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1、2019/4/29,1,5.2. Gas Exchange,Definition Diffusion of oxygen and carbon dioxide between the alveoli and the blood inside the pulmonary capillaries. What we have to know? Basic principles of gas diffusion Factors that determine the rate of gas diffusion,2019/4/29,2,Partial Pressure of Individual Gase
2、s in a Gas Mixture,Above the absolute temperature, all molecules of all matter are continually undergoing motion. Pressure is caused by multiple impacts of moving molecules against a surface either in air, liquid or other forms of matter. The partial pressure is directly proportional to the concentr
3、ation of the gas molecule for air.,2019/4/29,3,Applied Gas Laws,Question At the sea level (barometric pressure is 760 mmHg), how much is the partial pressure of oxygen if we know air consists of 21% oxygen? Hints Daltons law states that the total pressure exerted by a gaseous mixture is equal to the
4、 sum of the partial pressures of each gas component in the gas mixture. In a mixture of gases, the partial pressure of a given gas is directly proportional to the volume that gas contributes to the total volume.,2019/4/29,4,Answers,PB = PN2 + PO2 + PHO2 + PCO2 (at the sea level, PB = 760 mmHg). Wher
5、e PB = barometric pressure at the sea level. Where PN2 is the partial pressure of nitrogen. PO2 is the partial pressure of oxygen (氧分压). PO2 in the air = 760 mmHg X 0.21 = 160 mmHg .,2019/4/29,6,Partial Pressures of Individual Gases at the Seal Level (PB = 760 mmHg),2019/4/29,7,Water Vapor (水蒸汽） Low
6、ers PO2 once Air Is Being Humidified（湿化）,Air is being warmed and humidified before reaching alveoli. Water vapor does not change the percentage of O2 or N2 but does low the partial pressure of O2 or N2. In humidified air, PO2= (760 47) X 0.21 = 150 mmHg PN2 = (760 47) X 0.79 = 563 mmHg,2019/4/29,8,T
7、he Partial Pressure of Oxygen in the Alveolar Space is down to 102 mmHg,Two reasons cause this “down” The existence of FRC results in dilution of air in the alveoli. The continuous uptake of oxygen by the blood in pulmonary capillaries.,2019/4/29,9,The Determinants of Alveolar PO2,2019/4/29,10,Facto
8、rs that Determine Alveolar PCO2,2019/4/29,11,Diffusion of Gases through the Respiratory Membrane,2019/4/29,12,2019/4/29,12,Alveolar-Capillary Membrane (Respiratory Membrane),2019/4/29,12,Six layers construct a barrier Alveolar fluid layer Alveolar epithelium Alveolar basement membrane Interstitial m
9、atrix Capillary basement membrane Capillary endothelium,2019/4/29,13,Gases Diffuse according to Their Concentration Gradients,Blood stays in the pulmonary capillaries for 0.75 seconds. It takes 0.25 sec for O2 to reach the equilibrium. PO2 in the pulmonary capillaries, pulmonary vein, and systemic a
10、rteries should be close or identical to that in alveolar space.,2019/4/29,14,Why is Arterial PO2 Lower than That of Alveolar Air?,Two contributors: a shunt and a low ventilation/perfusion ratio. The conducting airways have a separate circulation, called bronchial circulation. About half of venous bl
11、ood from the bronchial circulation returns to the pulmonary veins, resulting in an admixture of unoxygenated blood and oxygenated blood and a drop in arterial PO2.,2019/4/29,15,Quantitative Description of Diffusion,Ficks law: D = P.T.A.S/d.(MW)1/2 Where D = diffusion rate P = partial pressure gradie
12、nt (driving force) T = temperature A = membrane surface area S = solubility of gas in the blood d = distance of diffusion (the thickness of membrane); MW = molecule weight of the gas. S/(MW)1/2 is also defined as diffusion coefficient (扩散系数).,2019/4/29,16,Between Oxygen and Carbon Dioxide, Which Gas
13、 Diffuses Faster?,Conclusions At the same pressure gradient, carbon dioxide diffuses 24.3 times faster than oxygen. Under physiological pressure gradients, carbon dioxide will diffuse 2.35 times faster than oxygen.,2019/4/29,17,Factors that Affect Gas Diffusion Rate across the Respiratory Membrane,1
14、. The partial pressure gradient between two sides of the membrane Directly proportional 2. The diffusion coefficient of the gas 3. The thickness of respiratory membrane. 4. The surface area of respiratory membrane. Remove of the lung, emphysema,2019/4/29,18,Factors That Affect Partial Pressure Gradi
15、ent,Alveolar ventilation Larger alveolar ventilation helps to reduce the buffering power of FRC, enhancing PAO2 and reducing PACO2. Pulmonary blood flow Increased pulmonary blood flow rises O2 and CO2 partial pressure gradient. The kinetics of hemoglobin binding to oxygen and carbon dioxide.,2019/4/
16、29,19,Diffusing Capacity of the Respiratory Membrane,It is a parameter that is used for measure the ability of the respiratory membrane to exchange a gas between the alveoli and the pulmonary blood. Definition The volume of a gas that will diffuse through the respiratory membrane each minute for a partial pressure difference of 1 mm Hg.,2019/4/29,20,The Diffusing Capacity of the Respiratory Membrane is not Fixed,The diffusing capacity for oxygen in young, healthy men is 21 ml/min/mmHg.
