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Erythrocytes, also known as red blood cells, are the most common type of blood cell and the vertebrate's primary means of delivering oxygen (O2) to the body tissues—via blood flow through the circulatory system. They take up oxygen in the lungs or gills and release it into tissues flowing through the peripheral circulatory system. The term erythrocyte was coined by German pathologist Rudolf Virchow in 1847. In humans, mature red blood cells are flexible, biconcave discs about 7–8 micrometers (μm) in diameter and lacking most organelles, including the nucleus. They contain a high concentration of hemoglobin, a protein that binds oxygen. The shape of erythrocytes allows for a large surface area to volume ratio, facilitating efficient diffusion of oxygen and carbon dioxide. Erythrocytes are produced in the bone marrow through a process called erythropoiesis, which is regulated by the hormone erythropoietin. After circulating in the bloodstream for about 120 days, they are removed from circulation by macrophages, primarily in the spleen and liver. The breakdown products of hemoglobin, such as bilirubin, are then processed and excreted. The production and destruction of erythrocytes are part of a continuous cycle that maintains a stable population of these vital cells. Their normal count in the blood is a key indicator of health.