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Bohr Effect

Bohr Effect. Normal dissociation curve An increase or decrease in acidity, temperature, or CO 2 concentration causes a shift in the curve This is due to an alteration in the molecular structure of Hb

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Bohr Effect

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  1. Bohr Effect • Normal dissociation curve • An increase or decrease in acidity, temperature, or CO2 concentration causes a shift in the curve • This is due to an alteration in the molecular structure of Hb • Specifically, the Bohr effect is the presence of H+ ions in contracting muscle unloads O2 from Hb

  2. Bohr effect is the reduced effectiveness of hemoglobin to hold O2, especially in PO2 ranges of 20-50 mm Hg • @PO2 in alveoli, Bohr effect in pulmonary capillary blood is negligible • Allows Hb to load completely with O2 as the blood passes through the lungs, even during maximal exercise

  3. Red-Blood-Cell 2,3-DPG • 2,3-diphophoglycerate is produced within the RBC during glycolysis (anaerobic) • Binds loosely with subunits of Hb molecule • Reduces the affinity for O2, shifting the curve • Enhances the unloading of O2 in the tissue • Unlike the response of H+ ions to unload O2 quickly, 2,3-DPG operates at a slower rate, allowing adaptations to gradual changes in O2 availability

  4. If PO2 decreases, more O2 is released to the tissues • High levels of 2,3-DPG in RBCs for those who live at high altitudes and those with cardiopulmonary disorders • Half-life is small, ~6 hours if return to low altitudes • Endurance training may increase 2,3-DPG after maximal exercise of short duration, while training has no benefit during prolonged, steady-rate exercise • Females appear to have higher levels, may compensate for lower Hb levels

  5. Myoglobin • Iron-protein found in skeletal and cardiac tissue • High concentration in red or oxidative fibers • Similar to Hb in its ability to combine reversibly with O2 • Each myoglobin molecule contains only one iron atom, carries one O2 molecule • Mb + O2 MbO2

  6. Oxygen release at low pressures • Myoglobin is an extra source of oxygen in muscle, may facilitate the transfer of O2 to the mitochondria • Especially in the beginning of exercise and during intense exercise, when there is a sharp drop in cellular PO2 • Dissociation curve is rectangular hyperbola • Myoglobin binds and retains oxygen at low pressures much more readily than Hb

  7. Greatest release of oxygen occurs from MbO2 when the tissue PO2 drops to 5 mm HG or less • Myoglobin has no Bohr effect

  8. Training • Slow twitch or slow oxidative fibers have more myoglobin • Myoglobin is some mammals is related to activity level • Results in humans are unclear

  9. CO2 transport in blood • Dissolved (~10%) • 20% CO2 bound to hemoglobin (carbamino-hemoglobin) • 70% as bicarbonate

  10. Regulation of blood pH • Buffer system – seconds • Phosphate buffer system • Carbonic/carbonate system • Blood proteins, esp. Hb • Respiratory system – minutes • Ventilation rate is controlled to keep sufficient CO2 in blood to maintain pH • Kidneys – days • Excrete bicarbonate (HCO3-) at a rate that optimizes pH

  11. Functions of the respiratory system • Delivery of O2 to tissues • Disposal of CO2 produced by the tissues • Maintenance of a stable blood pH @7.4

  12. Control of ventilation during exercise • Humoral stimuli: changes in physical and chemical properties in blood from normal values at rest • Neural stimuli: originates in the brain center – Respiratory Center • Mental conditions, e.g., emotions • Inflation and deflation (stretch) of the lungs • Muscle contraction and limb movement or tension development

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