Red blood cell

A red blood cell, or erythrocyte, is the most common cell in the blood, making up about a quarter of all the cells in the body. Mature human red blood cells have no nucleus, no mitochondria and few other organelles, which leaves room for hemoglobin, the protein that carries oxygen. Each cell is a flexible, biconcave disc about 7 to 8 micrometers across, thin in the middle and thicker at the rim. This shape gives a large surface for gas exchange and lets the cell bend to squeeze through narrow capillaries. A microliter of blood holds about 5 million of them.

Red blood cells are made in the red bone marrow, a process called erythropoiesis. When blood oxygen falls, the kidneys release the hormone erythropoietin, which speeds up production. Because they lack mitochondria, red cells make their energy without oxygen and deliver all the oxygen they carry to the tissues. Each one circulates for about 120 days before macrophages in the spleen, liver and bone marrow remove it and recycle its iron.

Counting red cells and measuring hemoglobin are part of a routine blood test. Too few red cells, or too little hemoglobin, is anemia; too many is polycythemia. Sickle cell disease and malaria both damage red blood cells directly.