Inside the flat bones is the bone marrow, where all the blood cells are produced. There are three types of cell: red blood cells (erythrocytes), white blood cells (leukocytes) and platelets (thrombocytes).
Red blood cells contain haemoglobin, which gives them their characteristic red colour. Haemoglobin is important for the transport of oxygen from the lungs to the body tissues and for returning carbon dioxide from the periphery back to the lungs.
White blood cells form part of the immune system’s defences and protect the body against infection by viruses, bacteria, fungi. They can be divided into granulocytes and lymphocytes.
Platelets help to stop bleeding.
The bone marrow constantly produces new blood cells, which enter the bloodstream once they have matured. They only have a limited lifespan: erythrocytes have a lifespan of about four months, leukocytes only a few hours and platelets a few days.
Blood cell development (simplified)
Normally, regulatory mechanisms produce as many new blood cells as the body needs. This means that the production of red cells can be increased if their number is reduced, for example due to anaemia or bleeding. If the body needs to fight an infection, the bone marrow produces more white blood cells.
The bone marrow contains a very small number of blood stem cells from which new blood cells are developed. Not only are fully mature cells produced, but new stem cells are constantly produced to maintain this ongoing cycle of cell generation.
If the stem cells are not damaged and the environment of the stem cells (the ‘stem cell niche’) is intact, normal production of healthy blood cells and stem cells can take place. But if either of these two components is defective, cell production is disrupted and there may even be complete bone marrow failure.
If cell production in the bone marrow is disrupted, a sufficient number of blood cells cannot be produced (bone marrow hypoplasia). Depending on whether the production of erythrocytes, leukocytes or thrombocytes is disrupted, symptoms such as pallor, a tendency to infection or bleeding can occur. If development of all three cell types is disrupted (pancytopenia), this is referred to as bone marrow failure (bone marrow aplasia).