Ruben Scarlet-McRea of Kaikorai Primary School asks :-

How does the body makes new blood cells?

Jim Faed, a haematologist and transfusion medicine specialist at the University of Otago, responded.

Most blood cells have a very short life span. The body continually produces many of these cells each day just to maintain normal numbers in blood. Production of blood cells is controlled by proteins called cytokines; they are like a set of signals which can turn production up or down.

In the case of red blood cells, control of production is managed by the kidney. The kidneys are very sensitive to the amount of oxygen being delivered by blood cells. If bleeding has been severe and the person has lost so many red cells that oxygen delivery is too low, the kidney increases production of a cytokine called erythropoietin. This protein signals the bone marrow to make even more red cells and these are delivered into the blood. Later, when red cell numbers in blood and oxygen delivery to tissues return to normal, production of erythropoietin is reduced to normal levels again.

Similar controls are used to increase production of white cells and platelets but in this case many different signal proteins are involved. The main signal for increasing platelet production is called thrombopoietin. Two of the important signal proteins for making white blood cells are called G-CSF and Interleukin 3. Some of these cytokines are now available as medicines but they are very expensive. They can be used to help boost production in some severe conditions when the bone marrow may not be making enough cells. They are not needed by people who have a healthy bone marrow and can turn up production of these cells when needed.