Gas exchange takes place in lungs during breathing.pptx

Gas Exchange

1. Gas exchange in lungs R. Rakesh Prabu MPT 1 st Year (Ortho)

2. Introduction Oxygen is essential for the cells. Carbon dioxide, which is produced as waste product in the cells must be expelled from the cells and body. Lungs serve to exchange these two gases with blood.

3. E xchange of respiratory gases in lungs In the lungs, exchange of respiratory gases takes place between the alveoli of lungs and the blood. Oxygen enters the blood from alveoli and carbon dioxide is expelled out of blood into alveoli. Exchange occurs through bulk flow diffusion Exchange of gases between blood and alveoli takes place through respiratory membrane .

4. Bulk Flow Bulk flow is the diffusion of large quantity of substances from a region of high pressure to the region of low pressure. It is due to the pressure gradient of the substance across the cell membrane . Partial pres sure of oxygen is greater in the alveolar air than in the alveolar capillary blood. So, oxygen moves from alveolar air into the blood through the respiratory membrane .

5. Partial pressure of carbon dioxide is more in blood than in the alveoli. So, it moves from the blood into the alveoli through the respiratory membrane

6. Factors influencing gas exchange Gas uptake from alveoli is determined by three factors 1. Diffusion properties of the alveolar-capillary membrane 2. Partial pressure gradient for the gas 3. Pulmonary capillary blood flow

7. 1.Diffusion properties of the alveolar-capillary membrane Alveolar-capillary membrane (also called respiratory membrane ) forms the blood-gas interface that separates blood in the pulmonary capillaries from the gas in the alveoli . 1. Diffusion of gases between alveolar air and pulmonary capillary blood takes place through alveolar-capillary membrane . 2. The alveolar-capillary membrane is exceedingly thin, and is mainly composed of alveolar epithelium, interstitial fluid layer, and capillary endothelium . 3. As the blood perfuses the alveolar capillaries and air ventilates the alveoli, oxygen and carbon dioxide move across the blood-gas interface by diffusion.

8. Layers of Alveolar-Capillary Membrane From interior of the alveoli to the capillary blood, the alveolar capillary interface consists of six layers. However, O2 from haemoglobin molecule in the red cells of pulmonary capillaries to enter into alveolar lumen (or the transport in the reverse direction), passes through 10 layers 1. Alveolar surfactant 2. Layer of alveolar epithelial cells 3. Basement membrane of alveolar epithelium 4. A thin layer of interstitial fluid 5. Basement membrane of capillary endothelium 6. Layer of capillary endothelial cells 7. Plasma 8. Red cell membrane 9. Intraerythrocyte fluid 10. Haemoglobin molecule

9. Thickness of alveolar-capillary membrane The thickness of alveolar-capillary membrane is normally 0.2 to 0.5μ . The thinness of the membrane allows easy diffusion of gases through it . Higher the thickness may cause lesser diffusion

10. Diffusing Capacity Diffusing capacity is defined as the volume of gas that diffuses through the respiratory membrane each minute for a pressure gradient of 1 mm Hg. Portion Layers Alveolar portion 1. Monomolecular layer of surfactant, which spreads over the surface of alveoli 2. Thin fluid layer that lines the alveoli 3. Alveolar epithelial layer, which is composed of thin epithelial cells resting on a basement membrane Between alveolar and capillary portions 4.Capillary portion Capillary portion 5. Basement membrane of capillary 6. Capillary endothelial cells

11. Diffusing Capacity for Oxygen and Carbon Dioxide Diffusing capacity for oxygen is 21 mL/minute/1 mm Hg. Diffusing capacity for carbon dioxide is 400 mL/minute/1 mm Hg. Thus , the diffusing capacity for carbon dioxide is about 20 times more than that of oxygen.

12. Factors Affecting Diffusion of Gases Diffusion of gases through alveolar-capillary membrane depends mainly on six factors . 1. Difference in partial pressure of gas on both sides of the membrane, Example- partial pressure gradient for the gas 2. Diffusing capacity of the membrane for the gas 3. Surface area of the membrane: Surface area for diffusion decreases in conditions like emphysema and increases in conditions in which there is more opening of number of capillaries as occurs in exercise .

13. 4. Solubility of the gas: Solubility of the gas in the membrane is an important factor for diffusion. For example, CO 2 being more soluble diffuses easily. 5. Thickness of the membrane: Diffusion is inversely proportional to the thickness of the alveolar-capillary membrane . When thickness is doubled the diffusion is halved . 6. Molecular weight of the gas: Diffusion is inversely proportional to the molecular weight of the gas . Thus, gas with smaller molecule diffuses easily through the membrane .

14. 2.Partial pressure gradient for the gas Diffusion Gradient The diffusion of gases depends on the difference of partial pressure of the individual gas across the alveolar-capillary membrane . This is also called as the diffusion gradient for the gas . For example, oxygen diffuses easily across the membrane because of a greater difference in PO2 between the alveoli and pulmonary capillaries (oxygen diffusion gradient ). Normally , the diffusion gradient for oxygen is about 60 mm Hg, which is the difference between the PO2 of the alveolar air (160 mm Hg) and the arterial blood ( 100 mm Hg). The diffusion gradient for carbon dioxide across alveolar-capillary membrane (PVCO2 – PACO2) is about 6 mm Hg , which is much lower than that of oxygen.

15. Gases are dissolved in the liquid when they are exposed to such liquids like plasma. In this dissolved state in the liquid, gases exert a partial pressure . 1. Henry’s law states that the amount of gas dissolved in a liquid at a given temperature is directly proportional to the partial pressure and the solubility of the gas . 2. Fick’s law explains the diffusion of gases across the alveolar-capillary membrane. Fick’s law states that the volume of gas diffusing per minute across a membrane is directly proportional to the membrane surface area , the diffusion coefficient of the gas, and the partial pressure difference of the gas, and inversely proportional to membrane thickness.

16. Diffusion Coefficient Diffusion coefficient is defined as a constant (a factor of proportionality), which is the measure of a substance diffusing through the concentration gradient. It is also known as diffusion constant. It is related to size and shape of the molecules of the substance.

17. The diffusion coefficient of a gas is directly proportional to its solubility and inversely proportional to the square root of its molecular weight. Therefore , a highly soluble molecule or a smaller molecule diffuses rapidly. For example, the diffusion coefficient of carbon dioxide in aqueous solutions is about 20 times greater than that of oxygen because of its higher solubility, even though it is a larger molecule than O2 .

18. Diffusion of Oxygen from Atmospheric Air into Alveoli Partial pressure of oxygen in the atmospheric air is 159 mm Hg and in the alveoli, it is 104 mm Hg. Because of the pressure gradient of 55 mm Hg, oxygen easily enters from atmospheric air into the alveoli

19. Diffusion of Oxygen from Alveoli into Blood When blood passes through pulmonary capillary, RBC is exposed to oxygen only for 0.75 second at rest and only for 0.25 second during severe exercise. So , diffusion of oxygen must be quicker and effective. Fortunately , this is possible because of pressure gradient. Partial pressure of oxygen in the pulmonary capillary is 40 mm Hg and in the alveoli, it is 104 mm Hg. Pressure gradient is 64 mm Hg. It facilitates the diffusion of oxygen from alveoli into the blood

20. Diffusion of Carbon Dioxide from Blood into Alveoli Partial pressure of carbon dioxide in alveoli is 40 mm Hg whereas in the blood it is 46 mm Hg. Pressure gradient of 6 mm Hg is responsible for the diffusion of carbon dioxide from blood into the alveoli

21. Diffusion of Carbon Dioxide from Alveoli into Atmospheric Air In atmospheric air, partial pressure of carbon dioxide is very insignificant and is only about 0.3 mm Hg whereas, in the alveoli, it is 40 mm Hg. So , carbon dioxide enters passes to atmosphere from alveoli easily.

22. 3.Pulmonary capillary blood flow Flow of blood in the pulmonary capillary significantly influences the oxygen uptake . 1. Normally, the transit time (time taken by the red cells to pass through the capillary), is approximately 0.75 sec , during which the gas tension in the blood equilibrates with the gas tension in the alveoli. 2. With increase in cardiac output, blood flow through the pulmonary capillaries increases that decreases the transit time. For example, nitrous oxide (N2O), an anesthetic gas easily diffuses across the alveolar-capillary membrane and equilibrates in 0.1 s.

23. 3. Thus, the only way to increase the transfer of N2O from alveolar air into the blood is by increasing the blood flow. The amount of N2O that can be taken up is entirely limited by blood flow, not by diffusion of the gas . Therefore, uptake of N2O is flow-limited . 4. On the other hand, diffusion of carbon monoxide (CO ) is different. CO also readily diffuses across the alveolar-capillary membrane, but due to its high affinity for hemoglobin it rapidly binds to hemoglobin . This decreases the partial pressure of CO in blood, for which the equilibrium for CO across the alveolarcapillary membrane is never reached in 0.75 s ( transit time ). Therefore, CO transfer is diffusion-limited ( not limited by the blood flow).

24. 5. The capillary oxygen tension equilibrates with alveolar oxygen tension well within the transit time, normally in one third of the available time, i.e. in 0.3 s. Thus, a wide safety margin is available for oxygen to ensure that the end-capillary PO2 is fully equilibrated with alveolar PO2. 6. During severe exercise, though the transit is reduced but still time is adequate to fully oxygenate the blood. Though , oxygen binds with hemoglobin less avidly than CO, oxygen equilibrates fast. Thus, oxygen uptake is flow limited.

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