Most of the bones in our body are hollow.  They have an outer hard shell, or casing, which is what we think of as the bone, but inside is a soft spongy material, which is the bone marrow.  The bones that make up our pelvis are particularly rich in bone marrow.

The bone marrow produces the cells (stem cells) that develop into the different types of blood cells: red cells, white cells and platelets. 
The usual arrangement for donating bone marrow for a transplant goes like this: about a week or two before the bone marrow is taken, the donor may have 1-2 pints of blood taken. This will be given back to them when the bone marrow is collected.

The collection of bone marrow is carried out under a general anaesthetic, or an anaesthetic given into the spinal cord, so it is painless. It involves the removal of some marrow from inside the bones at the back and front of the hip bones (the pelvis) or, sometimes from the breastbone (sternum).

The doctor will insert a special needle through the skin and into the bone. The marrow, which is quite liquid, is then sucked out into a syringe. The bone marrow is taken from a number of different places over the pelvis, usually through several small punctures, which quickly heal. It is then stored until it is needed. There may be some bruising for a few weeks afterwards.

Approximately one litre of bone marrow will be removed in this way from an adult - about 10-15% of the body's total. This leaves plenty for the donor's needs.

Giving bone marrow in this way normally means a stay in hospital overnight to recover from the general anaesthetic. Usually there will some soreness for a few days afterwards, but mild painkillers, like paracetamol, will ease this.

So donating bone marrow does not involve the removal of any pieces of the bones themselves.