Main Topic: Diffusion of Gases Across Pulmonary Membrane
by Baraaz minding
1. 1. Pressure of Gas Mixture at Sea Level
2. - Water vapour partial pressure = 47 mmHg at 37°C
3. 2. Partial Pressure of the Gas (Gas Tension)
4. - Calculation: (Atmospheric pressure - water vapour partial pressure) = 713 × % of the gas in the gas mixture.
5. 3. Structure of Pulmonary Membrane (Respiratory membrane / Alveolo-capillary membrane)
6. - It is composed of 6 layers, 2 of them are fluid.
7. - Gaseous exchange occurs by simple diffusion
8. A. Pressure Gradient of Gases Across the Membrane
9. - Pressure gradient increases during muscular exercise because:
9.1. 1. PO₂ is increased in alveolar air due to hyperventilation
9.2. 2. PO₂ is decreased in venous blood due to increased O₂ consumption by tissues
10. - Venous blood equilibrates completely with tissues in the same way.
11. - In the body, respiratory gases diffuse through aqueous media e.g., pulmonary membrane, plasma membrane, and RBC wall.
12. - Thus, in lung diseases, diffusion of O₂ is affected earlier than that of CO₂
12.1. → Hypoxia occurs many years before hypercapnia
13. - It is the area through which diffusion of gases takes place
14. - Increases in muscular exercise due to:
14.1. 1. More expansion of alveoli
14.2. 2. Opening of closed capillaries
14.3. 3. Dilation of already opened capillaries
15. - Atmospheric pressure (Barometric pressure) at sea level = 760 mmHg
16. - Total pressure of a gas mixture at sea level = 760 - 47 = 713 mmHg
17. - Definition: The pressure exerted by the gas when present in a gas mixture.
18. - Example:
18.1. - In alveolar air:
18.1.1. - CO₂ = 5.6% → Partial pressure of CO₂ (PCO₂) = 713 × 5.6% = 40 mmHg
18.1.2. - O₂ = 14% → Partial pressure of O₂ (PO₂) = 713 × 14% = 100 mmHg
19. - It is the membrane through which gas exchange between alveolar air and blood in pulmonary capillaries takes place.
20. 4. Factors Affecting Diffusion of Gases Through Pulmonary Membrane
21. (1) Directly Proportional with:
22. - Alveolar air vs. venous blood:
22.1. - PO₂ (normal):
22.1.1. - Alveolar air = 100 mmHg
22.1.2. - Venous blood = 40 mmHg
22.1.3. - Pressure gradient = 60 mmHg
22.2. - PCO₂ (normal):
22.2.1. - Alveolar air = 40 mmHg
22.2.2. - Venous blood = 46 mmHg
22.2.3. - Pressure gradient = 6 mmHg
23. - Arterial blood leaving the lungs is in complete equilibration with alveolar air
23.1. - Because the blood spends sufficient time in pulmonary capillaries for equilibration
23.1.1. - 0.75 seconds during rest
23.1.2. - 0.4 seconds during severe exercise
23.1.3. - Blood needs only:
23.1.3.1. - 0.3 sec in case of O₂
23.1.3.2. - 0.07 sec in case of CO₂ to be fully equilibrated with alveolar air.
24. B. Solubility in Aqueous Media
25. - In aqueous media, CO₂ diffuses about 20 times as rapidly as O₂
25.1. - This is because solubility of CO₂ in water is greater than O₂
26. C. Surface Area of Pulmonary Membrane
27. - Normally about 70 square meters (70 m²)
28. - Decreased in:
28.1. 1. Old age (due to decreased number of capillaries)
28.2. 2. Lung diseases whether:
28.2.1. - Obstructive (e.g., emphysema)
28.2.2. - Restrictive (e.g., lung collapse)