Red Blood Cell Physiology PDF

Red Blood Cell Physiology Sept. 2023 Content Red cell Metabolism Haemoglobin synthesis Characteristics of the Human RBC Iron Cycling Haemoglobinopathies Types of Anaemia Erythrocytes, leukocytes and thrombocytes SEM x1,825 Normal erythrocyte Biconcave disc 7-8 µm in diameter 2.5 µm in thickest part...

Red Blood Cell Physiology Sept. 2023 Content Red cell Metabolism Haemoglobin synthesis Characteristics of the Human RBC Iron Cycling Haemoglobinopathies Types of Anaemia Erythrocytes, leukocytes and thrombocytes SEM x1,825 Normal erythrocyte Biconcave disc 7-8 µm in diameter 2.5 µm in thickest part 1.0 µm in thinnest part 90 µm3 in volume 140 µm2 in surface area High surface area to volume ratio Facilitates gas delivery and cell deformability Erythrocytes Structure – Biconcave, anucleate Components – Hemoglobin – Lipids, ATP, carbonic anhydrase Function – Transport oxygen from lungs to tissues and carbon dioxide from tissues to lungs Erythrocytes Mature erythrocytes Have no nucleus Shaped like biconcave disks Do not contain ribosomes, mitochondria & other organelles typical of most body cells Primary component is haemoglobin (Hb). – Each RBC contains approx. 200 to 300 Million Hb molecules Most numerous of the formed elements. Red Cell Metabolism Immature RBCs – reticulocytes & normoblasts – metabolize pyruvate via the Tricarboxylic Acid (TCA) cycle and electron transfer chain to CO2 and H2O. Mature RBC lack mitochondria – Metabolize pyruvate to lactate Red Cell Membrane Metabolism Mature RBCs have no mitochondria hence there is no metabolism via the respiratory chain, TCA cycle, oxidation of fatty acids or ketone bodies. RBCs depend on glucose as their energy source Energy in the form of ATP is obtained from anaerobic glycolysis – Embden Meyerhof pathway – Hexose monophosphate shunt / Pentose phosphate pathway Red Cell Metabolism Chief metabolite of the RBC is glucose which undergoes degradation to pyruvate and subsequently to lactate along the glycolytic (Embden-Meyerhof) pathway. 90% glucose metabolized by Glycolysis 10% glucose enters the pentose phosphate pathway The Hexose Monophosphate shunt produces NADPH2 which provides reducing power to protect RBCs from oxidative stress. The HMP shunt is used to produce ribose-5-phosphate and nicotinamide adenine dinucleotide phosphate (NADPH2). It is an alternative pathway to glycolysis Embden-Meyerhof (Glycolytic) pathway Provide ATP for the erythrocytes Provide NADH needed for the reduction of functionally dead metHb (oxidized haemoglobin Fe3+) to functionally active reduced haemoglobin (Fe2+) 2,3-DPG bind to Hb decreasing its affinity for oxygen, thereby making it available to the tissues By-products of Red Cell Metabolism Net gain of 2 moles of ATP for every mole of glucose metabolized 2, 3 – Diphosphoglycerate Reduced Nicotinamide Adenine Dinucleotide (NADH) – Reduction of NAD during glycolysis Role of ATP in RBC membrane Maintenance of cell shape and deformability Phosphorylation of sugars Maintenance of ATPase pumps Hexose-monophosphate shunt Production of NADPH – “reducing power” linked with glutathione for the elimination of peroxide Protection of sulphydril (-SH) groups on the cell membrane and in Hb molecule from oxidation G6PD deficiency Sex linked, affecting males; female carriers show half normal G6PD levels Disorder is most common in West Africa, Southern Europe, Middle East and South East Asia Patients are usually asymptomatic and show severe haemolysis (acute) when exposed to oxidant stress Common triggers include antimalarials and analgesics, fava beans and infections Erythrocyte membrane metabolism G6PD protect cells from oxidant stress Deficiency results in hemolysis on exposure to oxidant stress e.g. drugs Sex-linked affecting males; females are carriers Pyruvate kinase deficiency – patients unable to generate adequate ATP Pyruvate kinase (PK) deficiency Red blood cells unable to generate adequate amounts of ATP Red cells become rigid and haemolyse easily Clinical symptoms of anaemia (chronic) are mild as block in metabolism lead to increased production of 2,3-DPG that facilitate oxygen release Folate and B12 necessary for DNA synthesis B12 deficiency cause folate to be trapped in methyl form The resultant deficiency in the methylene FH4 affects DNA synthesis RNA synthesis is unaffected Large red cells with nuclear retention formed Check Point Membrane proteins Defective spectrin, actin, ankyrin or glycophorin composition could result in changes in cell shape e.g. spherocytosis or elliptocytosis. Pathological Changes in Erythrocyte Morphology Pathological Changes in Erythrocyte Morphology Anisocytosis –generally caused by anaemia Macrocytes – RBCs are larger than usual Microcytes – small, hypochromic red blood cells Hypochromic cells – iron deficiency anaemia; low Hb levels; gastrectomy, vit. C deficiency Pathological Changes in Erythrocyte Morphology Spherocytes – RBCs are round instead of biconcave because they have lost pieces of their cell membrane – May be evidence of hemolysis; – Indicative of increased RBC destruction because spherocytes are sequestered and destroyed by mononuclear phagocyes in the spleen, liver & other organs. Leptocytes - are pale-staining with hemoglobin confined to a thin, flat, cell membrane; seen in thalassemias. Normoblasts may be seen in erythroblastosis fetalis; cancer that has spread to the bone marrow; condition causing excessive breakdown of Hb (thalassemia. Check Point Complete Blood Count (CBC) A CBC measures the different cells that make up your blood, including: Red blood cells White blood cells Hemoglobin Platelets Typical Red Blood Cell Parameters Haematocrit (Packed Cell Volume) Mean Corpuscular Volume (MCV) Mean corpuscular hemoglobin (MCH) - a measurement of the average amount of hemoglobin in each red blood cell. Mean Corpuscular Haemoglobin Concentration (MCHC) - a measurement of the average amount of hemoglobin in a single red blood cell (RBC). – The MCHC test measures the concentration of hemoglobin in a RBC relative to the size of the cell itself. MCHC is a calculation that helps describe how much space inside of each RBC is made of hemoglobin. Red Cell Distribution Width Hematocrit Haematocrit: the ratio of the volume of red blood cells to the total volume of blood. Determined by centrifugation the percentage by volume of red cells in the blood. Reference ranges – Male 42 - 52% – Female 36 - 46% Packed Cell Volume (PCV) or Hematocrit Modern instruments calculate PCV from RBC count and average cell size Characteristics of Human RBCs Types of Anaemia Characteristics of Human RBCs Increased erythropoietic activity seen as reticulocyte count greater than 2.5% The blood cells enter the circulation by a process of diapedesis Diagnoses of Blood Disorders Multiple tests are used to aid in the diagnoses and treatment of blood disorders: – Complete blood count (CBC): Measures all blood components and helps diagnose anemia, inflammatory diseases, and blood cancer – Platelet count: Usually conducted as a component of CBC and is used to diagnose bleeding disorders – Blood smear: A drop of blood is collected and smeared across a microscope slide. This test diagnoses diseases like leukemia, anemia, and malaria. – Blood enzyme tests – Coomb’s Test – Bone marrow biopsy Requirements for Synthesis of Haemoglobin The production of 2 polypeptide chains of globin The production of 4 moleules of the Oxygen-binding, compound haeme. One haeme molecule is inserted into each globin polypeptide chain – The synthesis of haeme requires: (i) The production of the tetrapyrrole ring compound protoporphyrin (ii) the binding of an atom of iron to the protoporphyrin molecule. Synthesis of Haemoglobin The developing normoblast possesses everything it needs o make haemoglobin except iron. The plasma transport protein, transferitin must deliver iron to the normoblast. Daily Iron Cycle Iron Cycling 70 Kg man requires 21 mg of iron daily for HB synthesis A 3-fold rate in the increase of erythropoiesis requires 63 mg iron daily. Sources of iron: – Breakdown of senescent RBCs – Bodily iron stores Lack of iron for increased haemoglobin synthesis limits the increase in erythropoiesis. As haeme accumulates, it decreases further protoporphyrin synthesis through feedback inhibition of production of the first enzymes of the reaction sequence, delta-ALA synthetase. Types of Haemoglobin HbA – 2  2 β (95%) HbF – 2  2 γ (0.5-0.8%) HbA 2 – 2  2 δ (2-3%) Haemoglobin Normal adult Hb A – 2  globin chains – 2 β globin chains – Heme: porphyrin plus Fe++ Minor Hb’s 2  + 2 δ : Hb A2 2  + 2 γ: Hb F Hemoglobin Consists of: – 4 globin molecules: Transport carbon dioxide (carbonic anhydrase involved), nitric oxide – 4 heme molecules: Transport oxygen Iron is required for oxygen transport Types of hemoglobin Oxyhemoglobin: O2 Reduced (deoxy) hemoglobin: no O2 Methemoglobin: oxidized Carboxyhemoglobin: 218 times greater affinity to CO than O2 Sulfhemoglobin: sulfur drugs Haemoglobin Pathology Haeme production problem: porphyria Fe++ problems: hemochromatosis Globin problem: sickle cell disease, thalassemia HAEMOGLOBINOPATHIES These are disorders due to abnormalities of globin prodution 2 main subgroups: – (i) characterized by the production of abnormal haemoglobins – (ii) abnormality is due primarily to suppression of normal globin production. HAEMOGLOBINOPATHIES Qualitative – substitution of one amino acid molecule e.g. sickle cell gene Quantitative – defective rate of globin chain synthesis e.g. thalassaemias THALASSAEMIAS o- thal major – complete absence of  chain synthesis. Death occurs in utero. +- thal minor – Absence of one  chain. Moderate anemia - thal carrier – No anemia βo- thal major – Severe anemia β+- thal minor – Mild anemia Sickle Cell Anaemia Valine replaces glutamic acid on 6th position on the β polypeptide chain Membrane looses deformability and becomes sickled shaped Could cause obstruction in the microcirculation Inheritance is autosomal, recessive. Check Point