RBC (Red Blood Cells): are the oxygen-carrying cells using Hemoglobin to hold the oxygen until it is exchanged for carbon dioxide. Hematocrit is a measure of the volume of whole blood taken up by the red blood cells and expressed as a percent. MCV, MHC, MCV and RDW all reflect the size, shape and contents of red cells. Anomolies can be due to smoking, genetics and lower oxygen levels.