Red Blood Cells Serum are the most common cells in your bloodstream that gives blood its red color. When seen through a microscope, they look similar to doughnuts with a miserable center instead of a hole. Every cell is filled with hundreds of millions of hemoglobin molecules. Every hemoglobin is, consecutively, a striking spherical structure made up of about 10,000 nitrogen, oxygen, hydrogen, carbon, and sulfur atoms, in addition to four heavier atoms of iron that provides blood its oxygen-carrying capability. Hemoglobin aids the transfer of carbon dioxide from the tissues to the lungs, where it is breathed out.