Erythrocytes (red blood cells)
Erythrocytes are red cells without a nucleus, shaped like biconcave discs; this shape enlarges the surface and improves gas exchange. A cubic millimetre of blood has about 4–6 million erythrocytes; they live about 120 days, are made in red bone marrow and destroyed in the liver and spleen. The red colour is due to haemoglobin: it contains iron, binds oxygen in the lungs forming oxyhaemoglobin, releases oxygen in tissues and helps carry carbon dioxide. A fall in red cells or haemoglobin is anaemia; if there is tiredness and pale skin see a doctor, and complete food (meat, legumes, greens) and fresh air help. Blood groups depend on A and B antigens on the erythrocytes: group I (O) – no antigen, II (A), III (B), IV (AB). Only doctors perform transfusions, matching group and Rh factor: group I can give to others and group IV can receive from all, but in practice the same group is chosen. The Rh factor is another protein on erythrocytes; if present the person is Rh-positive (Rh+), if absent Rh-negative (Rh−); an Rh-negative person is given Rh-negative blood.
If you know your family’s blood groups, ask adults and make a table (do not test blood yourself). Show with a picture which antigen each group has.