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Gas in Blood
Physiology and Pharmacology
| Question | Answer |
|---|---|
| O2 carriage by blood - dissolved | Solubility of O2 in water is 0.23 ml/l/kPa For arterial Po2 of 14 kPa Dissolved O2 = 3 ml/l So 15 ml/min - not enough to supply 250 ml/min demands |
| Haemoglobin | Protein of molecular weight 64500 Tetramer of 4 subunits each of molecular weight 16000 For oxygen binding the central components are the 4 heme structures, one associated with each chain |
| Heme | Pyrrole ring - 4 Pyrrole rings held together by methene bridge - symmetrical structure Crucial item is iron ion in centre, held in place by two ionic and two covalent bonds Oxygen binds reversibly to iron 2+ - oxygenation |
| Sigmoid Hb-O2 dissociation curve | Myoglobin reacts with a single O2 with a hyperbolic dissociation curve Haemoglobin, which reacts with 4 O2 molecules is sigmoid due to cooperativity - intermediate states of oxidation can occur |
| Shifts in O2 dissociation curve | Right shift cause by increased temp, increased 2,3 DPG, decreased pH and increased CO2 2,3 DPG is essential for maintaining P50 |
| Bohr effect | Physiological - increase in CO2 and H+ in systemic capillaries helps unload O2 Effect is small at rest - important in exercise Helps unload O2 at tissues and reload at lungs |
| Oxygen delivery | Rate at which O2 is delivered to systemic circulation - typically around 926 ml/min Combines both dissolved O2 and that bound to Hb In intensive care - keep )2 delivery above 600 ml/min |
| Oxygen consumption | The difference between O2 delivery from lung and O2 return to lung Roughly 250 ml/min Hb returns to lungs still 70% saturated - gives a reserve of O2 for exercise etc |
| Causes of inadequate O2 uptake | Hypoxaemia - low Pao2 and Sao2 due to low inspired O2 Stagnant hypoxia - low Q e.g. cardiac failure Anaemic hypoxia - low Hb - good saturation but not enough Histotoxic hypoxia - high Pvo2 and Svo2 due to inability to use O2 |
| Body's oxygen stores | Not good - only around 1550 ml Apnoea with sealed airway at FRC - unconsciousness within 90s With open airway and pure oxygen you do not need to breathe for O2 diffusion - only to move CO2 |
| CO2 carriage | Carried in blood in solution and reacted to form bicarbonate and carbamino CO2 10% as dissolved CO2 85% as bicarbonate 5% as carbamino CO2 |
| Dissolved CO2 | Solubility of CO2 is 5.2 ml/l/kPa Dissolved CO2 to lungs is 156 ml This is below resting CO2 production of 200 ml/min |
| Carbamino CO2 | Amino groups at the end of proteins combine directly with CO2 Most is Hb - 4 amino termini gives 8 mM CO2 Only around 20-40 ml/l blood is carried this way As it is affected by oxygenation it is important - Hb is better at binding CO2 than HbO2 |
| Bicarbonate and carbonic acid | A slow reaction catalysed by carbonic anhydrase in RNC and pulmonary endothelium H2CO3 broken down to give HCO3- as H+ is buffered via binding to Hb which drives the reaction forward |
| Chloride shift | As HCO3- is moved into a rbc to combine with H+ Cl- is moved out to maintain electrostatic equilibrium Occurs faster in rbcs due to the ready source of protons from Hb |
| Haldane effect | Dissociation curve for Hb-CO2 is higher in lower O2 saturations as Hb buffers protons better than HbO2 and Hb forms carbamino compounds better than HbO2 Makes the physiological dissociation curve steeper than at fixed So2 as this changes in lungs |
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