We must have oxygen to live. Every living animal cell needs oxygen for its vital metabolic activities (the process of converting fuel to energy), and every cell must also get rid of its carbon dioxide, the gaseous waste of its metabolism.
In the simple animal forms each cell gets oxygen for itself out of the surrounding environment and gives off carbon dioxide in the same way.
In the more highly developed organisms a special mechanism makes the exchange of oxygen and carbon dioxide on behalf of the entire body, and a carrier fetches the oxygen and carries away the waste for all the cells. The carrier is the blood in its circulatory system, and the special exchange mechanism is the respiratory system with the lungs as the key organs.
We have developed from water dwelling creatures and still spend the first nine months from our conception lying in a bath of warm fluid, called the amnion, receiving the oxygen necessary for the cells to do their work from our mothers.
In fact, we are still essentially water dwellers, carrying our watery environment around within us, inside our skins. Because we have evolved lungs instead of gills we are able to live on land, but the air must be sufficiently rich in oxygen-about 20 per cent. Above 8,000 feet breathing begins to become difficult and the symptoms of mountain sickness, headache, nausea and vomiting may appear.
If we did nothing but rest, needing only a minimum supply of air, we would still need 300 quarts of oxygen every day. In a single minute of ordinary activity half a pint of oxygen has to be transferred from the air to the blood.
For this half pint the lungs must process about five quarts of air every minute. An athlete running a race at sea level breathes as much as 120 quarts of air a minute to get the oxygen he needs to keep him going, which shows the importance of healthy lungs.