RbcS

Red blood cells (RBCs), also called erythrocytes, are the most common cell type in blood. Their primary function is to ferry oxygen from the lungs to tissues throughout the body and to carry carbon dioxide, a waste product of metabolism, from tissues back to the lungs for exhalation. This gas transport is accomplished mainly by hemoglobin, a protein with iron-containing heme groups that reversibly binds oxygen and carbon dioxide. In the lungs, hemoglobin loads oxygen to form oxyhemoglobin; in tissues it releases oxygen and binds carbon dioxide to form carbaminohemoglobin, aided by the chloride shift.

RBCs have a characteristic biconcave shape, which increases surface area and enables deformability as they traverse narrow capillaries. In humans, mature mammalian RBCs lack nuclei and most organelles, maximizing space for hemoglobin but limiting their ability to synthesize new proteins. They are about 6-8 micrometers in diameter and have a lifespan of about 120 days in circulation.

Production occurs in the red bone marrow through erythropoiesis, regulated chiefly by erythropoietin produced by the kidneys in response to hypoxia. New RBCs enter circulation as reticulocytes and mature over a day or two. Old and damaged RBCs are removed by macrophages in the spleen, liver, and bone marrow; heme is degraded to bilirubin, and iron is recycled for new hemoglobin synthesis.

RBCs carry blood group antigens (notably ABO and Rh), which are clinically important for transfusion. Abnormal RBCs are central to various disorders, including anemia, polycythemia, and inherited conditions such as sickle cell disease or thalassemias.