They have a lot of free space inside the red blood cells and that makes them perfect for storing as much oxygen as possible. Now, the protein that actually carries the oxygen inside the red blood cell is a protein known as hemoglobin. And hemoglobin can carry up to four different oxygen molecules. Now, inside a single red blood cell, we have about 280,000,000 of these individual hemoglobin proteins. And because hemoglobin carries four oxygens, four multiplied by that gives us over 1 billion of these oxygen molecules, is carried in a single red blood cell. So red blood cells are very, very efficient at their job.