Red blood cells, also called erythrocytes, are specialised blood cells whose principal function is transporting oxygen from the lungs to tissues. They contain large amounts of haemoglobin, an iron-containing protein that binds oxygen reversibly. Red blood cells also contribute to carbon dioxide transport and acid-base regulation. They are the most numerous cellular component of normal blood and are highly specialised for gas transport rather than for cell division, immune defence or clot formation. Their abundance reflects the continuous oxygen requirements of tissues throughout the body.
Mature human red blood cells have a distinctive biconcave disc shape and normally lack a nucleus and most other organelles. Losing the nucleus provides more internal space for haemoglobin and allows the cell to remain flexible. The biconcave shape increases the surface-area-to-volume ratio and facilitates rapid gas exchange. Red blood cells are also deformable, allowing them to pass through capillaries that may have an internal diameter comparable to or smaller than the cell itself. These structural characteristics are closely related to their role in efficient oxygen transport through the microcirculation.
Haemoglobin is central to red-cell function. Each haemoglobin molecule contains four globin chains and four haem groups, each containing an iron atom capable of reversibly binding oxygen. Oxygen loading occurs predominantly in the lungs, where oxygen partial pressure is relatively high, and unloading occurs in tissues where oxygen demand and local chemical conditions favour release. Haemoglobin's affinity for oxygen is influenced by factors including oxygen concentration, carbon dioxide, acidity and temperature. These relationships help match oxygen unloading to metabolically active tissues.
Red blood cells also participate in carbon dioxide transport. Some carbon dioxide binds to haemoglobin, while much of the carbon dioxide entering red blood cells is converted through carbonic anhydrase into bicarbonate, which is transported in plasma. The reaction is reversible and is important for both carbon dioxide carriage and acid-base regulation. In the lungs, bicarbonate is converted back into carbon dioxide, which diffuses into the alveoli and is exhaled. Red blood cells therefore participate in respiratory gas transport in both directions rather than functioning solely as oxygen carriers.
Red blood cells are produced through erythropoiesis in the bone marrow. The kidneys sense reduced oxygen availability and respond by producing more erythropoietin, which stimulates red-cell production. Adequate iron, vitamin B12, folate, amino acids and other nutrients are required for normal erythropoiesis. Mature red cells circulate for approximately 120 days under typical conditions before being removed primarily by macrophages in the spleen, liver and bone marrow. Iron and other useful components are recycled, particularly for new haemoglobin production.
The number and quality of red blood cells directly influence oxygen-carrying capacity. Anaemia is a condition in which the blood's oxygen-carrying capacity is reduced, commonly because of too few red cells, insufficient haemoglobin or abnormalities affecting the cells. Causes include iron deficiency, vitamin deficiencies, blood loss, chronic disease, inherited haemoglobin disorders and impaired bone-marrow production. Conversely, an unusually high red-cell concentration can occur in situations such as chronic hypoxia, dehydration or certain diseases. A red-cell count is interpreted together with haemoglobin, haematocrit and red-cell indices rather than in isolation.
Red-cell morphology can provide important diagnostic information. Mean corpuscular volume describes average cell size and helps classify anaemias as microcytic, normocytic or macrocytic. Other indices describe haemoglobin content and variation in cell size. Conditions such as iron deficiency commonly produce smaller cells, whereas vitamin B12 or folate deficiency can produce larger cells. Abnormal shapes may occur in inherited or acquired disorders, including sickle cell disease, in which haemoglobin abnormalities can cause red cells to adopt a characteristic sickle shape under certain conditions. Such changes can impair flexibility, oxygen delivery and blood flow.
Red blood cells should be distinguished from white blood cells and platelets because their functions and structures are fundamentally different. Red cells specialise in gas transport, white cells participate principally in immunity, and platelets participate principally in haemostasis. The term erythrocyte derives from Greek roots meaning red cell. Their red colour comes from haemoglobin, while the absence of a nucleus in mature human erythrocytes distinguishes them from many other cells. Their specialised structure demonstrates how cellular anatomy can become highly adapted to a specific physiological function.
