Looking at the Bone Marrow
Most blood cells are made in the bone marrow, a spongy material that fills the center of your bones. These blood cells then circulate in a fluid known as plasma.
Your bone marrow contains important information about blood formation, so it may be monitored by your health care provider. There are two ways this is done: by taking a liquid sample of the bone marrow through a needle, known as bone marrow aspiration, or by removing a small, intact piece of the marrow known as a bone marrow biopsy.