1. RESPIRATORY SYSTEM LECTURE-5 (GAS EXCHANGE) Dr. Mohammed Sharique Ahmed Quadri Assistant Prof. physiology Al maarefa college 1

2. Gas Exchange Gas Exchange takes place in alveoli and then at tissue level.  Why we are breathing? To provide a continuous supply of oxygen to tissues. Gas Exchange in Alveoli It is diffusion of O 2 and CO 2 through a permeable membrane of alveoli. It is down the partial pressure gradients of O 2 and CO 2. 2

3. Partial Pressures First we will see Composition of Atmospheric Air (Inspired Air ) - Nitrogen (N 2 ) – 79% - Oxygen (O 2 ) – 21% - Carbon dioxide (CO 2 ) – 0.04% [0% for practical purpose] - Water Vapor - Other gases and pollutants These gases exert an atmospheric pressure of 760mmHg at sea level. 3 NEGIGIBLE PERCENTAGE

4. Partial Pressure  What is Partial Pressure [P gas ] ? It is the pressure exerted by each gas in a mixture of gases, is known as ‘Partial Pressure’ of gas.  NOTE – Gases dissolved in a liquid such as blood also exert a partial pressure. Greater the partial pressure of the gas in the liquid, more of that gas is dissolved. 4

5. Partial Pressures The pressure exerted by a particular gas is directly proportional to the percentage of that gas in a total air mixture. Example: N 2 is 79%, - Partial Pressure of N 2 will be 79% of 760 = (79 * 760)/100 = 600mmHg - O 2 is 21% = (21*760)/100 = 160mmHg 5

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7. Partial Pressure Gradient A difference in partial pressure between capillary wall and surrounding structure is known as ‘Partial Pressure Gradient’. Partial Pressure Gradient exist between alveolar air and pulmonary capillary blood.  NOTE – A gas always diffuses down its partial pressure gradient i.e. from area of high partial pressure to area of low partial pressure. 7

8. Partial Pressure We will see the Alveolar Air Composition and Partial Pressure of O 2 and CO 2 in the alveolar air. Alveolar Air Partial Pressure O 2 – 14% - Partial pressure 100mmHg CO 2 – 6% - Partial pressure 40mmHg N 2 – 80% - Partial pressure 573mmHg H 2 O – 47mmHg ( Total Pressure = 760mmHg ) 8

9. 9 VALUES OF PO 2 AND PCO2 IN INSPIRED AIR, ALVEOLI,TISSUE CELLS AND BLOOD

10. P O 2 and P CO 2 Gradients Across Pulmonary Capillaries As blood passes through the lungs, it picks up O 2 and gives CO 2 by diffusion down the partial pressure gradient between blood and alveoli. Pulmonary Capillary has Venous blood [Deoxygenated]. Pulmonary Capillary blood P O 2 – 40mmHg P CO 2 - 46mmHg 10

11. P O 2 and P CO 2 Gradients Across Pulmonary Capillaries As blood flows through Pulmonary capillaries, it is exposed to alveolar air. As alveolar air has P O 2 at 100mmHg, which is higher than P O 2 of 40mmHg in the blood of Pulmonary capillaries [Venous blood]. Therefore O 2 diffuses from alveoli to capillary blood till no further gradient exist i.e. P O 2 in blood capillary becomes 100mmHg [oxygenated blood]. 11

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13. P O 2 and P CO 2 Gradients Across Pulmonary Capillaries Partial Pressure of CO 2 in Pulmonary capillary [Venous blood] is 46mmHg, whereas alveolar P CO 2 is only 40mmHg, therefore, CO 2 diffuses from the blood into the alveoli. Blood leaving the Pulmonary capillary has P CO 2 of 40mmHg. 13

14. P O 2 and P CO 2 Gradients Across Pulmonary Capillaries  SUMMARY Blood leaving the lungs [oxygenated] P O 2 - 100mmHg P CO 2 - 40mmHg Blood coming to the lungs from tissues [Venous Blood] P O 2 - 40mmHg P CO 2 - 46mmHg 14

15. Diffusion constant means ‘solubility of gas’. 15

16. Gas Diffusion Applied Aspect In Emphysema – there is loss of alveolar walls, resulting in large but less alveoli, therefore lung surface area decreases, therefore, decreased gas exchange. Pulmonary fibrosis --Increased thickness of alveolar membrane due to pulmonary fibrosis leads to decrease exchange of gases.  NOTE – Diffusion constant of CO 2 is 20 times more than O 2 in alveolar membrane as CO 2 is more soluble, therefore, in diseased lung O 2 transfer is more seriously affected than CO 2 transfer. 16

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18. Gas Exchange across Systemic Capillaries Gas exchange across systemic capillaries also occurs down partial pressure gradient. In Systemic capillaries O 2 and CO 2 move to tissues by diffusion down their partial pressure gradients. 18

19. Gas Exchange across Systemic Capillaries Arterial Capillary P O 2 – 100mmHg Arterial Capillary P CO 2 – 40mmHg Oxygen is given to tissues and CO 2 diffuses from tissues to blood. Tissue - P O 2 – 40 - P CO 2 – 46 Venous Capillary Blood - P O 2 – 40 - P CO 2 – 46 This blood [Venous] goes to right side of the heart and then to lungs to get oxygen and give out CO 2 and cycle repeats. 19

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21. Composition OF INSPIRED, Alveolar, EXPIRED AIR 21

22. References Human physiology by Lauralee Sherwood, seventh edition Text book physiology by Guyton &Hall,11 th edition Text book of physiology by Linda.s contanzo,third edition

23. Thank you 23