Blood Cell Formation

Blood is a body fluid in humans and other animals that transports metabolic waste products away from cells while delivering necessary substances such as nutrients and oxygen to them. It is made up of blood cells suspended in blood plasma in vertebrates.

Components of Blood

Blood accounts for 7% of human body weight, with an average density of around 1060 kg/m³, which is very close to the density of pure water, which is 1000 kg/m³. 

The average adult's blood volume is approximately 5 litres (11 US pt) or 1.3 gallons, and it is made up of plasma and formed elements. 

The formed elements are two types of blood cells or corpuscles: red blood cells (erythrocytes) and white blood cells (leukocytes), as well as platelets, which are involved in clotting. 

Red blood cells account for approximately 45% of total blood volume, plasma for approximately 54.3%, and white cells for approximately 0.7%.

Plasma -

• Plasma accounts for 55% of total blood volume. It is 90% water, salts, lipids, and hormones, with a high concentration of proteins (including its main protein albumin), immunoglobulins, clotting factors, and fibrinogen.

• Plasma serves several functions, including transporting blood cells and nutrients, regulating the body's water and mineral salts, irrigating tissues, fighting infections, and coagulating blood.

White Blood Cells -

• Each cubic millimetre of blood contains between 6,000 and 8,000 white cells. Leukocytes are white cells that are slightly larger than red cells. They cleanse the body and protect it from infection. When an infection is detected in any part of the body, the immune system mobilises to fight it.

Red Blood Cells -

• A pinhead-sized drop of blood contains approximately 5 million red blood cells (erythrocytes). They are small biconcave discs without a nucleus that get their red colour from haemoglobin, an iron-containing protein.

• Women's blood volume is made up of between 37 and 43%  red cells, while men's blood volume is made up of between 43% and 49% red cells. Red blood cells are responsible for transporting oxygen throughout the body.

Platelets -

• Platelets, also known as thrombocytes, are much smaller than red and white blood cells. Platelets aid in blood clotting and wound healing. When a blood vessel ruptures, platelets join forces with fibrin, which is derived from fibrinogen, to form a clot.

Blood Cell Formation

Hemopoiesis (hematopoiesis) is the process by which blood elements are formed. Hemopoiesis occurs in the red bone marrow of the epiphyses of long bones (such as the humerus and femur), flat bones (such as the ribs and cranial bones), vertebrae, and the pelvis. 

Hemopoietic stem cells (hemocytoblasts) divide within the red bone marrow to produce various “blast” cells. Each of these cells matures into a distinct formed element.

The Role of Hematopoietic Stem Cells: 

HSCs are self-renewing cells. When they multiply, at least some of their daughter cells remain as HSCs, ensuring that the pool of stem cells is not depleted. 

Myeloid and lymphoid progenitor cells, the other daughters of HSCs, can commit to any of the alternative differentiation pathways that lead to the production of one or more specific types of blood cells, but they cannot self-renew. 

This is one of the body's most important processes. Hematopoiesis is the process by which different blood cells develop from HSCs to mature cells.

Blood Cells are of Three Types-

1. Erythrocytes are red blood cells that carry oxygen. They are derived from common myeloid progenitors.

2. Lymphocytes are the adaptive immune system's foundation. They are derived from common lymphoid progenitors and are commonly referred to as white blood cells. T-cells and B-cells make up the majority of the lymphoid lineage.

3. Myelocytes, which include granulocytes, megakaryocytes, and macrophages, are descended from common myeloid progenitors and play a variety of roles including innate immunity, adaptive immunity, and blood clotting.

Erythropoiesis

The process of producing erythrocytes, known as erythropoiesis, begins with the formation of proerythroblasts from hematopoietic stem cells. 

Several stages of development occur over three to five days as ribosomes proliferate and haemoglobin is synthesised. Finally, the nucleus is ejected, resulting in depression in the cell's centre. 

Young erythrocytes, known as reticulocytes, enter the bloodstream while still containing some ribosomes and endoplasmic reticulum and develop into mature erythrocytes after another one or two days.

Erythropoietin (EPO), a hormone primarily produced by the kidneys, stimulates bone marrow production of erythrocytes (stimulates erythropoiesis). When there is insufficient oxygen delivered to body cells, a condition known as hypoxia, the kidneys increase EPO secretion, which stimulates an increase in erythrocyte production.

In healthy people, the average rate of erythrocyte production is two million cells per second. Iron, vitamin B12, and folic acid are all required for normal production. Vitamin B12 and folic acid are required for proper DNA development in erythroblasts. This DNA is in charge of organising the heme molecule, of which iron will become a component. Erythroblast reproduction also requires proper DNA development. Pernicious anaemia can be caused by a deficiency in either vitamin B12 or folic acid.

How are the Liver and Red Blood Cell Production Interconnected?

The yolk sac is the first site of blood formation in the human embryo. Later in embryonic life, the liver becomes the most important red blood cell-forming organ, but it is quickly surpassed by the bone marrow, which is the only source of both red blood cells and granulocytes in adulthood.

In response to low oxygen levels, the kidney and liver produce erythropoietin. Furthermore, erythropoietin is bound by circulating red blood cells; low circulating numbers result in a relatively high level of unbound erythropoietin, which stimulates bone marrow production.

To maintain normal blood sugar levels, the liver breaks down glycogen into glucose and releases it into the blood. Vitamins A, D, E, K, and B12 are stored in the liver. It also stores iron in the form of ferritin, which it releases to allow the body to produce new RBCs.

Leukopoiesis

For the formation of white blood cells medical term used is Leukopoiesis. Colony-stimulating factors (CSFs), which are hormones produced by mature white blood cells, stimulate leukopoiesis, the process of making leukocytes. The division of hematopoietic stem cells into one of the following "blast" cells initiates the development of each type of white blood cell:

1. Myeloblasts divide to form eosinophilic, neutrophilic, or basophilic myelocytes, which give rise to the three types of granulocytes.

2. Monocytes are formed as a result of the development of monoblasts.

3. Lymphoblasts are responsible for the development of lymphocytes.

Thrombopoiesis

Thrombopoiesis, or platelet formation, begins with the formation of megakaryoblasts from hematopoietic stem cells. Megakaryoblasts divide without cytokinesis to form megakaryocytes, which are large cells with a multilobed nucleus. As the plasma membrane folds into the cytoplasm, the megakaryocytes fragment into segments.

Location for Blood Formation in Human Body

Blood formation in human body occurs in developing embryos in blood islands, which are aggregates of blood cells in the yolk sac. Blood formation occurs in the spleen, liver, and lymph nodes as development progresses. When bone marrow matures, it eventually takes on the task of producing the majority of the blood cells for the entire organism.

Secondary lymphoid organs, such as the spleen, thymus, and lymph nodes, do, however, undergo lymphoid cell maturation, activation, and some proliferation. Hematopoiesis occurs in the marrow of long bones in children, such as the femur and tibia. It is most common in adults in the pelvis, cranium, vertebrae, and sternum.

Maturation

As a stem cell matures, changes in gene expression limit the cell types it can become and bring it closer to a specific cell type. These changes are frequently tracked by observing the presence of proteins on the cell's surface. Each subsequent change brings the cell closer to the final cell type while limiting its ability to become a different cell type.

Determination

The location of differentiation appears to be a determinant of cell determination. The thymus, for example, provides an ideal environment for thymocytes to differentiate into a wide range of functional T cells. Blood cells in the bone marrow are randomly assigned to specific cell types, including stem cells and other undifferentiated blood cells. The hematopoietic microenvironment influences some cells to survive while others undergo apoptosis and die. Bone marrow can change the number of different cells produced by regulating this balance between cell types.