Diagram showing the development of different blood cells fromhematopoietic stem cells to mature cells.
Ablood cell (also called ahematopoietic cell,hemocyte, orhematocyte) is acell produced throughhematopoiesis and found mainly in theblood. Major types of blood cells includered blood cells (erythrocytes),white blood cells (leukocytes), andplatelets (thrombocytes). Together, these three kinds of blood cells add up to a total 45% of the blood tissue by volume, with the remaining 55% of the volume composed ofplasma, the liquid component of blood.[1]
Red and white human blood cells as seen under a microscope using a blue slide stainThe darker red blood syringes have deoxygenated blood, whereas the brighter red have oxygenated blood.
Red blood cells orerythrocytes primarily carryoxygen and collectcarbon dioxide through the use ofhemoglobin.[2] Hemoglobin is aniron-containing protein that gives red blood cells their color and facilitates transportation ofoxygen from thelungs to tissues andcarbon dioxide from tissues to the lungs to be exhaled.[3] Red blood cells are the most abundant cell in the blood, accounting for about 40–45% of its volume. Red blood cells are circular, biconcave, disk-shaped and deformable to allow them to squeeze through narrow capillaries. They do not have a nucleus. Red blood cells are much smaller than most other human cells.
RBCs are formed in the red bone marrow fromhematopoietic stem cells in a process known aserythropoiesis. In adults, about 2.4 million RBCs are produced each second. The normal RBCs count is 4.5 to 5 millions 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 thespleen.
Mature red blood cells are unique among cells in the human body in that they lack a nucleus (althougherythroblasts do have a nucleus).
The condition of having too few red blood cells is known asanemia, while having too many ispolycythemia.
Erythrocyte sedimentation rate (ESR) is the rate at which RBCs sink to the bottom (when placed in a vertical column after adding an anticoagulant). Normal values of ESR are:
Artificially colored electron micrograph of blood cells. From left to right: erythrocyte, thrombocyte, leukocyte.
White blood cells orleukocytes, are cells of theimmune system involved in defending the body against bothinfectious disease and foreign materials. They are produced and derived from multipotent cells in thebone marrow known ashematopoietic stem cells. Leukocytes are found throughout the body, including the blood andlymphatic system. There are a variety of types of white blood cells that serve specific roles in the human immune system. WBCs constitute approximately 1% of the blood volume.[4]
The condition of having too few white blood cells isleukopenia, while having too many isleukocytosis. 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 ofinfection.[5] Manyhematological cancers are based on the inappropriate production of white blood cells.
Platelets, orthrombocytes, are very small, irregularly shaped clear cell fragments, 2–3 μm in diameter, which derive from fragmentation ofmegakaryocytes. 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 (99% of population analyzed) for platelets is 150,000 to 450,000 per cubic millimeter.[6] 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 astroke,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, which can be either a low number of platelets (thrombocytopenia), a decrease in function of platelets (thrombasthenia), or an increase in the number of platelets (thrombocytosis). There are disorders that reduce the number of platelets, such asheparin-induced thrombocytopenia (HIT) orthrombotic thrombocytopenic purpura (TTP), that typically cause thromboses, or clots, instead of bleeding.
Platelets release a multitude of growth factors includingplatelet-derived growth factor (PDGF), a potent chemotactic agent, andTGF beta, which stimulates the deposition of extracellular matrix. Both of these growth factors have been shown to play a significant role in the repair and regeneration of connective tissues. Other healing-associated growth factors produced by platelets includebasic fibroblast growth factor (bFGF),insulin-like growth factor 1 (IGF-1), platelet-derived epidermal growth factor, andvascular endothelial growth factor (VEGF). Local application of these factors in increased concentrations through platelet-rich plasma (PRP) has been used as an adjunct to wound healing for several decades.
A complete blood count (CBC) is a test panel requested by a doctor or other medical professional that gives information about the cells in a patient's blood. A scientist or lab technician performs the requested testing and provides the requesting medical professional with the results of the CBC. In the past, counting the cells in a patient's blood was performed manually, by viewing a slide prepared with a sample of the patient's blood under a microscope. Today, this process is generally automated by use of an automated analyzer, with only approximately 10-20% of samples now being examined manually. Abnormally high or low counts may indicate the presence of many forms of disease, and hence blood counts are amongst the most commonly performed blood tests in medicine, as they can provide an overview of a patient's general health status.
In 1658 Dutch naturalistJan Swammerdam was the first person to observe red blood cells under a microscope, and in 1695,microscopistAntoni van Leeuwenhoek, also Dutch, was the first to draw an illustration of "red corpuscles", as they were called. No further blood cells were discovered until 1842 when French physicianAlfred Donné discovered platelets. The following year leukocytes were first observed byGabriel Andral, a French professor of medicine, andWilliam Addison, a British physician, simultaneously. Both men believed that both red and white cells were altered in disease. With these discoveries,hematology, a new field of medicine, was established. Even though agents for staining tissues and cells were available, almost no advances were made in knowledge about the morphology of blood cells until 1879, whenPaul Ehrlich published his technique for staining blood films and his method for differential blood cell counting.[7]
^Maton, Anthea; Jean Hopkins; Charles William McLaughlin; Susan Johnson; Maryanna Quon Warner; David LaHart; Jill D. Wright (1993).Human Biology and Health. Englewood Cliffs, New Jersey, US: Prentice Hall.ISBN0-13-981176-1.
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^Hajdu, Steven I. (2003). "A Note from History: The Discovery of Blood Cells".Ann Clin Lab Sci.33 (2):237–8.PMID12817630.
