Red Blood Cells and Haemoglobin PDF

RED BLOOD CELLS AND HAEMOGLOBIN Dr. Amal Uzrail ERYTHROCYTE STRUCTURE  An average lifespan of 120 days.  The normal concentration of erythrocytes in the blood is 3.9–6.5 X 1012/L.  Are not nucleated and contain no organelles.  Contain millions of molecules of haemoglobin, an oxygen-carryin...

RED BLOOD CELLS AND HAEMOGLOBIN Dr. Amal Uzrail ERYTHROCYTE STRUCTURE  An average lifespan of 120 days.  The normal concentration of erythrocytes in the blood is 3.9–6.5 X 1012/L.  Are not nucleated and contain no organelles.  Contain millions of molecules of haemoglobin, an oxygen-carrying pigment that gives blood its red color  Have a characteristic biconcave discoid shape on blood smears=> This gives a 20–30% larger surface area than a sphere of the same volume ERYTHROCYTE FUNCTION  The primary function of erythrocytes is the carriage of O2 & CO2 between the lungs and tissues.  The large surface area facilitates this function.  pH buffering also. GAS EXCHANGE AND TRANSPORT  1000 ml of O2 transported by hemoglobin (Hb)/ min => most transported by Hb, few dissolved in plasma.  The O2 content of the blood depends on:  The concentration of Hb  The affinity of Hb for O2  The solubility of O2 in the blood (small effect)  GAS EXCHANGE AND TRANSPORT  CO2is carried in 3 forms:  ~90% as bicarbonate  ~5% in the form of carbamino compounds (CO2 combines with the amino groups of plasma proteins and haemoglobin)  ~5% in physical solution (CO2 is > 20 X more soluble in blood than is O2) ELECTROLYTES BALANCE  Chloride,potassium and hydrogen ions are transported across the red-cell membrane=>In blood stored for transfusion, the extracellular potassium level is quite high due to disruption of active transport. ERYTHROPOIESIS ( PRODUCTION OF RED BLOOD CELLS FROM THE BFU-GEMM (GRANULOCYTE, ERYTHROCYTE, MONOCYTE, MEGAKARYOCYTE) PROGENITOR( REGULATION OF ERYTHROPOIESIS  By Erythropoietin (EPO):  EPO (hormone)is a heavily glycosylated polypeptide.  It is 165 amino acids in length and weighs 30,400 kDa.  It is secreted by: 1. Endothelial cells of the peritubular capillaries in the renal cortex (90%)  2.Kupffer cells and hepatocytes in the liver (10%) CONTROL OF ERYTHROPOIETIN DRIVE  The major stimulus for secretion is hypoxia. This can be caused by any factor that gives rise to decreased oxygen transport to tissues relative to tissue demand. IRON METABOLISM 1. Uptake and excretion of iron:  The total iron store of the body is around 4 g, mainly as haemoglobin.  The daily requirement is normally around 1 mg.  Absorption is controlled by proteins in the gut.  The rate of iron transfer from epithelial cells to plasma responds to iron requirements, e.g. it is high when stores are low or the rate of erythropoiesis is high. 2. IRON TRANSPORT AND STORAGE PROTEINS  Free iron is toxic => incorporated into haem or bound to protein within the body.  Haem consists of an iron atom at the centre of a protoporphyrin ring.  Transferrin transports up to two molecules of iron to tissues that have transferrin receptors, e.g. bone marrow.  Both ferritin and haemosiderin store iron in its ferric form.  Ferritin is a water-soluble compound, consisting of protein and iron.  Haemosiderin is insoluble and consists of aggregates of ferritin that have partially lost their protein component IRON OVERLOAD  Iron overload can occur as a result of: Increased absorption Parenteral administration.  Excessiron can result in organ damage if it is deposited in the tissues. The heart, liver and endocrine organs are particularly at risk. INCREASED IRON ABSORPTION  This can be either primary or secondary, and results from the following:  Primary/hereditary haemochromatosis—an autosomal recessive disorder characterized by excessive intestinal absorption of iron  Massive ineffective erythropoiesis as seen in thalassaemia syndromes or congenital dyserythropoietic anaemia  Dietary excess—rare in the developed world but sometimes seen in sub Saharan Africa. HAEM METABOLISM  Haem belongs to a family of compounds known as the porphyrins, which are characterized by the presence of a tetrapyrrole ring.  Haem is an iron-containing derivative, the iron ion (Fe2þ) being located at the centre of the tetrapyrrole ring of protoporphyrin IX.  The haem group is responsible for the oxygen-binding properties of haemoglobin. 1. HAEM BIOSYNTHESIS  Haem synthesis occurs in the mitochondria of immature red cells in the bone marrow by a process outlined in fig.  2. HAEM BREAKDOWN  Degradation occurs in the macrophages of the spleen, bone marrow and liver. HAEMOGLOBIN  Haemoglobin is composed of four globin chains held together by non-covalent interactions (Fig. 2.9).  Each globin chain has a hydrophobic crevice, or haem pocket, which contains the haem molecule.  Each haemoglobin can, therefore, carry 4 molecules of oxygen. The haem pocket allows O2 binding, while protecting the iron atom from oxidation.  Different types of haemoglobin are present at different stages of development (Fig. 2.10). Adult haemoglobin (HbA) contains two a- and two b-chains, which are arranged as two dimers, written 2(ab).  The globin chains interact with each other in an allosteric fashion, i.e. they bind with each other away from their active sites.  The other major haem-containing protein in humans is myoglobin, which consists of a single chain associated with a haem group. It is found principally in muscle, where it provides an oxygen reserve. The four haemoglobin subunits are structurally similar to myoglobin. THE GENETICS OF HAEMOGLOBIN  The genes encoding the ε-, γ-, σ- and β - chains are found on chromosome 11.  The ζ- and two copies of the a-chain genes are found on chromosome 16.  Each globin gene has three exons separated by two introns.  The different globin chains are synthesized separately and then come together to form a functional Hb molecule. PHYSIOLOGICAL PROPERTIES OF HAEMOGLOBIN  Eachhaemoglobin molecule (Hb) can bind four molecules of oxygen, one at each haem site. In terms of oxygenation, Hb can exist in two configurations:  Relaxed (R-) Hb: Occurs when Hb is oxygenated => the globin chains are able to move against each other, which will allow O2 release.  Taut (T-) Hb: Occurs when O2 is unloaded=> the metabolite 2,3-diphosphoglycerate (2,3-DPG) enters the centre of the deoxyhaemoglobin molecule, reducing its affinity for O2. Deoxyhaemoglobin is characterized by a relatively large number of ionic and hydrogen bonds between the αβ dimers, which restrict the movement of the globin chains. PHYSIOLOGICAL PROPERTIES OF HAEMOGLOBIN  Binding of one O2 molecule increases the affinity for oxygen at the remaining haem groups. It is this property of Hb which causes the characteristic sigmoidal (S-shaped) dissociation curve.  The oxygen dissociation curve is a plot of partial pressure of oxygen (x axis) against oxygen saturation (y axis). OXYGEN DISSOCIATION CURVE FOR HAEMOGLOBIN OXYGEN DISSOCIATION CURVE FOR HAEMOGLOBIN  Changes in CO2, Hþ, 2,3-DPG and temperature shift the position of the haemoglobin curve but do not generally alter its shape.  Hþ and 2,3-DPG bind to and stabilize deoxyhaemoglobin, favouring the unloading of oxygen. Oxygen binding to myoglobin is not altered by these factors.  Haemoglobin variants also have an effect on the oxygen dissociation curve, e.g. sickle-cell haemoglobin shifts the curve to the right=>This shift to the right enables the patient to have a normal exercise tolerance in spite of a low Hb count. THE CYTOSKELETON OF THE RED CELL:  The erythrocyte plasma membrane is supported by a dense, fibrillar, protein shell—the cytoskeleton.  The red-cell cytoskeleton:  Maintains cell shape and confers strength to the erythrocyte membrane, allowing the cell to withstand the stresses of the circulation  Permits flexibility, which is important in erythrocyte circulation. The proteins of the plasma membrane, categorized into integral and peripheral, are important constituents of the cytoskeleton.  The band numbers refer to their mobility on electrophoresis THE CYTOSKELETON OF THE RED CELL  Integral proteins:  These proteins penetrate the lipid bilayer of the cell membrane and are closely associated with it. These include band 3 protein and glycophorins.  Peripheral proteins:  Peripheral proteins are loosely attached to the lipid bilayer and include spectrin, ankyrin, band 4.1 protein and actin. THE CYTOSKELETON OF THE RED CELL  Dysfunction within the peripheral proteins is the basis of some inherited diseases that result in anaemia.  The two most common are hereditary spherocytosis and hereditary elliptocytosis.  Surface proteins:  There are numerous surface proteins interacting with plasma.  Many are linked by the glucosyl phosphatidylinositol (GPI) anchor.  Somatic mutation in the gene for phosphatidylinositol glycan protein A (PIG-A) results in the condition paroxysmal nocturnal haemoglobinuria (PNH). METABOLISM OF RED CELLS GLYCOLYSIS AND THE EMBDEN– MEYERHOF PATHWAY  Thisis the glycolytic pathway common to all cells of the human body whereby glucose is metabolized to lactate.  There is a net yield of two ATP molecules, but no net NADH production. THE HEXOSE MONOPHOSPHATE SHUNT  This is also known as the pentose phosphate pathway.  Under normal conditions, 5% of the glucose metabolized by the red cell passes through an oxidative pathway of metabolism, the hexose monophosphate (HMP) shunt.  There is no net ATP yield, but two NADPH molecules are produced per molecule of glucose-6-phosphate entering the shunt. PREVENTION OF HAEM OXIDATION When haemoglobin is oxidized (Fe2+ => Fe3+) it is known as methaemoglobin (metHb).  Excess metHb is caused by:  Toxic substances  Abnormal haemoglobins resistant to enzymatic reduction (M haemoglobins)  NADH methaemoglobin reductase deficiency (rare). The reduced haemoglobin can bind to albumin and has a reduced oxygen-carrying capability.  NADH from the Embden-Meyerhof pathway and NADH methaemoglobin reductase are important in ensuring that iron remains in its reduced form  FULL BLOOD COUNT AND RETICULOCYTE COUNT  Blood samples are added to ethylenediaminetetraacetic acid (EDTA), an anticoagulant.  The samples are tested by an automated analyser, which provides the following information:  Hb concentration, haematocrit, red-cell count, mean cell volume (MCV), mean cell haemoglobin (MCH) and mean cell haemoglobin concentration (MCHC)  White-cell count with differential  Platelet count: some laboratories produce additional parameters  RDW (red cell distribution width) : a measure of the range of red blood cell size in a sample.  Red-cell parameters and the diagnostic inferences of abnormalities of the full blood count are shown in Figure 2.14. PERIPHERAL BLOOD FILM  Examination of a peripheral blood filmis a simple haematological investigation, which can provide a significant amount of information.  Blood is evenly spread into a film on a glass slide, which is then dried and stained, most often with a Romanowsky stain.  The peripheral blood film shows the morphology of blood cells and can show inclusions within the cells. NORMAL RED BLOOD CELLS