Inside a bone
If you could peek inside a bone, you'd see layers, a bit like a tree trunk.
On the outside is a thin skin with nerves and blood vessels. Under that is compact bone, which is hard and solid. It makes a strong outer wall.
Inside that is spongy bone. It's still hard, but it's full of tiny holes, like a honeycomb. That makes it strong but light.
And in the middle of long bones, and in all the little holes of the spongy bone, is the soft marrow.
Red marrow, the blood factory
Red marrow makes all your new blood cells. It makes red blood cells, which carry oxygen. It makes white blood cells, which fight germs. And it makes platelets, which help a cut stop bleeding.
It works fast! Your body makes about 2 million new red blood cells every second.
Why so many? Each red blood cell lasts about 4 months. Then it wears out, and a new one from your marrow takes its place.
Yellow marrow, the store
Yellow marrow is mostly fat. It is kept in the middle of long bones, like the bones in your arms and legs.
A new baby's marrow is almost all red, making blood as fast as the baby grows. As you get older, much of the marrow in your arms and legs slowly turns yellow.
In grown-ups, red marrow is mostly in flat bones like the hips, ribs, breastbone, skull and backbone, and in the ends of long bones.
If your body ever needs a lot more blood, some yellow marrow can turn back into red marrow and get to work.
Stem cells, the seeds of blood
Red marrow is full of special cells called stem cells. A stem cell is like a seed. It can grow into any kind of blood cell your body needs.
When you have a cold, your marrow can make extra white blood cells to fight the germs.
A gift that helps others
Sometimes a person is sick and their marrow can't make healthy blood. A healthy grown-up can give some of their own blood stem cells to help. This is called a donation, and it is a very kind gift.
Doctors take good care of the person who gives. Afterward, the giver's body makes new stem cells to replace the ones they gave.
Grown-ups can sign up to be donors. Kids can help by learning about it and sharing what they know with their families.