Red blood cells
Red blood cells (erythrocytes) provide the system for oxygen delivery from the lungs to the tissues, and evolution has produced a very specialised cell for this purpose. The erythrocyte lacks organelles and a nucleus and is no more than a membrane enclosing a solution of protein and electrolytes. Over 95% of the protein content is the oxygen transport protein haemoglobin, the remainder being enzymes required to maintain haemoglobin in a functional, reduced state and enzymes for glycolysis. The biconcave shape of erythrocytes increases the surface-area-to-volume ratio, making gas exchange more efficient, and also makes the cell more deformable (compared with a sphere) and therefore more able to navigate the microvasculature. Under normal circumstances the average survival of erythrocytes is 120 days.
Haemopoiesis starts in the yolk sac in the 2-week-old embryo. At 6 weeks the liver, and to a lesser extent the spleen, start to produce haemopoietic cells and by 12–16 weeks the liver is the main haemopoietic tissue. The bone marrow starts to produce blood cells at 20 weeks. At birth haemopoietic (red) marrow occupies all bones. Fat gradually replaces the red marrow until in adults red marrow is confined to the axial skeleton only.
Manufacture of erythrocytes (erythropoiesis) occurs predominantly in the bone marrow from the seventh month of gestation. A homeostatic mechanism ensures that under physiological conditions the rate of production equals the rate of destruction, but at the same time there is a capability to respond to demands such as hypoxia, haemorrhage or haemolysis by increasing production.