The red cells are the oxygen transporting carriers. They are filled with a red-coloured chemical called haemoglobin. This is normally a dark-coloured substance. But it has an incredibly high affinity for oxygen. Exposed to it, it will instantly absorb large quantities, and be transformed from its sombre colour to a bright, fiery red substance we call oxy haemoglobin.
It occurs in the lungs, at a spot doctors call the cell interface of the tiny air sac. At this point, the air we breathe comes into close proximity with the blood cell travelling through a microscopically narrow tube called a capillary. Only a thin membrane separates the blood from the air. At this point the blood haemoglobin is filled with carbon dioxide, another gas collected by the blood on its journey through the body. Immediately the blood cell contacts oxygen, it releases the carbon dioxide and absorbs the oxygen. At this moment its colour changes from a dull, bluish tint to a brilliant red. From here it continues on its merry way, to enter the arterial circulation and be pumped by the heart around the body once more.
Ultimately the blood enters similar small tubes again, called capillaries, not in the lungs, but this time in various organs of the body, or muscles. Here the reverse takes place. As the body requires oxygen, this is absorbed from the red cells. It combines with the body’s fuel usually to produce energy. In turn, carbon dioxide and other by-products of the system’s metabolism are channelled back into the bloodstream. Carbon dioxide is taken up by the haemoglobin, and so the long trip back to the air sacs of the lungs recommences, and the process continues unabated.
Other impurities are thrown into the fluid part of the blood, and these are finally filtered out in the kidney system, to be excreted in fluid we call urine.