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Hematopoiesis refers to the process through which the body manufactures blood cells. It occurs in different sites of the body depending on the stage of life
Hematopoiesis refers to the process through which the body manufactures blood cells. It begins in the embryo and continues for life. Each of the blood cell types that are formed during hematopoiesis has specific functions in the body:
- Red blood cells (RBCs): RBCs, also called erythrocytes, are bright red due to the presence of the iron-containing pigment called hemoglobin. They carry oxygen from the lungs to various cells and tissues in the body, which use oxygen to produce energy and release carbon dioxide as a waste product. Carbon dioxide is carried through the RBCs and blood plasma to the lungs, where it is eliminated from the body.
- White blood cells (WBCs): WBCs, also called leukocytes, are mainly involved in providing immunity. Based on their microscopic appearance, WBCs are of various types:
- Granulocytes: They include neutrophils, eosinophils, and basophils.
- Monocytes: They are the largest WBCs and can transform into macrophages and dendritic cells that help in immune responses.
- Lymphocytes: They include T and B cells.
- Platelets: Also called thrombocytes, platelets are the smallest blood cells and play a role in blood clotting, preventing excessive blood loss after an injury.
Where does hematopoiesis occur?
Hematopoiesis occurs in different sites of the body depending on the stage of life.
- In the early embryonic stage, hematopoiesis mainly occurs in the yolk sac, which is a small, membranous structure that surrounds the embryo. Hematopoiesis in the yolk sac may begin at about 2 weeks of gestation.
- As the pregnancy advances, the site of hematopoiesis shifts from the yolk sac to the liver of the fetus.
- As the fetus matures, hematopoiesis occurs in the bone marrow.
- After birth, hematopoiesis occurs in both the bone marrow and lymphatic tissues (the thymus, spleen, and lymph nodes).
- In most adults, hematopoiesis is limited to the ends of long bones of the limbs, spine, and pelvic crests.
The various types of blood cells originate from primitive cells (also called stem cells) that have the ability to form different types of blood cells (pluripotent cells). In a healthy adult:
- Red blood cells, platelets, and granulocytes (which make up 60%-70% of white blood cells) are formed in the bone marrow.
- Lymphocytes (which make up 20%-30% of white blood cells) are formed in the lymphatic tissues.
- Monocytes are formed in the reticuloendothelial tissue of the spleen, lymph nodes, liver, and other organs.
Why is hematopoiesis important?
Hematopoiesis is a vital process that leads to the formation of blood cells that perform various crucial roles in the body, such as:
- Transporting oxygen to various cells
- Transporting carbon dioxide for elimination through the lungs
- Immunity against infections and various cancers
- Tissue repair
- Prevention of excessive blood loss
Problems with hematopoiesis can lead to several health conditions including cancer, blood clotting disorders, anemia, and frequent infections.
