Anemia is defined as a low number of red blood cells (RBCs or erythrocytes) in the circulation. Red blood cells are a type of cell that contains hemoglobin, which is an iron-containing molecule that binds oxygen efficiently. When we breathe in and out, oxygen is drawn from the air in our lungs and absorbed into the blood, where it is bound to hemoglobin in our red blood cells. As these RBCs circulate throughout the body, the hemoglobin can then release oxygen in the tissues where it is required to maintain life. RBCs contain hemoglobin, which gives blood its characteristic red color.