1/6/07

Erythrocytes (Red Blood Cells)

Red Blood Cells

Source: Wikipedia

Also known as erythrocytes (from Greek erythros for "red" and kytos for "hollow", with cyte nowaday translated as "cell"). Primary function of transporting gases and other metabolites. Principle gases carried: oxygen (lungs to tissues), and carbon dioxide (tissues to lungs).

Erythrocytes consist mainly of hemoglobin, which is a conplex molecule containing heme groups (which contain iron atoms at their center - thus making blood appear red). The iron component of hemoglobin temporarily link to oxygen molecules in the lungs or gills. Oxygen can easily diffuse through the red blood cell's membrane. Hemoglobin also carries Carbox Dioxide (return waste product) back from the tissues. In mammals, erythrocytes are anucleate which means that they do not have a cell nucleus, and this no DNA (in comparison, all other vertebrates have nuclei, except for salamanders). Erythrocytes do not have many other organelles either, and thus lack the ability of repair giving them very small life spans.

The diameter of the typical erythrocyte is 6-8 ┬Ám (smaller than most other cells). A typical erythrocyte contains about 270 million hemoglobin molecules, which each contain 4 heme groups.

Women have 4-5 million erythrocytes per cubic milliliter (micrometer), and men have about 5-6 million. Collectively, red blood cells store 3.5 grams of iron.

The process by which red blood cells are made is called erythropoeisis. Erythrocytes are continuously being produced in the red bone marrow of large bones, at a rate of about 2 million per second. (In the embryo, the liver is the main site of red blood cell production.) The production can be stimulated by the hormone erythropoietin (EPO), which is used for doping in sports (because more erythrocytes = more capacity to store oxygen).

After leaving the bone marrow, they are known as reticulocytes which comprise about 1% of circulating red blood cells (they are called reticulocytes because of the reticular, or mesh-like, network of ribosomal RNA visible when stained). The number of reticulocytes is a good indicator of bone marrow activity, because it represents recent production. This means that the reticulocyte count, and the reticulocyte production index that can be calculated from it, can be used to determine whether a production problem is contributing to the anaemia, and can also be used to monitor the progress of treatment for anaemia.

Erythrocytes develop from stem cells through reticuloctyes to mature erythrocytes 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 types of humans are 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.

Other than taking EPO, 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 system which is not equipped to deal with blood of the resulting higher viscosity.

Importance in disease:

Sickle Cell Anaemia

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