Red blood cells, white blood cells and plasma are all formed inside of bones in the red bone marrow. Stem cells within the bone marrow constantly produce blood cells and work harder when the body is ill or bleeding to make up for blood cells lost.
Red Bone Marrow
At the time of birth, bones contain only red bone marrow. Slowly, as the body ages, this marrow is replaced by inactive marrow that is known as yellow bone marrow. Red bone marrow is responsible for producing and destroying blood cells. About a third of the marrow's production consists of red blood cells and the rest is white blood cells and platelets.
Blood-producing stem cells, called hematopoietic stem cells or HSCs, are located in the bone marrow. These stem cells will either become red blood cells, white blood cells or platelets. HSCs are able to renew themselves, can move into blood in circulation and can self-destruct. This process is known as hemopoiesis.
Formation of Red Blood Cells
A hemotopoietic stem cell will split during mitosis, forming an immature red blood cell. The production of red blood cells is called erythropoiesis. The stem cell divides three or four times and produces either eight or 16 cells. This takes about one week.
Formation of White Blood Cells
The process of stem cells forming white blood cells is called granulopoiesis. This process also takes place in red bone marrow. The production of these cells can take about two weeks, except in cases of infection or illness, in which the process speeds up considerably. The red marrow also contains a collection of "backup" white blood cells in case of infection.
Rate of Blood Cell Production
Blood cells do not last long and are constantly being replaced. HCSs produce 10 to 100 trillion blood cells daily. Only about one in every 100,000 cells in the bone marrow is an HCS, but their production rates are very prolific. If the body needs emergency replacements, the process speeds up. Red blood cells, the most numerous cells in the blood are formed at a rate of about 2 million each second. White blood cells and platelets only make up about 1 percent of the blood and are produced at a slower rate.