Oxygen, like other gases, will pass from one medium to another.  In the lungs, oxygen is passed from air to the blood.  This occurs by passive diffusion.  Like water flowing downhill, oxygen passes from the oxygen-rich air to the oxygen-poor blood without the use of energy.  However, the amount of oxygen passed to the blood would inadequate without the assistance of an oxygen transporting protein called hemoglobin.  Hemoglobin, the protein that gives blood its red color, increases the oxygen carrying capacity of blood by about 70 times!  Red blood cells are packed with hemoglobin, and it is these cells that deliver oxygen to the body tissues.

What is hemoglobin?

Hemoglobin is a protein made of four subunits.  Each of these subunits is specialy designed to hold a single oxygen molecule.  Red blood cells are packed with hemoglobin... a total of 270 million hemoglobin molecules per red blood cell, with each carrying four heme groups.  There are about 30 trillion red blood cells floating around in the average adult.  So... 30 trillion red blood cells, multiplied by 270 million hemoglobin molecules, multiplied by 4 heme groups... well that's... hmmm... a BIG number!

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