呼吸科呼吸系统 .ppt

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1、5.2.5.2.Gas Exchange Gas Exchange 2024/6/111lDefinitionlDiffusion of oxygen and carbon dioxide between the alveoli and the blood inside the pulmonary capillaries.lWhat we have to know?lBasic principles of gas diffusionlFactors that determine the rate of gas diffusion Partial Pressure of Individual G

2、ases in a Gas Partial Pressure of Individual Gases in a Gas MixtureMixtureAbove 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 partia

3、l pressure is directly proportional to the concentration of the gas molecule for air.2024/6/112Applied Gas LawsApplied Gas LawsQuestionnAt the sea level(barometric pressure is 760 mmHg),how much is the partial pressure of oxygen if we know air consists of 21%oxygen?HintsnDaltons law states that the

4、total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each gas component in the gas mixture.nIn a mixture of gases,the partial pressure of a given gas is directly proportional to the volume that gas contributes to the total volume.2024/6/113AnswersAnswersPB=PN2+

5、PO2+PHO2+PCO2(at the sea level,PB=760 mmHg).WherePB=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.2024/6/114Partial Pressures of Individual Gases at the Partial Pressures of Indivi

6、dual Gases at the Seal Level(PSeal Level(PB B=760 mmHg)=760 mmHg)2024/6/116Ambient Dry air(mm Hg)Moist Tracheal Air(mmHg)Alveolar Air(mmHg)Systemic Arterial Blood(mmHg)Mixed Venous Blood(mmHg)O21601501029540CO200404046Water Vapor047474747N2600563571571571Total760760760760704Water Vapor(Water Vapor(水

7、蒸汽）水蒸汽）Lowers POLowers PO2 2 once Air once Air Is Being HumidifiedIs Being Humidified（湿化）（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,nPO2=(760 47)X 0.21=150 mmHgn

8、PN2=(760 47)X 0.79=563 mmHg2024/6/117The Partial Pressure of Oxygen in the The Partial Pressure of Oxygen in the Alveolar Space is down to 102 mmHg 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 o

9、xygen by the blood in pulmonary capillaries.2024/6/118The Determinants of Alveolar PThe Determinants of Alveolar POO2 22024/6/119Factors that Determine Alveolar PCOFactors that Determine Alveolar PCO2 2 2024/6/11102024/6/1111Diffusion of Gases through the Diffusion of Gases through the Respiratory M

10、embraneRespiratory Membrane2024/6/11122024/6/1112Alveolar-Capillary Membrane(Respiratory Membrane)2024/6/1112Six layers construct a barrier1.Alveolar fluid layer 2.Alveolar epithelium 3.Alveolar basement membrane4.Interstitial matrix5.Capillary basement membrane 6.Capillary endotheliumGases Diffuse

11、according to Their Gases Diffuse according to Their Concentration GradientsConcentration Gradients2024/6/11131.Blood stays in the pulmonary capillaries for 0.75 seconds.2.It takes 0.25 sec for O2 to reach the equilibrium.3.PO2 in the pulmonary capillaries,pulmonary vein,and systemic arteries should

12、be close or identical to that in alveolar space.Why is Arterial POWhy is Arterial PO2 2 Lower than That of Lower than That of Alveolar Air?Alveolar Air?2024/6/1114nTwo contributors:a shunt and a low ventilation/perfusion ratio.nThe conducting airways have a separate circulation,called bronchial circ

13、ulation.nAbout half of venous blood 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.Quantitative Description of DiffusionQuantitative Description of DiffusionFicks law:D=P.T.A.S/d.(MW)1/2Whe

14、re D=diffusion rate P=partial pressure gradient(driving force)T=temperatureA=membrane surface area S=solubility of gas in the bloodd=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).2024/6/1115Between Oxygen and C

15、arbon Dioxide,Between Oxygen and Carbon Dioxide,Which Gas Diffuses Faster?Which Gas Diffuses Faster?ConclusionslAt the same pressure gradient,carbon dioxide diffuses 24.3 times faster than oxygen.lUnder physiological pressure gradients,carbon dioxide will diffuse 2.35 times faster than oxygen.2024/6

16、/1116GasSolubility coefficient at 37C(ml/dl/mmHg)Partial pressure gradient(mmHg)CO20.592X63.552O20.0244X621.5128CO2/O224.3X0.0972.35Factors that Affect Gas Diffusion Rate across the Factors that Affect Gas Diffusion Rate across the Respiratory MembraneRespiratory Membrane 1.The partial pressure grad

17、ient between two sides of the membraneDirectly proportional2.The diffusion coefficient of the gas3.The thickness of respiratory membrane.4.The surface area of respiratory membrane.Remove of the lung,emphysema2024/6/1117Factors That Affect Partial Pressure Gradient Factors That Affect Partial Pressur

18、e Gradient Alveolar ventilationnLarger alveolar ventilation helps to reduce the buffering power of FRC,enhancing PAO2 and reducing PACO2.Pulmonary blood flownIncreased pulmonary blood flow rises O2 and CO2 partial pressure gradient.The kinetics of hemoglobin binding to oxygen and carbon dioxide.2024

19、/6/1118Diffusing Capacity of the Respiratory Diffusing Capacity of the Respiratory Membrane 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.DefinitionThe volume of a gas that will diffuse throug

20、h the respiratory membrane each minute for a partial pressure difference of 1 mm Hg.2024/6/1119The Diffusing Capacity of the Respiratory The Diffusing Capacity of the Respiratory Membrane is not FixedMembrane is not FixedThe diffusing capacity for oxygen in young,healthy men is 21 ml/min/mmHg.Exerci

21、se increases the diffusing capacity.2024/6/1120Ventilation/perfusion ratioVentilation/perfusion ratioThe ratio of alveolar minute ventilation(VA)to minute pulmonary blood flow(Q)is ventilation/perfusion ratio.It is used to indicate matching of air flow and blood flow for gas exchange.The ventilation

22、 is alveolar minute volume whereas minute pulmonary blood flow equals heart output.As VA is 4 L/min and cardiac output is 5 L/min in normal adults,VA/Q is approximately 0.8.2024/6/11212024/6/1122VA/QExplanationSignificanceHypoventilation and/or pulmonary blood flowA-V shunt(Hypoxia with or without C

23、O2 accumulation)Hyperventilation and/or pulmonary blood flowIncrease in alveolar dead space(Hypoxia with or without CO2 accumulation)Abnormal VA/Q Ratios Alter Gas TensionsGravity Causes Uneven Pulmonary Blood Gravity Causes Uneven Pulmonary Blood Flow in the Upright PersonFlow in the Upright Person

24、2024/6/1123Gravity Causes a Mismatch of Regional Gravity Causes a Mismatch of Regional Ventilation and Blood Flow in the LungsVentilation and Blood Flow in the Lungs2024/6/1124Ventilation-Perfusion Ratios Affect Capillary Ventilation-Perfusion Ratios Affect Capillary Blood Gas TensionBlood Gas Tensi

25、on2024/6/1125Venous Admixture Venous Admixture The mixing of unoxygenated blood with oxygenated blood is called venous admixture.Venous admixture has two reasons:A shunt and a low VA/Q.A shunt has a anatomic structure that allows blood to bypass alveoli through a channel,such as from the right to th

26、e left heart through an atrial or ventricular septal defect.An anatomic shunt is often called a right-left shunt.2024/6/1126SummarySummaryThe uptake of oxygen is determined by diffusion properties of the alveolar-capillary membrane,the O2 partial pressure gradient,and pulmonary blood flow.The transf

27、er of oxygen in the lung is limited primarily by blood flow.Diffusion capacity of the lung(DL)is defined as the volume of given gas diffused across the respiratory membrane per minute under a partial pressure gradient of 1 mmHg.2024/6/1127Summary-ContinuedSummary-Continued The ratio of alveolar minute ventilation(VA)to minute pulmonary blood flow(Q)is ventilation/perfusion ratio.It is used to indicate matching of air flow and blood flow for gas exchange.Gravity causes a mismatch of regional ventilation and blood flow in the lungs.2024/6/1128