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Bohr Effect

Bohr Effect. Alterations in hemoglobin’s structure Shift to the right in the oxyhemoglobin dissociation curve Loading of O 2 is not affected the flat upper portion is not altered Unloading of O 2 is enhanced along steep lower portion, more O 2 is unloaded at a given PO 2 with the shift.

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Bohr Effect

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  1. Bohr Effect • Alterations in hemoglobin’s structure • Shift to the right in the oxyhemoglobin dissociation curve • Loading of O2 is not affected • the flat upper portion is not altered • Unloading of O2 is enhanced • along steep lower portion, more O2 is unloaded at a given PO2 with the shift

  2. Myoglobin and Muscle Oxygen Storage • Skeletal & cardiac muscle contain compound myoglobin. • Each myoglobin contains only one heme in contrast to 4 in hemoglobin (Hb). • Myoglobin binds and retains O2 at low pressures. • Facilitates oxygen transfer to mitochondria at start of exercise and intense exercise when cellular PO2↓ greatly.

  3. Carbon Dioxide Transportin the Blood • Dissolved in plasma • CO2 is 20 times more soluble than O2 • 7% to 10% of CO2 is dissolved • Combined with amino compounds • hemoglobin is most common • Haldane effect: Hb’s de-oxygenation enables bind CO2 • about 20% of CO2 is carried as carbamino compounds • Bicarbonate • about 70% carried as bicarbonate

  4. Carbon Dioxide Transportin the Blood

  5. Formation of Bicarbonate at Tissue Level • CO2 diffuses into RBC • Enzyme, carbonic anhydrase, absent in plasma but present in RBC drives reaction of CO2 + H2O => H2CO3 • H2CO3 dissociates a proton =>HCO3- + H+ • CO2 + H2O => H2CO3 => HCO3- + H+ • HCO3- moves into plasma via HCO3- / Cl-anion exchanger to prevent electrical imbalance • Hb acts as buffer and accepts the H+

  6. Bicarbonate in the Lungs • In lungs, carbon dioxide diffuses from plasma into alveoli; lowers plasma PCO2. • HCO3- + H+recombine to form carbonic acid. • H2CO3dissociates to H2O and CO2, allowing carbon dioxide to exit through the lungs. • CO2 + H2O <= H2CO3 <= HCO3- + H+

  7. Ventilatory Regulation Two factors regulate pulmonary ventilation: • Neural input from higher brain centers provides primary drive to ventilate • Gaseous and chemical state of blood: humoral factors

  8. Pulmonary Ventilation Control • Clusters neurons in medulla oblongata referred to as respiratory center. • Inspiratory center activates diaphragm & intercostals. • Expiratory center inhibits inspiratory neurons. • Stretch receptors assist regulation of breathing • Pneumotaxic & apneustic centers contribute (depth).

  9. Humoral Factors • Chemoreceptors are specialized neurons. • Chemoreceptors monitor blood conditions, provide feedback • Peripheral located in aortic arch and bifurcation of common carotid respond to CO2, “temperature”-no, H+ • Central located in medulla affected by PCO2 & H+ • Specialized receptors in lungs sensitive to stretch and irritantsact to provide feedback • Interaction among factors controls ventilation • CO2 production is closely associated with ventilation rate

  10. Receptor Location and Function • Central chemoreceptors located within the medulla • respond to changes in PCO2 & H+ in cerebral spinal fluid • ventilation increases with elevations of PCO2 or H+

  11. Receptor Location and Function • Peripheral chemoreceptors located in aortic arch and common carotid arteries • respond to changes in PO2, PCO2 and H+ • at sea level changes in PO2 have little effect on VE

  12. Ventilatory Control at Rest • Carbon dioxide pressure in arterial plasma (PaCO2) provides the most important respiratory stimulus at rest. • Urge to breathe after 40 s breath-holding results mainly from increased arterial PCO2. • Hyperventilation decreases Alveolar PCO2 to 15 mm Hg, which decreases PaCO2 below normal, allows longer breath holding.

  13. Ventilatory Control in Exercise • Very rapid increase at start of exercise • Chemical stimuli cannot explain initial hyperpnea during exercise. • Nonchemical factors mediate the rapid response • Cortical: motor cortex • Peripheral: mechanoreceptors in joints, tendons and muscles

  14. Integrated Response • Control of breathing is not result of a single factor but of combined result of several chemical and neural factors. • Composite of ventilatory response to exercise.

  15. References • Axen and Axen. 2001. Illustrated Principles of Exercise Physiology. Prentice Hall. • Kapit, Macey, Meisami. 1987. Physiology Coloring Book. Harper & Row. • McArdle, Katch, Katch. 2006. Image Collection Essentials of Exercise Physiology, 3rd ed. Lippincott William & Wilkens.

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