Physiology Topics
Red blood cells (erythrocytes) are destroyed and replenished at a rate of about 2 million per second. Myeloid cells become proerythroblasts in the bone marrow. Proerythroblast have large endoplasmic reticulum and many ribosomes and mitochondria for the generation of hemoglobin. After it is full of hemoglobin, the nucleus is expelled and the cell becomes a reticulocyte. Reticulocytes pass to the blood stream and develop into erythrocytes in 1-2 days.
The heme portion of hemoglobin is made of Fe+3 and the photopigment biliverdin. Two-thirds of the body iron is in red blood cells as part of hemoglobin. Dietary iron ad recycled iron from erythrocyte degradation (Fe+3) is transported in the blood by the plasma protein transferrin. Iron is stored in muscle, liver, and spleen bound to the proteins ferritin and hemosiderin.
Vitamin B12 is necessary for the formation of hemoglobin. Intrinsic factor allows for the absorption of vitamin B12, then complexes with it to form erythrocyte maturation factor.
Red blood cell formation is stimulated by erythropoietin, a hormone released by the kidney in response to low oxygen levels in blood. Erythropoietin stimulates the production of proerythroblasts and accelerates the maturation of erythrocytes. Its secretion is stimulated by high altitude, blood loss and exercise, and it must be replaced in patients with kidney failure.
Erythropoietin abuse among athletes is done to increase performance by increasing red blood cells and oxygenation of muscle. The blood becomes very viscous, and couple with profuse sweating during events may lead to high blood pressure and heart failure. Blood doping, the removal and storage of blood cells to be reinjected a few days before an event, has the same effect as erythropoietin abuse.
Old red blood cells are phagocytized and broken down into amino acids (mostly from globin), and heme for reuse. Iron is removed from heme and biliverdin is converted to bilirubin and transported to the liver, where it is conjugated to glucoronic acid and excreted in bile. Bilirubin is toxic and if not excreted promptly will cause jaundice. Since it is a photosensitive molecule, newborns with high bilirubin levels (undeveloped liver function) are put under UV light to degrade the toxin.
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