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Published byRhoda Fletcher Modified over 9 years ago
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Gas Exchange Partial pressures of gases Composition of lung gases Alveolar ventilation Diffusion Perfusion = blood flow Matching of ventilation to perfusion
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Partial Pressure of Gas P = nRT V gas equation P gas 1 + P gas 2 + P gas 3……= Total P (e.g. 760 mmHg) partial pressures of gases
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Gas Transfer
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. Henry’s law: conc dissolved gas partial pressure of gas x solubility index Also: Dissolved gas in equilibrium with gas combined with other agents e.g. hemoglobin Gases in liquids diffuse from area of higher partial pressure to area of lower partial pressure.
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P A > P B O2O2 CO 2 mixed venous blood venous 40 mmHg 100 mmHg P A O 2 100 mmHg 46 mmHg 40 mmHg P A CO 2 40 mmHg Pulmonary vein = arterial
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Gas diffusion cont. Rate of diffusion pressure difference surface area solubility index membrane thickness molecular weight Pressure difference:≈ 60 mmHg for O 2 ≈ 6 mmHg for CO 2 Surface area:large depends on ventilation: perfusion matching Solubility index if:O 2 = 1, CO 2 = 20 Membrane thickness:small
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Respiratory membrane
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Fig. is a scanning electron micrograph of mouse lung to show an interalveolar septum. Red blood cells
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A surface view of capillaries in the alveolar wall. Cross-sectional view of the alveolar wall and their vascular supply.
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Diffusing capacity: = volume of gas that diffuses through respiratory membrane per minute for each mmHg of gas partial pressure difference with exercise — capillary diameter ~ better matching of ventilation to perfusion (blood flow)
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Alveolar Ventilation Air in alveoli + airways Dead space = air not involved in gas exchange ~ anatomical: air in airways (in ml ≈ body weight in pounds) ~ alveolar: air in alveoli with blood supply little gas exchange ventilation wasted very small normally V T = V A + V D tidal volume air to alveoli dead space 500 ml = 350 ml + 150 ml
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Minute/total ventilation = V T breaths / min Alveolar ventilation = VAVA = (V T V D ) breaths / min (500 150 ) 12 ≈ 4.2 L / min
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Ventilation : Perfusion Matching Pulmonary circulation flow rate in systemic circulation = flow rate in pulmonary circulation blood pressure in pulmonary circulation mean ≈ 15 mmHg << blood pressure in systemic circulation mean ≈ 100 mmHg
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Even with exercise Mean P < 30 mmHg
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Shunts = blood that returns to L side of heart without being oxygenated ~ anatomical: ≈ 2% directly to heart from central chest tissues ~ physiologic / pathologic: blood from areas of lung where blood flow > airflow so some blood flow wasted
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Factors which tend to disturb ventilation / perfusion ratio: non-uniform pulmonary blood flow non-uniform ventilation pathological gravity effects
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Effects of Gravity at normal breathing volumes: 1. change in volume at lung bases is greater than at apices i.e. alveolar ventilation > alveolar ventilation (at bases)(at apices) 2. blood flow (perfusion) > blood flow (perfusion) (at bases)(at apices) but Overall Baseperfusion > ventilation Apexventilation > perfusion
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Protective reflexes which match ventilation with perfusion: 1.Hypoxic pulmonary vasoconstriction – NB if blood flow > airflow 2. PCO 2 control of airway smooth muscle if airflow > blood flow PO 2 in lung tissue vasoconstriction blood flow PCO 2 in lung tissue broncho-constriction air flow
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Gas Exchange (summary) Alveolar vs total ventilation Alveolar gas composition (partial pressure) Gas transfer partial pressure of gas in liquid rate of gas diffusion diffusing capacity pulmonary circulation ventilation : perfusion matching dead space shunts protective reflexes
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