Blood cells are also called hematopoietic cells. They are broadly classified into three types: They are Red blood cells, white blood cells, and platelets. Together, these add up to a total of 45% of the blood tissue by volume, with the remaining 55% of the volume composed of plasma, the liquid component of blood.
Red Blood Cells (RBC)
RBC also is known as Erythrocytes. They primarily carry oxygen and collect carbon dioxide through the use of hemoglobin. Hemoglobin is an iron-containing protein that gives red blood cells their color and facilitates the transportation of oxygen from the lungs to tissues and carbon dioxide from tissues to the lungs to be exhaled. They are the most abundant cell in the blood, accounting for about 40-45% of their volume. Erythrocytes are circular, biconcave, disk-shaped, and deformable to allow them to squeeze through narrow capillaries. They do not have a nucleus. Erythrocytes are much smaller than most other human cells.
RBCs are formed in the red bone marrow from hematopoietic stem cells in a process known as erythropoiesis. In adults, about 2.4 million RBCs are produced each second. The normal RBCs count is 4.5 to 5 million per cu. mm. RBCs have a lifespan of approximately 100-120 days. After they have completed their lifespan, they are removed from the bloodstream by the spleen.
White Blood Cells (WBC)
White blood cells or leukocytes are cells of the immune system involved in defending the body against both infectious disease and foreign materials. They are produced and derived from multipotent cells in the bone marrow known as hematopoietic stem cells. Leukocytes are found throughout the body, including the blood and lymphatic system. There are a variety of types of leukocytes that serve specific roles in the human immune system. WBCs constitute approximately 1% of the blood volume.
White blood cells are divided into granulocytes and agranulocytes, distinguished by the presence or absence of granules in the cytoplasm. Granulocytes include basophils, eosinophils, neutrophils, and mast cells. Agranulocytes include lymphocytes and monocytes.
The condition of having too few white blood cells is leukopenia while having too many is leukocytosis. There are individual terms for the lack or overabundance of specific types of white blood cells. The number of white blood cells in circulation is commonly increased in the incidence of infection. Many hematological cancers are based on the inappropriate production of white blood cells.
Platelets are very small, irregularly shaped clear cell fragments, 2–3 µm in diameter, which derive from the fragmentation of megakaryocytes. The average lifespan of a platelet is normally just 5 to 9 days. Platelets are a natural source of growth factors. They circulate in the blood of mammals and are involved in hemostasis, leading to the formation of blood clots. Platelets release thread-like fibers to form these clots.
The normal range for platelets is 150,000 – 450,000 per cubic millimeter. If the number of platelets is too low, excessive bleeding can occur. However, if the number of platelets is too high, blood clots can form thrombosis, which may obstruct blood vessels and result in such events as a stroke, myocardial infarction, pulmonary embolism, or blockage of blood vessels to other parts of the body, such as the extremities of the arms or legs. An abnormality or disease of the platelets is called a thrombocytopathy.