Red blood cell

Image:Redbloodcells.jpg Red blood cells are the most common type of blood celland are the vertebratebody's principal means of delivering oxygenfrom the lungsor gillsto body tissues via the blood.

Red blood cells are also known as RBCs or erythrocytes (from Greekerythros for "red" and kytos for "hollow", with cyte nowadays translated as "cell"). A schistocyte is a red blood cell undergoing fragmentation, or a fragmented part of a red blood cell.

Vertebrate erythrocytes

Erythrocytes consist mainly of hemoglobin, a complex molecule containing hemegroups whose ironmolecules temporarily link to oxygen molecules in the lungs or gills and release them throughout the body. Hemoglobin also carries some of the waste product carbon dioxideback from the tissues. (In humans, less than 2% 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 musclecells.

The color of erythrocytes is due to the heme group of hemoglobin. The blood plasma is straw-colored alone, but the red blood cells change colors due to the state of the hemoglobin: when combined with oxygen the resulting oxyhemoglobin is scarlet and when oxygen has been released, the resulting deoxyhemoglobin is darker, appearing bluish through the blood vessel walls.

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 viscousblood and longer transport ways of oxygen.

Mammalian erythrocytes

Erythrocytes in mammalsare anucleate when mature, meaning that they don't have a cell nucleusand thus no DNA. In comparison, the erythrocytes of nearly all other vertebrateshave nuclei; the only known exception is salamandersof the Batrachoseps genus. Mammalian erythrocytes also lose their other organellesincluding their mitochondriaand produce energy by fermentation, via glycolysisof glucosefollowed by lactic acidproduction. Like most cell types, red cells do not have an insulin receptorand thus glucose uptake is not regulated by insulin.

Mammalian erythrocytes are biconcavedisks: flattened and depressed in the center, with a dumb-bell shaped cross section. This shape (as well as the loss of organelles) optimizes the 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 camelfamily 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 elevated, as for instance during inflammation.

The spleenacts as a reservoir of red blood cells, but this effect is somewhat limited in humans. In some other mammals such as dogsand 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 disk is 6–8 µm; they are thus much smaller than most other human cells. A typical erythrocyte contains about 270 million hemoglobin molecules, with each carrying four heme groups.

Adult humans have roughly 2–3 × 10¹³red blood cells at any given time (women have about 4 million to 5 million erythrocytes per cubic millimeter (microliter) of blood and men about 5 million to 6 million; people living at high altitudes with low oxygen tension 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 150,000–400,000 plateletsin 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 marrowof large bones. (In the embryo, the liveris the main site of red blood cell production.) The production can be stimulated by the hormoneerythropoietin(EPO), 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 their 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 typesof humans are due to variations in surface glycoproteinsof erythrocytes.

Red blood cells can be separated from blood plasmaby 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 −78 °C.) This practice is hard to detect but may endanger the human cardiovascular systemwhich is not equipped to deal with blood of the resulting higher viscosity.

Diseases and diagnostic tools

Image:Sicklecells.jpg

Blood diseasesinvolving 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 anemiais the most common anemia; it occurs when the dietary intake or absorption of ironis insufficient, and hemoglobin, which contains iron, cannot be formed
  • Sickle-cell diseaseis a genetic disease that results in abnormal hemoglobin molecules. When these release their oxygen load in the tissues, they become insoluble, leading to mis-shaped red blood cells. These sickle shaped red cells are rigid and cause blood vessel blockage, pain, strokes, and other tissue damage.
  • Thalassemiais a genetic disease that results in the production of an abnormal ratio of hemoglobin subunits.
  • Spherocytosisis 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 anemiais an autoimmune diseasewherein the body lacks intrinsic factor, required to absorb vitamin B12from food. Vitamin B12 is needed for the production of hemoglobin.
  • Aplastic anemiais caused by the inability of the bone marrowto produce blood cells.
  • Hemolysisis the general term for excessive breakdown of red blood cells. It can have several causes.

• The malariaparasite spends part of its life-cycle in red blood cells, feeds on their hemoglobin and then breaks them apart, causing fever. Both sickle-cell diseaseand thalassemiaare more common in malaria areas, because these mutations convey some protection against the parasite.
• Polycythemias(or erythrocytoses) are diseases characterized by a surplus of red blood cells. The increased viscosity of the blood can cause a number of symptoms.
  • In polycythemia verathe increased number of red blood cells results from an abnormality in the bone marrow.
• Several microangiopathic diseases, including disseminated intravascular coagulationand thrombotic microangiopathies, present with pathognomic (diagnostic) RBC fragments called schistocytes. These pathologies generate fibrinstrands that sever RBCs as they try to move past a thrombus.

Several blood testsinvolve 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 typeneeds to be determined to prepare for a blood transfusionor an organ transplantation.

History

In 1658, the DutchJan Swammerdamwas the first to describe red blood cells; he had used an early microscope.

External links

• Database of vertebrate erythrocyte sizes

cs:?ervená krvinka de:Erythrozyt es:Eritrocito eo:Eritrocito fr:Érythrocyte ko:??? ia:Erythrocyto it:Globulo rosso he:?? ?? ???? lt:Raudonasis kraujo k?nelis mk:????????? nl:Rode bloedcel nn:Raud blodlekam ja:??? pl:Erytrocyt ru:?????????? sr:?????? ????? ????? fi:Punasolu sv:Röda blodkroppar ta:??????? ???????? vi:H?ng c?u tr:Eritrosit zh:???

This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Red+blood+cell Wikipedia article Red blood cell.