GAS EXCHANGE

1. GAS EXCHANGE Dr. JAWAD NAWAZ

2. Diffusion • Random movement of molecules of gas by their own kinetic energy • Net diffusion from higher conc. to lower conc • Molecules try to equilibrate in all empty places

3. Partial pressure • The pressure exerted by the gas molecules on a surface In atmospheric air • PO2160mmHg • PCO 2 0.3mmHg • PN2 600mmHg

4. Pressure of gases dissolved in water and tissues • Partial pressure in fluid develop same way as in air • Partial pressure= conc. of dissolved gas/solubility coefficient HENRY’S LAW • Solubility coefficients of different gases O2=0.024 CO2=O.57 CO=0.018 N2=0.012 H=0.008 • Water solubility of CO2 20 times more than that of O2 • Partial pressure of carbon dioxide is less than one twentieth that exerted by oxygen.

5. Water vapor pressure • In airway passage air gets humidified, water vapors mixed up with inspired air • At body temp. 370C pH2O =47mm Hg • pH2O directly proportional to temperature • In fever pH2O is more

6. Rate of diffusion D=Δ P×A×S/d×√MW Δ P=Partial pressure difference A=cross-sectional area S=solubility of gas d= distance √MW=molecular weight • Diffusion coefficient=S/ √MW • Two gases at same partial pressure, rate of diffusion proportional to diffusion coefficient

8. Expired Air

9. Respiratory Unit • Respiratory Lobule • Respiratory bronchiole • Alveolar ducts • Atria • Alveoli

10. 300 millions alveoli • Diameter 0.2 milliliter • Sheet of flowing blood

11. Respiratory Membrane or Pulmonary Membrane Membranes of all the terminal portions of the lungs

12. Factors That Affect the Rate of Gas Diffusion Through the Respiratory Membrane • Thickness of membrane • Surface area of membrane • Diffusion coefficient • Partial pressure difference of the gas • Edema & Fibrosis • Emphysema • Solubility of gas/ √ Mol. Weight • partial pressure of gas in the alveoli and partial pressure of the gas in the pulmonary capillary blood

13. Diffusion Capacity Volume of a gas that will diffuse through the membrane each minute for a partial pressure difference of 1 mmHg • Diffusing capacity for oxygen 21 ml/min/mm Hg at rest 65 ml/min/mm Hg during exercise • Diffusing capacity for carbon dioxide 20 times more than O2 400 to 450 ml/min/mm Hg at rest 1200 to 1300 ml/min/mm Hg during exercise

14. Measurement of Diffusing Capacity • Alveolar Po2 • Po2 in the pulmonary capillary blood • Rate of oxygen uptake by the blood Diffusing capacity(DC) of CO=Volume of CO absorbed pCO DC of O2 = DC of CO × 1.23 = 17× 1.23= 21ml/min/mmHg

15. Ventilation – Perfusion Ratio The imbalance between alveolar ventilation and alveolar blood flow • Va Alveolar ventilation • Q Blood flow • Va/Q • When the ventilation(Va) is zero, yet there is still perfusion (Q) of the alveolus, Va/Q is zero • When there is adequate ventilation (Va) but zero perfusion (Q),Va/Q is infinity.

18. Physiological Shunt • When Va/Q is below normal • Shunted blood • Bronchial vessels • The total quantitative amount of shunted blood per minute is called the physiologic shunt • The greater the physiologic shunt, the greater the amount of blood that fails to be oxygenated as it passes through the lungs. • Lower part of lung Va/Q is 0.6 times below normal

19. Physiological Dead Space • When Va/Q is ∞ • Alveolar wasted ventilation or alveolar dead space • Anatomical dead space • The sum of these two types of wasted ventilation is called the physiologic dead space • When the physiologic dead space is great, much of the work of ventilation is wasted effort because so much of the ventilating air never reaches the blood • Upper part of Lung Va/Q 2.5 times more than normal