Natural Sciences-Blood Cells

From Rastafari Education Daily,
A regular column by kaya, Sep 03, 2005         

Summary: An overview of some of the components of the circulatory system, starting with the Blood Cells. To I Jah has made INI so perfeckly it is miraculous jus to ponder each cell! This series is just a simple overview. If you are Rastafari an would like more detail information, help wid a course in anatomy or hematology, please contack I and I will gladly help as much as I am able.

 
Blood is a liquid tissue. Suspended in the watery PLASMA are seven types of cells and cell fragments: Red blood cells (RBCs) or erythrocytes Platelets or thrombocytes five kinds of White blood cells (WBCs) or leukocytes Three kinds of granulocytes Neutrophils Eosinophils Basophils Two kinds of Leukocytes without granules in their cytoplasm Lymphocytes Monocytes If one takes a sample of blood, treats it with an agent to prevent clotting, and spins it in a centrifuge, the red cells settle to the bottom the white cells settle on top of them forming the "buffy coat". The fraction occupied by the red cells is called the hematocrit. Normally it is approximately 45%. Values much lower than this are a sign of anemia. Functions of the blood Blood performs two major functions: Transport through the body of oxygen and carbon dioxide Food molecules (glucose, lipids, amino acids) Ions (e.g., Na , Ca2 , HCO3−) Wastes (e.g., urea) Hormones Heat Defense of the body against infections and other foreign materials. All the WBCs participate in these defenses. The formation of blood cells They are produced in the bone marrow (some 1011 of them each day in an adult human!). Arise from a single type of cell called a multipotent stem cell. These stem cells are very rare (only about one in 10,000 bone marrow cells); are attached (probably by adherens junctions) to osteoblasts lining the inner surface of bone cavities; produce, by mitosis, two kinds of progeny: more stem cells (A mouse that has had all its blood stem cells killed by a lethal dose of radiation can be saved by the injection of a single living stem cell!). Cells that begin to differentiate along the paths leading to the various kinds of blood cells. Which path is taken is regulated by the need for more of that type of blood cell which is, in turn, controlled by appropriate cytokines and/or hormones. Examples: Interleukin-7 (IL-7) is the major cytokine in stimulating bone marrow stem cells to start down the path leading to the various lymphocytes (mostly B cells and T cells). Erythropoietin (EPO), produced by the kidneys, enhances the production of red blood cells (RBCs). Thrombopoietin (TPO), assisted by Interleukin-11 (IL-11), stimulates the production of megakaryocytes. Their fragmentation produces platelets. Granulocyte-monocyte colony-stimulating factor (GM-CSF), as its name suggests, sends cells down the path leading to both those cell types. In due course, one path or the other is taken. Under the influence of granulocyte colony-stimulating factor (G-CSF), they differentiate into neutrophils. Further stimulated by interleukin-5 (IL-5) they develop into eosinophils. Interleukin-3 (IL-3) participates in the differentiation of most of the white blood cells but plays a particularly prominent role in the formation of basophils (responsible for some ALLERGIES). Stimulated by macrophage colony-stimulating factor (M-CSF) the granulocyte/macrophage progenitor cells differentiate into monocytes, the precursors of macrophages. Red Blood Cells (erythrocytes) The most numerous type in the blood. Women average about 4.8 million of these cells per cubic millimeter (mm3; which is the same as a microliter ) of blood. Men average about 5.4 x 106 per µl. These values can vary over quite a range depending on such factors as health and altitude. (Peruvians living at 18,000 feet may have as many as 8.3 x 106 RBCs per µl.) RBC precursors mature in the bone marrow closely attached to a macrophage. They manufacture hemoglobin until it accounts for some 90% of the dry weight of the cell. The nucleus is squeezed out of the cell and is ingested by the macrophage. No-longer-needed proteins are expelled from the cell in vesicles called exosomes. RBCs are terminally differentiated; that is, they can never divide. They live about 120 days and then are ingested by phagocytic cells in the liver and spleen. Most of the iron in their hemoglobin is reclaimed for reuse. The remainder of the heme portion of the molecule is degraded into bile pigments and excreted by the liver. Some 3 million RBCs die and are scavenged by the liver each second. Red blood cells are responsible for the transport of oxygen and carbon dioxide. Oxygen Transport In adult humans the hemoglobin (Hb) molecule consists of four polypeptides: two alpha (α) chains of 141 amino acids and two beta (β) chains of 146 amino acids Each of these is attached the prosthetic group heme. There is one atom of iron at the center of each heme. One molecule of oxygen can bind to each heme. The reaction is reversible. Under the conditions of lower temperature, higher pH, and increased oxygen pressure in the capillaries of the lungs, the reaction proceeds to the right. The purple-red deoxygenated hemoglobin of the venous blood becomes the bright-red oxyhemoglobin of the arterial blood. Under the conditions of higher temperature, lower pH, and lower oxygen pressure in the tissues, the reverse reaction is promoted and oxyhemoglobin gives up its oxygen. Carbon Dioxide Transport Carbon dioxide (CO2) combines with water forming carbonic acid, which dissociates into a hydrogen ion (H ) and a bicarbonate ions: CO2 H2O ↔ H2CO3 ↔ H HCO3− 95% of the CO2 generated in the tissues is carried in the red blood cells: It probably enters (and leaves) the cell by diffusing through transmembrane channels in the plasma membrane. (One of the proteins that forms the channel is the D antigen that is the most important factor in the Rh system of blood groups.) Once inside, about one-half of the CO2 is directly bound to hemoglobin (at a site different from the one that binds oxygen). The rest is converted — following the equation above — by the enzyme carbonic anhydrase into bicarbonate ions that diffuse back out into the plasma and hydrogen ions (H ) that bind to the protein portion of the hemoglobin (thus having no effect on pH). Only about 5% of the CO2 generated in the tissues dissolves directly in the plasma. (A good thing, too: if all the CO2 we make were carried this way, the pH of the blood would drop from its normal 7.4 to an instantly-fatal 4.5!) When the red cells reach the lungs, these reactions are reversed and CO2 is released to the air of the alveoli. Anemia Anemia is a shortage of RBCs and/or the amount of hemoglobin in them. Anemia has many causes. One of the most common is an inadequate intake of iron in the diet.

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