An erythrocyte, commonly called a red blood cell, is a specialized cell whose principal function is to transport oxygen in the blood. Its abundant hemoglobin binds oxygen reversibly, allowing uptake in respiratory organs and release to tissues. Erythrocytes also contribute to carbon dioxide transport. In humans, they are the most numerous cellular components of blood and normally circulate within blood vessels. Mature human erythrocytes are small, flexible, biconcave discs without nuclei. (openstax.org)
Structure and cellular specialization
A typical human erythrocyte measures approximately 7–8 micrometres across. Its centre is thinner than its rim, producing the characteristic biconcave shape. This geometry provides a relatively large surface area for gas exchange and short distances for diffusion. Flexibility allows the cell to deform while passing through narrow capillaries and recover its shape afterward. (openstax.org)
The cell membrane is supported by a cytoskeleton containing structural proteins such as spectrin. This framework combines mechanical strength with deformability. During maturation, the cell expels its nucleus and removes most organelles. Mature human erythrocytes lack mitochondria and functional ribosomes, so they cannot divide or replace proteins through ordinary protein synthesis. Their interior is largely devoted to hemoglobin, an oxygen-binding protein. (openstax.org)
The absence of mitochondria also distinguishes their metabolism from that of many other human cells. They obtain adenosine triphosphate through glycolysis rather than mitochondrial oxidative metabolism, preserving the oxygen they carry for delivery to other tissues. (openstax.org)
Gas transport
Hemoglobin contains four globin chains, each associated with an iron-containing heme group. Each heme can bind one oxygen molecule, giving a hemoglobin molecule a maximum capacity of four oxygen molecules. Oxygen binds in the lungs and is released as blood passes through tissues. Oxygenated hemoglobin is bright red, whereas deoxygenated hemoglobin is darker red; neither form makes blood blue. (openstax.org)
Oxygen unloading responds to local chemical conditions. Increased carbon dioxide and decreased pH reduce hemoglobin’s affinity for oxygen, a relationship known as the Bohr effect. This helps couple oxygen delivery to tissue activity: metabolically active tissues produce carbon dioxide and acid, favouring oxygen release from hemoglobin. (ncbi.nlm.nih.gov)
Erythrocytes also facilitate the transport of carbon dioxide produced by tissues. Some carbon dioxide binds directly to globin, forming carbaminohemoglobin, but much is transported as bicarbonate in blood plasma. Within erythrocytes, carbonic anhydrase, an enzyme, accelerates the reversible conversion of carbon dioxide and water to carbonic acid, which dissociates into bicarbonate and hydrogen ions. The reactions reverse during passage through the lungs, permitting carbon dioxide exhalation. Hemoglobin also buffers hydrogen ions generated during this process. (ncbi.nlm.nih.gov)
Production and life cycle
Erythrocyte production, termed erythropoiesis, is part of hematopoiesis. In adults it occurs primarily in red bone marrow, where hematopoietic stem cells generate progressively specialized erythroid precursors. Developing cells accumulate hemoglobin, undergo nuclear condensation, and eventually expel their nuclei. The resulting immature cells, called reticulocytes, complete their maturation by removing residual cellular machinery. Their abundance in circulating blood provides information about recent erythrocyte production. (pmc.ncbi.nlm.nih.gov)
Production is regulated principally by erythropoietin, a hormone produced mainly by the kidneys in response to reduced oxygen availability. It promotes the survival and development of erythroid precursors. Effective production requires iron for heme synthesis and adequate folate and vitamin B12 for precursor-cell DNA synthesis. Mature erythrocytes themselves no longer perform this nuclear synthesis. (en.wikipedia.org)
Human erythrocytes circulate for approximately 120 days. Ageing or damaged cells are removed by macrophages, particularly in the spleen, liver, and bone marrow. Globin is broken down into amino acids, and much of the iron is recovered for reuse. The remaining heme structure is converted through biliverdin to bilirubin, which is subsequently processed by the liver for excretion. (openstax.org)
Blood groups and transfusion
Erythrocyte surfaces carry antigens that determine blood groups. The ABO system reflects differences in surface carbohydrate structures, while Rh antigens are associated with membrane proteins. These inherited differences are central to compatibility in blood transfusion. (ncbi.nlm.nih.gov)
If transfused erythrocytes bear antigens recognized by a recipient’s antibodies, immune reactions can destroy the cells. Maternal antibodies can also cross the placenta and attack fetal erythrocytes when particular blood-group incompatibilities occur. Blood grouping therefore concerns molecular differences in erythrocyte surfaces rather than differences in their basic oxygen-transport function. (ncbi.nlm.nih.gov)
Laboratory measurements and disorders
A complete blood count assesses erythrocyte number, hemoglobin concentration, and related measurements. Hematocrit is the proportion of blood volume occupied by erythrocytes. Red-cell indices describe average cell volume, average hemoglobin content, hemoglobin concentration within cells, and variation in cell size. These measurements characterize erythrocyte populations rather than serving as interchangeable measures of oxygen delivery. (medlineplus.gov)
Anemia reduces blood’s oxygen-carrying capacity and can arise from inadequate erythrocyte production, blood loss, or excessive cell destruction. In sickle cell disease, inherited abnormalities of hemoglobin can produce poorly deformable cells that obstruct blood flow. Conversely, an elevated hematocrit can reflect increased erythrocyte abundance or reduced plasma volume, as occurs in dehydration. (nhlbi.nih.gov)
Variation among vertebrates
The nucleus-free human erythrocyte represents the mammalian pattern, not a universal vertebrate condition. Birds, reptiles, amphibians, and most fish retain nuclei in their mature erythrocytes. These nucleated cells differ in size, shape, and cellular capabilities from mammalian red cells. Research on avian erythrocytes demonstrates substantial variation among species, including associations between cell dimensions and energetically demanding activities such as migration and diving. (pmc.ncbi.nlm.nih.gov)