Red blood corpuscles / Erythrocytes

Erythrocytes, or red blood corpuscles, are the single most abundant cell type in the human body. They are small, circular, biconcave discs, and in humans they lack a nucleus once mature (unusually, camels and llamas keep a nucleus in their RBCs). Their red colour comes from haemoglobin (Hb), the oxygen-carrying pigment packed into their cytoplasm. A typical male has about 5.1-5.8 million RBCs per mm3 (per microlitre) of blood, and a typical female about 4.3-5.2 million per mm3 — this figure is called the total RBC count. Each red blood cell survives for about 120 days before being broken down.

RBCs are made (a process called erythropoiesis) from precursor cells called haemocytoblasts or reticulocytes. Before birth, this happens in the fetal liver and spleen; after birth, it shifts mainly to the red bone marrow. Vitamin B12, folic acid, iron and haem protein are all needed to make new RBCs. Once worn out, old red blood cells are broken down in the liver and spleen — which is why these two organs are sometimes nicknamed the 'graveyard of RBCs'. Too few RBCs, or too little haemoglobin, or both, is called anaemia; an abnormal increase in RBC number is polycythemia, and an abnormal decrease is erythrocytopenia. Production is kept in balance by the hormone erythropoietin, made by kidney cells, which stimulates the bone marrow to produce more RBCs when needed.

A mature erythrocyte is stripped down for its one job: it has no nucleus, no mitochondria and no other membrane-bound organelles, and its cytoplasm (called the stroma) is dominated by haemoglobin, the oxygen-carrying protein that gives both the cell and whole blood their red colour. It also contains the enzyme carbonic anhydrase, which (as covered in section 8.3) is essential for carrying carbon dioxide as bicarbonate.

Besides carrying O2 and CO2, erythrocytes also help maintain the blood's pH and viscosity, and contribute to blood clotting. The hematocrit — the fraction of total blood volume taken up by RBCs — is a standard clinical measurement and differs somewhat between men and women.