erythrocytes

Erythrocytes, or red blood cells (RBCs), are the most abundant cells in blood and are specialized for gas transport. In humans and many vertebrates they carry oxygen from the lungs to tissues and help carry carbon dioxide back to the lungs for exhalation.

Typically disk-shaped and highly flexible, mature erythrocytes are about 6-8 micrometers in diameter with a biconcave geometry, increasing surface area for gas exchange and facilitating passage through narrow capillaries. In mammals they are anucleate and largely devoid of organelles such as mitochondria and ribosomes, a configuration that maximizes hemoglobin content but restricts metabolic activity. During maturation, nuclei are extruded in erythropoiesis.

Red blood cells contain hemoglobin, a tetrameric protein with four heme groups that bind iron and reversible oxygen. Hemoglobin binds oxygen in the lungs (oxyhemoglobin) and releases it in tissues with lower oxygen tension; it also binds carbon dioxide and protons, helping transport CO2 from tissues to the lungs as carbaminohemoglobin and bicarbonate.

Production and lifespan: Erythropoiesis occurs in the bone marrow from erythroid progenitors, regulated by erythropoietin produced primarily by the kidneys in response to hypoxia. Mature erythrocytes circulate for about 120 days in humans and are removed by macrophages in the spleen, liver, and marrow. Iron is recycled and heme is degraded to bilirubin for elimination.

Across vertebrates, erythrocyte morphology varies: mammalian RBCs are anucleate, whereas many non-mammalian vertebrates retain nuclei in their erythrocytes. Normal erythrocyte counts and indices are used in clinical medicine to diagnose anemia, polycythemia, and other blood disorders.