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TRANSPORT OF OXYGEN AND OXYGENHEMOGLOBIN DISSOCIATION CURVE LEARNING OBJECTIVES By the end of this lecture the students should be able to: • • • • • • • Explain the steps of gas exchange in lungs and tissue Describe the process of oxygen diffusion Explain the transport of oxygen in blood. State the influence of oxygen on haemoglobin saturation. Describe the role of haemoglobin in oxygen transport. Explain the O2-Hb dissociation curve. Describe the factors affecting on O2-Hb dissociation curve. Lecture outline Overview of gas exchange Oxygen exchange in lungs and tissues • Diffusion through alveolar thin cells • Down diffusion gradient Higher in alveoli Lower in blood • Diffusion from blood to tissues Also down gradient First to ECF Next to tissue cells Oxygen transport in blood Influence of PaO2 on hemoglobin saturation • 98% saturated arterial blood contains 20 ml oxygen per 100ml of blood (20 vol %) • As arterial blood flow through capillaries , only 5 ml oxygen are released • Blood holds O2 reservoir Structure of hemoglobin Hemoglobin and its forms • 4 peptide subunits ( 2 + 2) – globin combines with 4 molecules of haem with Fe ++ • Hb A1 : 90 % of hemoglobin in adult • Hb A2: 2 – 3 % hemoglobin in adult • HbA1C : Glycosylated hemoglobin – an important marker to assess the blood glucose control • Hb F : 0.5% hemoglobin in adults – fetal hemoglobin , very high affinity with oxygen Role of hemoglobin in oxygen transport • Each hemoglobin molecule combines with 4 oxygen molecules in a rapid and reversible process • The hemoglobin - oxygen complex is called as OXYHEMOGLOBIN • Hemoglobin that has released oxygen is called as reduced hemoglobin Hemoglobin derivates unable to transport O2 • Methhemoglobin : Contains Fe3+ instead of Fe2+ • Carboxy hemoglobin – contains CO instead of O2 (cherry red in color) • Sulfa – hemoglobin- contains sulfur – green in color Oxygen –hemoglobin dissociation curve Saturation of hemoglobin • When all four haem are bound to oxygen , it is fully saturated 98 % saturated in alveolar arteries • When 1-3 haem molecules are bound to oxygen , then hemoglobin is called as partially bound hemoglobin Factor influencing hemoglobin saturation • The rate at which hemoglobin binds and releases oxygen depends on several factors: 1. Temperature 2. Blood pH 3. PCO2 level 4. 2,3-BPG BPG modify the structure of hemoglobin , alters its affinity for oxygenand enhances unloading. 2,3-bisphosphoglycerate • Very important for long term regulation of Hb affinity with O2 • 2,3-bisphosphoglycerate shunt is a pathway derived from glycolysis • Hypoxia stimulates 2,3 – BPG formation and thus improves O2 release Factor influencing hemoglobin saturation • Right shift means higher ability to Hb to release oxygen but lower ability to bind it • Only 20- 25 % of bound oxygen is unloaded during one systemic circulation • If O2 level in tissues drops: 1. More oxygen dissociates from Hb and is used up by cells 2. Respiratory rate or cardiac output need not to be increased Metabolic specialities of RBCs • • • • No organelle – no mitochondria Anaerobic glycolysis is the only source ATP formation 2,3- BPG shunt is unique for RBC 20% of glucose is metabolised via hexone monophosphate pathway