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Production of red blood cells occurs in the red bone marrow, and is stimulated by erythropoietin, EPO. EPO is secreted predominantly by the kidneys. The kidneys sense oxygen levels in the blood and adjust EPO secretion accordingly to the body’s needs.
Red cells live about 100 to 120 days. With age, the cells lose their elasticity. Without protein synthesis, they are unable to repair themselves. Worn-out red cells are detected in the spleen, which serves as a quality control center. The spleen has a network of very narrow channels which test the agility of erythrocytes. Healthy cells can bend and fold to squeeze through, while old cells, being rigid and fragile, get stuck and are destroyed by macrophages. Parts of the dead cells are salvaged to make new cells. Part of the heme is secreted into bile and disposed in feces.
The number of red blood cells is strictly regulated and has important clinical significance. Common measurements include red blood cell count, hematocrit, and hemoglobin concentration.
An imbalance between the rate of red cell production and death can result in their deficiency, known as anemia, or excess, known as polycythemia.
Anemia can be caused by blood loss, insufficient erythrocyte production, or their premature destruction.
Insufficient red cell production can result from:
+ deficiency of any of the nutrients that are required for their formation,
+ impaired kidney function, which leads to lower secretion of EPO,
+ or destruction of the bone marrow tissue responsible for red cell production. This can happen because of inherited mutations, autoimmune diseases, or exposure to chemicals, drugs or radiation; but causes are unknown for many cases. Reduced erythropoiesis is known as hypoplastic anemia, while complete cessation of red cell production is called aplastic anemia.
Inappropriate destruction of red blood cells, also called hemolytic anemia, can be inherited or acquired. The inherited forms are usually due to defects within red cells themselves, such as abnormalities in hemoglobin structure, while acquired hemolytic anemia can be caused by toxins, drugs, autoimmune diseases, infection, overactive spleen, or blood group mismatch.
Anemia results in low oxygen levels in the blood, known as hypoxemia. Mild anemia causes weakness and confusion, while severe anemia may lead to organ failure due to lack of oxygen and is life-threatening.
Excess red cell production, or polycythemia, can be primary or secondary. Primary polycythemia, or polycythemia vera, is a form of blood cancer, where the bone marrow produces too many blood cells. Secondary polycythemia, on the other hand, is a consequence of low oxygen state, which induces the kidneys to produce more erythropoietin, subsequently leading to more erythrocytes. Causes include smoking, air pollution, emphysema, living at high altitudes, and physical strenuous conditioning in athletes.
Excess red cells may increase blood volume, blood pressure, and viscosity. This augments the risks for blood clot formation, which may lead to heart attacks, strokes, and pulmonary embolism. The heart also has to work harder to manage larger amount of thicker blood and heart failures may result.
