Lecture – 5 Dr. Zahoor Ali Shaikh 1

 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

 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

 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 4

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 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. 6

 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

 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

 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 mmHg 9

 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]. 10

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 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. 12

 SUMMARY  Blood leaving the lungs [oxygenated] P O mmHg P CO mmHg  Blood coming to the lungs from tissues [Venous Blood] P O mmHg P CO mmHg 13

14  Diffusion constant means ‘solubility of gas’.

 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 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. 15

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 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. 17

 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. 18

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 Composition of Inspired Air, Alveolar Air, and Expired Air  Partial Pressure of O 2 and CO 2 in Alveolar Air  Partial Pressure of O 2 and CO 2 in Oxygenated [Arterial] and in Deoxygenated [Venous] blood  Partial Pressure of O 2 and CO 2 in tissues  Factors Affecting the Gas transfer (Diffusion) across the Alveolar Membrane  Gas Exchange across the Systemic Capillaries  Solubility of O 2 and CO 2 21

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