Blood Cells and Hematopoietic System

Blood Cells and Hematopoietic System

The hematopoietic system entails all organs and tissues that are essential for the production of cellular components of blood and the blood cells. The organs and tissues that contribute to the production of cellular components of blood are the bone marrow, liver, lymph nodes, thymus, and spleen. The blood cells produced by these organs and tissues include the erythrocytes (red blood cells), leukocytes (white blood cells) and thrombocytes (platelets)(Hart, 2012). In essence, this discussion aims at scrutinizing this system to the practical details. Key Highlights of the analysis include the hematopoiesis process and the function of the various components of the hematopoietic system. With such information, it is beyond doubt that an understanding of the system is inevitable.

Overview of the Functions the Organs and Tissues of Hematopoietic System

Lymph Nodes

The lymph nodes are essential for indicating infections since they inflame in the event of diseases(Rogers, 2011).

Thymus

The thymus provides a proliferation site of the lymphocytes upon leaving the bone marrow. It offers the right environment for the maturation of these cells(Rogers, 2011).

Spleen

The spleen is responsible for the immune responses for instance in the case of malaria. It also destroys old fragile red blood cells(Hart, 2012).

Bone Marrow

The bone marrow is the hematopoietic site of the body where all the blood cells originate.  The particular site of the hematopoiesis is red bone marrow. The bone marrow is in all long bones and flat bones(Hart, 2012).

Hematopoiesis

Source:(OpenStax College, 2013)

Hematopoiesis begins in the bone marrow with the undifferentiated blast cell called the hematopoietic (pluripotent) stem cell. The cell undergoes a series of differentiation to form the myeloid stem cell and lymphoid stem cells. The two stem cells differentiate further to result in the development of the various blood cells. The differentiation of the myeloid stem cells leads to the development of the erythrocytes, granulocytes (basophil, neutrophil, and eosinophil), monocytes and thrombocytes. On the contrary, the lymphoid stem cells differentiate to the two types of lymphocytes (T-lymphocyte and B-lymphocyte). As such, every blood cell type has its unique pathway that it follows as it differentiates from the hematopoietic stem cell(Hart, 2012).

Erythrocytes

Structure

The structure of the red blood cells enhances its suitability to carry out its various functions in the body. For instance, the shape of the erythrocytes is like that of biconcave discs, which appear flattened and have depressed centers. The shape is significant as it helps them to pass through the small microvasculature of the capillaries as they carry gasses across the body.

Additionally, the largest portion (97%) of an erythrocyte possesses a molecule known as hemoglobin. The hemoglobin is necessary for binding to the respiratory gasses and transporting them across the body(Hart, 2012).

Besides, the erythrocytes lack nuclei in their structure, which creates more room for the hemoglobin that is necessary for the transportation of respiratory gases.

Function

Primarily, the red blood cells have the sole responsibility of transporting respiratory gases across the human body. The hemoglobin is particularly essential in giving the erythrocytes the binding capability needed to bind to the oxygen and carbon dioxide gases.

Fate and Destruction of the Erythrocytes

The useful lifespan of the red blood cells is between 100 and 120 days. After this timeframe, destruction of the old erythrocytes follows, especially in the spleen. The macrophages engulf the old fragile red blood cells in the spleen and destroy them(Hart, 2012).

The hemoglobin is split off into two the heme and the globin. The iron component of the heme binds to a protein and is stored as ferritin while the rest of the heme enters the heme biosynthesis pathway. In the heme biosynthesis, it forms the bilirubin that enters the entero-hepatic system and eventually undergoes excretion as stercobilin in feces(Hart, 2012).

Disorders of Red Blood Cells

The erythrocytes are subject to certain conditions that are worth noting. One befitting example of such is Anemia. In this condition, an individual’s erythrocyte oxygen carrying capacity is too low that it cannot support the normal metabolic processes of the body. As such, an individual has a hemoglobin level that is below the required level. The normal ranges of hemoglobin levels for adult males are 13-18 g/dl while for the females is 12-16g/dl. A possible cause of this phenomenon can be excessive loss of blood (hemorrhagic anemia) like in traumatic cases or stab wounds. Another likely cause could be the low production of red blood cells like in the iron-deficiency anemia, pernicious anemia, and aplastic anemia. Lastly, increased red blood cells destruction is another cause that may contribute to anemia. Some cases that contribute to increased erythrocyte destruction include but not limited to sickle cell anemia and thalassemias(Rogers, 2011).

Additionally, polycythemia is the other disorder associated with erythrocytes. Characteristic of this condition is that the person has an abnormal excess of erythrocytes.  Consequently, this excess number of red blood cell increases the blood viscosity, causing it to sludge and assume a slow movement. The hematocrit level may increase up to 80% while the blood volume may double(Hart, 2012).

Leucocytes

The two classes of leukocytes include granulocytes and agranulocytes. Central to this classification is the existence of granules in their structure (granulocytes have granules while granulocytes have no granules). The granulocytes include comprising of the neutrophil, eosinophil, and basophil. On the contrary, the agranulocytes include monocytes and lymphocytes(Rogers, 2011).

Granulocytes

All the granulocytes are spherical in shape and large as compared to the erythrocytes. They have lobed nuclei and have cytoplasmic granules that give them their name. They comprise of neutrophil, eosinophil, and basophil(Rogers, 2011).

First and most importantly, the neutrophils account for 50-70% of all leucocyte population. They have fine granules that have the tendency to take up both the basic (blue) and acidic (red) dyes. The two colors give its cytoplasm a characteristic lilac color. The granules contain lysosomes (hydrolytic enzymes) and antimicrobial proteins (defensins) that are essential for immune function against bacteria. Their nuclei have three to six lobes, and because of this variability of the nuclei, they are known as polymorphonuclear white blood cells. Their body function is to destroy the bacteria. As such, the neutrophils level in the blood increase in times of acute bacterial infection(Hoffman, 2013).

Secondly, the eosinophil accounts for 2-4% of the total leucocyte population. They are approximately the same size as the neutrophils. They possess nuclei that have two lobes. Their granules stain red when exposed to the acidic dye (eosin). The granules contain digestive enzymes (lysosomes) and actively phagocytize antibody-antigen complexes. They are useful biomarkers of parasitic infection and allergic reactions such as asthma(Rogers, 2011).

Finally, basophils are rare of all the leucocytes as they account for 0.5-1% of the leucocyte population. The nuclei are U- or S-shaped with two to five lobes. Their granules stain blue-purple with basic dyes. The granules of the basophils contain histamine, serotonin, and heparin. Histamine is often released in response to tissue damage and pathogen invasion. The mast cells are similar to the basophils in structure and functions and are mostly inhabit the connective tissues(Hoffman, 2013).

Agranulocytes

The agranulocytes have no visible cytoplasmic granules in their structure. Their nuclei are spherical or kidney-shaped. They include the monocytes, which descends from the myeloid stem cell and lymphocytes that originate from the lymphoid stem cell(Hoffman, 2013).

Monocytes have large kidney-shaped nuclei that have to surround of abundant blue-gray staining cytoplasm. They are the largest of all the leucocytes and account for 3-8% of the total leucocyte population. The monocytes become macrophages when enlarged and upon leaving the bloodstream. The highly mobile macrophages are essential for engulfing microorganism and cellular debris(Hoffman, 2013).

On the other hand, the lymphocytes account for 25% of the total leucocyte population. They have a large dark purple nucleus occupying most of the cell volume. The lymphocytes are the only leucocytes that can return the blood stream upon leaving it. They comprise the T-lymphocytes and B-lymphocytes. They play a crucial role in immunity responses. For instance, the T cells function by acting directly to against the virus-infected cells. The B cells on the other hand contribute to one’s immunity by giving rise to plasma cells that are necessary for the production of antibodies(Houghton, 2007). Antibodies offer the immune function of this cell type by reacting to circulatory antigen (bacteria toxin).

Life Span of Leucocytes

The leucocytes have different life span with the agranulocytes having a higher life span as compared to the granulocytes. For instance, neutrophils have a life span of 6 hours to a few days. Eosinophil can last for about five days while the basophils can live for a few hours to a few days. On the contrary, the monocytes can last for months whereas the lymphocyte has the potential of lasting for hours to years(Rogers, 2011).

Leucocytes Disorders

The leucocytes are also associated with certain conditions. A case in point is the overproduction of abnormal white blood cells like in leukemia and infectious mononucleosis. Another disorder of the leucocytes is leukopenia (abnormally low white blood cell count) which is due to glucocorticoids and anticancer agents(Houghton, 2007).

Thrombocytes

Structure

Platelets mostly not cells, rather they are cytoplasm fragments of cells that are extraordinarily large known as megakaryocytes. They are approximately a quarter the diameter of lymphocytes in size. When viewed after blood smears every platelet shows a blue stained outer area surrounding an inner are with granules that usually stains purple.  Within the granules are a variety of chemicals that enable the clotting process. Some of the chemicals within the granules are Calcium ions, Adenosine diphosphate and other enzymes useful in the clotting process(Hoffman, 2013).

Formation

A hormone called thrombopoietin regulates the formation of platelets. The megakaryocytes that are essentially the parental cells of platelets are from the hematopoietic stem cell(Hoffman, 2013).

Function

Platelets are vital if not the largest component of the clotting process. Coagulation occurs in the blood plasma when a blood vessel lining is injured or when it ruptures. The platelets stick together forming a meshwork that seals the point of breakage. If platelets do not take part in any clotting process, they age rather quickly and degenerate in approximately ten days. During this period of inactivity, they just circulate freely by molecules such as prostacyclin and nitric oxide(Antovic&Blombäck, 2010).

Clotting begins with a vascular spasm. During this stage the smooth muscle contracts and causes vasoconstriction of the affected blood vessels. What follows is the formation of the platelet plug. In this juncture, an injury that caused to the blood vessel wall exposes the collagen fibers subsequently causing adherence of the platelets. The platelets then release chemicals in the blood that case other platelets to become sticky thus forming a platelet plug. The final stage comprises coagulation. The fibrin makes a mesh-like framework; its function is to trap the red blood cells ultimately forming the blood clot(Antovic&Blombäck, 2010). The bleeding is then halted.

Conclusion

In closure, indeed the hematopoietic system is a broad and sensitive topic that requires more attention for proper understanding. The areas covered in this discussion include hematopoiesis, functions of the various hematopoietic organs and tissues as well as the blood cells.

References

Antovic, J. &Blombäck, M. (2010). Essential guide to blood coagulation (1st ed.). Chichester, West Sussex, UK: Wiley-Blackwell.

Hart, M. (2012). Introduction to human disease (1st ed.). Sudbury: Jones & Bartlett Learning.

Hoffman, R. (2013). Hematology (1st ed.). Philadelphia, PA: Saunders/Elsevier.

Houghton, G. (2007). Blood (1st ed.). New York: PowerKids Press.

OpenStax College,. (2013). The Hematopoietic System of the Bone Marrow. Retrieved from http://cnx.org/content/col11496/1.6/

Rogers, K. (2011). Blood (1st ed.). Chicago: Britannica Educational Pub.

 

 

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