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RESPIRATION. Gas Exchange. PARTIAL PRESSURES. In a mixture of gasses, the total pressure distributes among the constituents proportional to their percent of the total The concentration of a gas can therefore be expressed as its partial pressure. Oxygen = 21% Nitrogen = 79%.

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RESPIRATION


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    1. RESPIRATION Gas Exchange

    2. PARTIAL PRESSURES • In a mixture of gasses, the total pressure distributes among the constituents proportional to their percent of the total • The concentration of a gas can therefore be expressed as its partial pressure

    3. Oxygen = 21% Nitrogen = 79% Po2 = 160 mm Hg PN2 = 600 mm Hg Total Pressure (at sea level) = 760mm Hg Partial Pressures in air

    4. Effect of water vapor • As fresh air enters the nose and mouth it is immediately mixed with water vapor • Since the total pressure remains constant, the water vapor lowers the partial pressure of all other gases • For this reason, the PO2 is lowered to about 149 mmHg

    5. DEAD SPACE VOLUME • At the height of expiration, about 150ml of gas still occupies the respiratory tree • This “old gas” is necessarily mixed with the incoming fresh air and further lowers the PO2 to about 100 mmHg

    6. GAS EXCHANGE ACROSS PULMONARY CAPILLARIES Inspired Air PO2 = 160mmHg • Both oxygen and carbon dioxide diffuse down their concentration (partial pressure) gradients PCO2 = 0.03mmHg LUNG PO2 = 100mmHg PCO2 = 40mmHg PULMONARY CAPILLARIES PO2 = 40mmHg PCO2 = 46mmHg PO2 = 100mmHg PCO2 = 40mmHg

    7. GAS EXCHANGE ACROSS SYSTEMIC CAPILLARIES • Both oxygen and carbon dioxide diffuse down their concentration (partial pressure) gradients TISSUE PO2 < 40mmHg PCO2 > 46mmHg SYSTEMIC CAPILLARIES PO2 = 40mmHg PCO2 = 46mmHg PO2 = 100mmHg PCO2 = 40mmHg

    8. Carbon dioxide/Bicarbonate Relationship CO2 + H2O <---> H2CO3 <---> H+ + HCO3- Carbon dioxide dissolved in water readily combines with water to form carbonic acid. The carbonic acid then dissociates into the hydrogen ion and bicarbonate ion. The former reaction is catalized by and enzyme called Carbonic Anhydrase in many tissues.

    9. GAS TRANSPORT IN BLOOD • Oxygen physically dissolved = 1.5% • Oxygen bound to hemoglobin = 98.5% • Carbon dioxide physically dissolved = 10% • Carbon dioxide bound to hemoglobin = 30% • Carbon dioxide as bicarbonate = 60%

    10. HEMOGLOBIN/OXYGEN DISSOCIATION Resting PO2 Systemic Normal PO2 Capillaries % Hemoglobin Saturation PO2 of blood (mmHg)

    11. Agents which shift the Hb/O Dissociation curve: The Bohr Effect

    12. UNDERSTANDING THE HB/O DISSOCIATION CURVE • The plateau: Provides a margin of safety in the oxygen carrying capacity of the blood • The steep portion: Small changes in Oxygen levels can cause significant changes in binding. This promotes release to the tissues.

    13. Agents which shift the Hb/O Dissociation curve: The Bohr Effect

    14. Carbon dioxide/Bicarbonate Relationship CO2 + H2O <---> H2CO3 <---> H+ + HCO3- Carbon dioxide dissolved in water readily combines with water to form carbonic acid. The carbonic acid then dissociates into the hydrogen ion and bicarbonate ion. The former reaction is catalized by and enzyme called Carbonic Anhydrase in many tissues.

    15. Carbon Dioxide Transport in the Blood: At the tissues Tissue Cell Carbonic Anhydrase Red Blood Cell CO2 + H2O ---> H2CO3 ---> H+ + HCO3 + Hb --->HbH + Hb ---> HbCO2 HbO2 -----> Hb + O2

    16. Carbon Dioxide Transport in the Blood: At the lungs Alveolus Carbonic Anhydrase Red Blood Cell CO2 + H2O <--- H2CO3 <--- H+ + HCO3- + Hb <---HbH + Hb <--- HbCO2 HbO2 <--- Hb + O2

    17. The Haldane Effect • Removal of oxygen from hemoglobin increases hemoglobin’s affinity for carbon dioxide • This allows carbon dioxide to “ride” on the empty hemoglobin

    18. RESPIRATORY CONTROL • Pons: Pneumotactic center • Pons: Apneustic center • Medulla: Dorsal respiratory group • Medulla: Ventral respiratory group

    19. Medulla: Dorsal respiratory group • Inspiratory neurons • Pacemaker activity • Expiration occurs when these cease firing

    20. Medulla: Ventral respiratory group • Both inspiratory and expiratory neurons • Inactive during normal quiet breathing • Rev up inspiratory activity when demands for ventilation are high

    21. Pons: Pneumotactic center • Fine tuning over medullary centers • Switches off inspiration

    22. Pons: Apneustic center • Fine tuning over medullary centers • Blocks switching off of inspiritory neurons

    23. CARBON DIOXIDE CONTROLLS RESPIRATION • High carbon dioxide generates acidity of blood in brain • Acidity of blood in systemic circulation is prevented from directly influencing the brain due to the blood/brain barrier’s impermeability to H+ • CO2 + H2O <---> H2CO3 <---> H+ + HCO3

    24. OXYGEN LEVELS MUST FALL DRASTICALLY TO AFFECT BREATHING • Receptors in carotid bodies • Below 60 mmHg for oxygen partial pressure, breathing is stimulated • This is a last-ditch, fail-safe mechanism only!