How blood is formed (Haemopoeisis, Haematopoeisis)

How blood is formed (Haemopoeisis, Haematopoeisis)


Blood is a fluid that maintains life through constant circulation availing the body with required nutrients, oxygen and waste removal

Blood is composed of cells and proteins making it thicker than pure water. The average person has about 5 liters of blood. A yellowish liquid called plasma makes up about half of the contents of blood. Plasma contains proteins that help blood to clot (Clotting factors) , transport substances through the blood, and perform other functions. Blood plasma also contains glucose and other dissolved nutrients.

About half of blood volume is composed of blood cells and these are Erythrocytes which carry oxygen to the tissues leukocytes which fight infection and Platelets the smaller cells that aid in blood coagulation

Blood formation (haemopoeisis)

Haemopoiesis or Blood cell formation is recurring body  process by which the cellular components of blood are replenished as required by the body

Blood cells do not come from the bloodstream itself but in particular blood-forming organs, that is the marrow of certain bones. The bone marrow makes all of the red blood cells, 60–70 percent of the white cells in an adult human. (i.e., the granulocytes), and all of the platelets. The lymphatic tissues, particularly the thymus, the spleen, and the lymph nodes, produce the lymphocytes (comprising 20–30 percent of the white cells). The reticuloendothelial tissues of the spleen, liver, lymph nodes, and other organs produce the monocytes (4–8 percent of the white cells). The platelets, which are small cellular fragments rather than complete cells, are formed from bits of the cytoplasm of the giant cells (megakaryocytes) of the bone marrow.

Blood formation in human embryo first happens is the yolk sac. Later in embryonic life, the liver  dominates red blood cell-forming organ, then soon taken over by the bone marrow, which in adult life is the only source of both red blood cells and the granulocytes. The RBCs and WBCs arise through a series of complicated, gradual, and successive transformations from primitive stem cells, which are able to form any of the precursors of a blood cell. Precursor cells are stem cells that have developed to the stage where they are determined to forming a particular kind of new blood cell.

Formation of Red Blood Cells (Erythropoiesis)

Formation of RBC (erythropoiesis) happens in the bone marrow under the influence of the hormone erythropoietin (EPO).

The Juxtaglomerular cells of the kidney are responsible for the production of erythropoietin in response to decreased oxygen supply to body organs and tissues (anemia and hypoxia) or higher levels of androgens. In Red blood cell production doesn’t Only require Erythropoiestin, it also requires adequate supply of the following:


vitamin B12,



If erythropoiesis is deemed insufficient in the bone marrow, this can call on extramedullary haematopoiesis that haematopoiesis occurring outside the marrow. This is usually observed in haemoglobulinopathies, in particular thalassaemias and myelofibrosis.

Process of RBC production

Formation of all cellular components of  blood begins with the Haemocytoblast or Haematopoietic stem cells (HSCs), This is a multipotent haematopoietic stem cell. Haemocytoblasts are powerful enough to self-renew any adult cell. They are situated in the bone marrow and can be mobilised into the circulating blood when needed.

Some haemocytoblasts differentiate into common myeloid progenitor cells, which go on to produce erythrocytes, as well as mast cells, megakaryocytes and myeloblasts.

The process of RBC formation involves a several stages, they first become normoblasts (eryhthroblasts), which are normally seen the bone marrow only, they then lose some organelles and their nucleus as they progress into reticulocytes, which can be thought of as immature RBCs. Some of the reticulocytes sent into the peripheral circulation. At last, the reticulocytes lose their remaining organelles as they mature into erythrocytes, which are fully mature RBCs. These normally survive for around 120 days.

While this maturation process happens, there is nuclear extrusion. mature erythrocytes have no nucleus.

Nucleated RBCs present in a sample of bone marrow can indicates the release of incompletely developed cells. This can occur in pathological conditions like


Severe anaemia

Haematological malignancy.

Relevance of RBC production

Underproduction of Red Blood Cells results onto anaemia. Anaemia is caused by low RBC production, or increased RBC removal. .

Reduced red cell production may be due to 3 main things:

Not enough building blocks for production of RBCs, the building blocks include iron, folate or B12 deficiency.

Failure of the stimulus  forexample EPO deficiency secondary to chronic kidney disease.

Bone marrow failure, forexample,  aplastic anaemia.

Overproduction of Red Blood Cells occurs in conditions such as polycythaemia rubra vera. It is a myeloproliferative disease which results from dysregulation at the level of the haematopoietic stem cell.

Formation of white blood cells (Leukocyte Haematopoiesis)

Haemocytoblasts or Haematopoietic stem cells (HSCs) which are found in the bone marrow and have that special ability to give rise to all mature blood cellular components through differentiation into other progenitor cells. They are self-renewing. When they proliferate, at least some daughter cells remain Haemocytoblasts , so the pool of stem cells does not become depleted over time.

Two non similar leukocyte lineages and two non-leukocyte lineages come from the progeny of Haemocytoblast. Following this split in differentiation, the subdivisions undergo differentiation into terminally-differentiated leukocytes, which typically do not divide independently. The lymphocyte lineage come from common lymphoid progenitor cells, which in turn become lymphoblasts before differentiating into T cells, B cells, and NK cells. The myelocytes are an offshoot of common myeloid progenitor cells, which also differentiate into the erythropoietic and magakaryotic progenitors. This diverse group differentiates into granulocytes and monocytes. Monocytes further differentiate into macrophages or dendritic cells upon reaching certain tissues.

Formation of platelets

Platelets are made in the bone marrow, the same as the RBCs cells and most of the WBCs.  Platelets are made from very big bone marrow cells called megakaryocytes.  As megakaryocytes develop into giant cells, they fragment and this gives rise to the ghe release of more 1,000 platelets per each megakaryocyte.  The hormone controlling megakaryocyte formation is thrombopoietin (often abbreviated as TPO).

Formation of Plasma

It constitutes 55% of total blood, it’s amazing in a way that  no organ produces it. Instead, it is formed from water and salts absorbed through the digestive tract. However plasma is made up of proteins that have distinct organs that produce them based on an individual’s stage of development.

In Embryo

In the embryo, the mesenchymal cells are in charge plasma cell production. The first protein to be synthesized is albumin, followed by globulin and the other plasma proteins.

In Adults

In adults the reticuloendothelial cells of the liver are responsible for plasma protein synthesis. The bone marrow, degenerating blood cells, general body tissue cells, and the spleen also contribute to the formation of plasma proteins. Gamma globulins originate from B lymphocytes, which in turn form immunoglobulins.

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