Red Blood Cells
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Introduction
Each cubic milimeter of blood contains about 5 million red blood cells, giving blood its colour and consistency.
RBCs make up to 3,000 trips per day from the lungs to the tissues.
Structure of Red Blood Cells
Red blood cells are biconcave discs with a diameter of 8 µm a thickness of 2 µm, and a volume of 90 fL. RBCs lack nuclei or mitochondria.
Thirty-three perecnt of a RBC's contents are hemoglobin. Energy requirements are supplied by glucose metabolism, with 2,3-BPG being produced.
Red blood cells, lacking mitochondria, ironically use anaerobic glycolysis for ATP production.
Pyruvate is converted into lactate through action of lactate dehydrogenase (LDH).
Red blood cells use the glucose monophosphate shunt (aka the pentose phosphate pathway), relying on the activity of glucose-6-phosphate dehydrogenase. G6PD deficiency can cause hemolytic anemia due to an inability to produce reducing agents and subsequent activity of oxidizing foes.
Life Span
Production
Embryos produce RBCs in the yolk sac. The fetus produces RBCs in the liver and spleen, which stops at the time of birth.
At birth, the increase in arterial oxygen saturation results in a decrease of erythropoietin activity and erythropoiesis. Hemoglobin levels drop until 6-8 weeks.
RBCs live for about 120 days (4 weeks), being replaced at a rate of more than 2 million per second. However, normal erythropoiesis is also defined by the ability of bone marrow to respond to states of acute or chronic anemia.
In acute anemia, the marrow will respond with a 3x increase in cell proudction within 7-10 days, with a marrow E/G ration of 1:1 and a reticulocyte index of 2-3. With chronic anemia, the reticulocyte index can climb higer, to 3-8.
Red blood cells develop from committed erythroid cells (CFU-E) in the bone marrow. It takes about five days for their progeny to fully differentiate.
Red blood cell production is regulated by the hormone erythropoietin, produced by juxtaglomerular cells in the kidney. Erythroblasts become filled with hemoglobin and develop into erythrocytes in about one week, losing their nuclei and other organelles.
Iron supplies are the most important for hemoglobin production, and marrow's response to hemorrhagic or hemolytic anemia is essentially a measure of iron supply. Normally, iron stores are sufficient to support production of up to 3x normal levels.
Destruction
Old RBCs are removed from circulation by macrophages (more specific?) in the liver and spleen. Hemoglobin is dismantled into its constituent amino acids, iron, which is returned to the bone marrow; and heme, which is converted in the liver to make bilirubin.
Over 300 red blood cell antigens have been identified, with the ABO and Rh blood groups being the most important in blood typing for transfusion.
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