Now, what would happen if we were to throw all the garbage from the kitchen, and the wash water from the kitchen sink, and the dirty water from the bathroom right into the well out of which we pumped our drinking water?
We should simply be poisoned within two or three days, if indeed we could manage to drink the disgusting mixture at all.
That is exactly what would happen to our body cells if they were not provided with some way of getting rid of their waste and dirt.
[Illustration: THE GREAT ESSENTIAL TO LIFE--AIR If the air, supplied to the diver through the tube, is cut off for three minutes, or even less, the diver cannot live.] Part of the waste that comes from our body cells is either watery, or easily dissolved in water; and this is carried in the blood to a special set of filter organs--the liver and the kidneys--and poured out of the body as the _urine_.

Another part of it, when circulating through the skin, is passed off in the form of that watery vapor which we call perspiration, or sweat.

But part of the waste can be got rid of only by burning, and what we call burning is another name for combining with oxygen, or to use one word--_oxidation_; and this is precisely the purpose of the carrying of oxygen by the little red blood cells from the lungs to the deeper parts of the body--to burn up, or oxidize, these waste materials which would otherwise poison our cells.

When they are burnt, or oxidized, they become almost harmless.
Why the Red Cells Carry only Oxygen to the Body.

But why do not the red cells carry air instead of just oxygen?
This is simply a clever little economy of space on nature's part.