Erythrocytes, commonly known as red blood cells, are the most abundant type of cells in the blood. Their primary function is to transport oxygen from the lungs to the body's tissues and return carbon dioxide from the tissues back to the lungs. Erythrocytes contain a protein called hemoglobin, which binds to oxygen and gives blood its red color. These cells are produced in the bone marrow and have a lifespan of about 120 days. Erythrocytes are unique in that they lack a nucleus, allowing for more space to carry hemoglobin. The production and regulation of erythrocytes are influenced by the hormone erythropoietin, which is produced by the kidneys in response to low oxygen levels.