Red blood cells, or erythrocytes, travel through circulating blood carrying oxygen to body tissues and organs while removing waste. These blood cells make up the largest part of the blood system.

As the red blood cells in blood travel through the lungs, oxygen molecules from the lungs attach to the hemoglobin, a protein in the blood cells that contains iron. The oxygen is then released to tissues and organs, and the hemoglobin bonds with carbon dioxide and other waste gases. These waste products are transported away and removed as blood continues to circulate.

Millions of red blood cells are contained in a single drop of blood. Red blood cells are constantly being produced in the bone marrow to replenish those that gradually wear out and die. The average life of a red blood cell is about 120 days.

A significant decrease in the number of red blood cells causes anemia and shortness of breath.