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Red blood cells are the most common type of blood cell and are vertebrate body's principal means of delivering oxygen from the lungs or gills to body tissues via the blood.Red blood cells are also known as RBCs erythrocytes from Greek erythros for "red" and kytos for "hollow", nowadays translated as "cellschistocyte red blood cell undergoing or a fragmented part of a red blood cell.

Vertebrate erythrocytes
Erythrocytes consist mainly of hemoglobin, a complex molecule containing heme groups whose iron molecules temporarily link to oxygen molecules in the lungs or gills and release them throughout Hemoglobin also carries some of the waste product carbon dioxide back from the tissues. (In humans, less than of the total oxygen, and most of the carbon dioxide, are held in solution in the blood plasma). A related compound, myoglobin, acts to store oxygen in muscle cells.The color of erythrocytes is to the heme group of hemoglobin. Individual RBCs are straw-colored, but in the aggregate they appear bright-red if the hemoglobin is bound to oxygen, and bluish-purple if not.The keeping of oxygen-binding proteins in cells rather than having them dissolved in body fluid was an important step in the evolution of vertebrates; it allows for less viscous blood and longer transport ways of oxygen.

Mammalian erythrocytes
Erythrocytes in mammals are anucleate when mature, meaning that they don't have a cell nucleus and thus no DNA. The erythrocyts of nearly all other vertebrates have nuclei; the only known exception are salamanders of the Batrachoseps genus.Erythrocytes also lose their other organelles including their mitochondria and produce energy by fermentation, via glycolysis of glucose followed by lactic acid production. Like most cell types, red cells do not have an insulin receptor and thus glucose uptake is not regulated by insulin.ammalian erythrocytes have a biconcave shape: flattened and depressed in the center. This shape optimizes cell for the exchange of oxygen with its surroundings. The cells are flexible so as to fit through tiny capillaries, where they release their oxygen load. Erythrocytes are circular, except in the camel family Camelidae, where they are oval.In large blood vessels, red blood cells sometimes occur as a stack, flat side next to flat side. This is known as rouleaux formation, and it occurs more often if the levels of certain serum proteins are elevatance during inflammation.The spleen acts as a of red blood cells, but this effect is somewhat limited in humans. In some other mammals such as dogs and horses, the spleen sequesters large numbers of red blood cells that are dumped into the blood during times of exertion stress, yielding a higher oxygen transport capacity.

Human erythrocytes
The diameter of a typical human erythrocyte is ; they are thus much smaller than most other human cells. A typical erythrocyte contains about million hemoglobin molecules, with each carrying four heme groups.Adult humans have roughly red blood cells at any given time (women have about 4 million to million erythrocytes per cubic millimeter of blood ad men about 5 million to people living at high altitudes with low oxygen concentration will have more. Red blood cells are thus much more common than the other blood particles: there are about 4,000–11,000 white blood cells and about platelets in a cubic millimeter of human blood. The red blood cells store collectively about 3.5 grams of iron; that's more than five times the iron stored by all the other tissues combined.

The process by which red blood cells are produced is called erythropoiesis. Erythrocytes are continuously being produced in the red bone marrow of large bones. In the embryo, the liver is the main site of red blood cell production The production can be stimulated by the hormone erythropoietin , which is used for doping in sports. Erythrocytes develop in about 7 days and live a total of about 120 days. The aging cells swell up to a sphere-like shape and are engulfed by phagocytes, destroyed and t materials are released into the blood. The main sites of destruction are the liver and the spleen. The heme constituent of hemoglobin is eventually excreted as bilirubin.The blood types of humans due to variations in surface glycoproteins of erythrocytes.

Red blood cells can be separated from blood plasma by centrifugation. During plasma donation, the red blood cells are pumped back into the body right away, and the plasma is collected. Some athletes have tried to improve their performance by doping their blood: first about 1 liter of their blood is extracted, then the red blood cells are isolated, frozen and stored, to be reinjected shortly before the competition. (Red blood cells can be conserved for 5 weeks at This practice is hard to detect but may endanger the human cardiovascular system which is not equipped to deal with blood of the resulting higher viscosity.

Diseases and diagnostic tools
Affected by Sickle-cell disease, red blood cells alter shape and threaten to damage internal organs.Blood diseases involving the red blood cells include:

Anemias or anaemias are diseases characterized by low oxygen transport capacity of the blood, because of low red cell count or some abnormality of the red blood cells or the hemoglobin. Iron deficiency anemia is the most common anemia; it occurs when the dietary intake or absorption of iron is insufficient, and hemoglobin, which contains iron, cannot be formed
Sicklecell disease is a genetic disease which leads to mis-shaped red blood cells.
Thalassemia is a genetic disease that results in the of abnormal hemoglobin molecules.
Spherocytosis is a genetic disease that causes a defect in the red blood cell's cytoskeleton, causing the RBCs to be small, sphere-shaped, and fragile instead of donut-shaped and flexible.
Pernicious anemia is an autoimmune disease where the body lacks intrinsic factor, required to absorb vitamin B12 from food. Vitamin B12 is needed for the production of hemoglobin.
Aplastic anemia is caused by the inability of the bone marrow to produce blood cells.
Hemolysis is the general term for excessive breakdown of red blood cells. It can have several causes.
The malaria parasite spends part of its life-cycle in red blood cells, feeds on their hemoglobin and then breaks them apart, causing fever. Both sickle-cell disease and thalassemia are more common in malaria areas, because these mutations convey some protection against the parasite.
Polycythemiasre diseases characterized by a surplus of red blood cells. The increased viscosity of the blood can cause a number of symptoms.
In polycythemia vera the increased number of red blood cells results from an abnormality in the bone marrow.
Several blood tests involve red blood cells, including the RBC count the number of red blood cells per volume of blood) and the hematocrit (percentage of blood volume occupied by red blood cells The blood type needs to be determined to prepare for a blood transfusion or an organ transplantation.

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