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Erythrocytes, commonly known as red blood cells, are the most abundant type of blood cell and the vertebrate's primary means of delivering oxygen 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 throughout the body. The counterpart of this function is the removal of the waste product, carbon dioxide, from the tissues and its transport to various organs to be released. Erythrocytes contain the protein hemoglobin, which binds oxygen and gives blood its red color.

The shape of erythrocytes is a biconcave disc, which increases the surface area to volume ratio, facilitating diffusion of oxygen and carbon dioxide. Mature erythrocytes in mammals lack a nucleus and most organelles, including mitochondria. This anucleated state maximizes the space available for hemoglobin. Their lifespan is typically around 120 days, after which they are removed from circulation by macrophages in the spleen and liver. The production of erythrocytes, a process called erythropoiesis, occurs in the bone marrow and is regulated by the hormone erythropoietin.

Erythrocyte count is a common blood test that can indicate various health conditions. A low count, known as anemia, can result from deficiencies in iron, vitamin B12, or folate, or from chronic diseases. A high count, or polycythemia, can be caused by dehydration, lung disease, or certain cancers. Diseases affecting erythrocytes, such as sickle cell anemia and thalassemia, are genetic disorders that alter hemoglobin structure or production, leading to impaired oxygen transport and other complications.