Bone marrow is the spongy tissue inside some of your bones, such as your hip and thigh bones. It contains stem cells. These are early blood cells that develop into red blood cells, which carry oxygen around your body, white blood cells, which fight infection, and platelets, which help with blood clotting.
You have two types of bone marrow: red and yellow. Red bone marrow is mainly found in the flat bones of your skeleton, such as your breastbone, ribs, pelvis (hip bone), shoulder blades and upper legs. Yellow bone marrow is found in the hollow shafts of long bones, such as those in your arms and legs.
Most people have about 2 thirds red bone marrow and 1 third yellow bone marrow. However, if you need treatment for certain conditions – such as leukaemia – doctors may need to take only red bone marrow from you.
Your bone marrow makes a constant supply of new blood cells throughout your life. For example:
* Every second, our bodies produce millions of new red blood cells in the red bone marrow to replace those that die or are lost from bleeding.
* White blood cell production increases when we have an infection so that there are enough to fight it off.
At birth all our bones contain red bone marrow but as we get older some gradually change to yellow bone marrow (a process called ‘replacement’). This usually starts at around 6 months old and is complete by the time we reach adulthood. In adults most yellow bone marrow is turned back to red when needed – for example during pregnancy or illness when extra blood cells are required.