Blood While we are considering the circulatory system it is logical to look at blood. Blood is a liquid with suspended cells 30-50% by volume. These cells are of three basic types, erythrocytes or rbcs, leucocytes or wbcs and thrombocytes or platelets. Two of these three cell types are odd, in being anucleate. All originate in the bone marrow, although the number of primary stem cell types is unresolved. Erythrocytes By far the commonest blood cells (4-6m/mm3) erythrocytes are classically biconcave enucleate discs 7-8* in diameter. They are red because the cytoplasm is packed with haemoglobin which transports oxygen. They also transport carbon dioxide. Their shape is variable: they are able to deform to squeeze through capillaries. Because they lack a nucleus they have a short lifespan (c 120d) after which their components are recycled (iron) or excreted (bilirubin). Young erythrocytes - reticulocytes (1%) contain a net made up of the remains of the RNA used in haemoglobin synthesis. Leucocytes
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