are given off to the muscle 
cells, and carbon dioxide and other waste 
substances pass off from these same 
muscle cells into the capillaries on the 
way into the veins. 
