The hematopoietic system is formed by bodies responsible for haematopoiesis, or the production of the cellular elements of blood: red blood cells, white blood cells and platelets. During embryonic development, this function is performed mainly in the spleen, liver and bone marrow, and after the birth the production of these elements is performed mainly by the bone marrow and the lymph nodes. The cellular elements supply oxygen (red blood cells), initiate coagulation (platelets), and protect against microbes and antigens (white blood cells).
What is the hematopoietic system?
The hematopoietic organs together form a system through which the body produces red blood cells, white blood cells and platelets, that is, the so-called elements of the blood.
The red blood cells (erythrocytes) constitute the largest portion of blood cells. They have a biconcave shape and a flexible membrane, which enables the red blood cells to change shape without breaking when passing through narrow blood vessels or capillaries. The number of red blood cells varies from person to person according to age, gender, and overall health.
The white blood cells (leukocytes) are present in various forms and each type of leukocyte is somewhat essential to the immune system. These cells help the body fight infections and inflammations, and they produce antibodies.
Platelets are the third category of blood cells and they are the smallest of the blood cells. They are formed from a bone marrow cell known as the megakaryocyte. Platelets are responsible for starting the repair process of small, damaged blood vessels and produce substances that are essential to coagulation.
The main producer of these elements is the bone marrow, in particular that located inside the vertebrae, sternum, ribs and shoulder blades. Among the white blood cells, however, the lymphocytes are produced especially at the level of the lymph glands, small spherical structures localized in different parts of the body.
Some experts believe that the hematopoietic system also includes another element, the reticule endothelial tissue, distributed throughout the body.
What function does the hematopoietic system serve?
The hematopoietic system allows the body to continuously renew red blood cells, platelets and white blood cells (granulocytes, monocytes and lymphocytes) in the blood. In this way the organism can always have the elements of the blood that it needs available, even if these have a well-defined lifetime.
In fact, the red blood cells, which allow the transport of oxygen to the tissues and carbon dioxide to the lungs, have a lifespan of about 120 days, after which they must be replaced. Other elements, such as the neutrophils, live much less and after six hours they need to be replaced by new such elements. The entire population of platelets is instead replaced on average every 9.9 days.
Pluripotent stem cells are present in the bone marrow from which the precursors of the different types of blood cells can originate. Pluripotent stem cells may in fact be formed from progenitor cells of lymphoid cells and myeloid stem cells, from which there can be formed red blood cells instead, megakaryocytes, which will form the platelets, mast cells and myeloblasts, which form granulocytes (basophils, neutrophils and eosinophils) and monocytes.
Different growth factors and hormones regulate the differentiation of the cells.
