Blood cells have a limited life span. Red blood cells live the longest, around four months. On average, white blood cells live a few days, while blood platelets live between eight to ten days. This means that new cells have to be produced continuously. And in large quantities. In total, your body produces 350 million blood cells every minute.

New blood cells are produced in your bone marrow. The bone marrow (a collection of immature and mature cells) and the blood cells are created in your bones’ marrow cavities. In adults, this happens mainly in the thorax, ribs, vertebrae, upper arms, and skull. In children, blood cells are produced in all bones.

All the different kinds of blood cells are produced from the same stem cells in the red bone marrow. Various substances acting on the blood cells make them develop into the different kinds of mature blood cells in a process that we call hematopoiesis.

As soon as blood cells are mature, they are released into the bloodstream.

Erythropoiesis

The development of red blood cells in the red bone marrow is called erythropoiesis. During this process, proerythroblast is formed into a normoblast (a precursor to the red blood cell).

After the normoblast has expelled its nucleus, the cell enters the bloodstream as a reticulocyte. Once they are in the bloodstream, reticolucytes (immature red blood cells) develop into erythrocytes (mature red blood cells).

Leukopoiesis

The development of white blood cells is called leukopoiesis. As the precursor to the white blood cell, a myeloblast can develop into neutrophil, eosinophil, and basophil granulocytes. These white blood cells end up in the bloodstream. They will stay there, but they can also attach to the wall of a blood vessel or penetrate into the various kinds of tissue of your body.

The monoblast creates monocytes that enter the bloodstream. A number of those monocytes migrate to the spleen to be stored there. Another group remains in the bloodstream and will penetrate into body tissue a few days later, following which the monocytes develop further into macrophages.

Thrombopoiesis

The development of blood platelets is called thrombopoiesis.
A megakaryoblast (a precursor to blood platelets) can ultimately develop into a megakaryocyte, which subsequently splits up into numerous small particles, the platelets, which end up in your bloodstream.

Lymphopoiesis

The development of lymphocytes is called lymphopoiesis. Lymphoid cells do not develop from the myeoloid stem cell, but instead from the lymphoid stem cell, which can develop into lymphoblast and then into B lymphocytes, T lymphocytes, and/or Natural Killer cells.

B lymphocytes enter the bloodstream and the lymphatic system. At an early stage in their development, T lymphocytes migrate from the bone marrow to the thymus gland to mature further.
All in all, there are quite a few different steps in the production process for all these different kinds of cells. If all these steps are completed correctly, you will have a normal blood count. But if one of the steps does not go the way it should, you may develop an abnormal blood count and ultimately even develop a disease.