呼吸科呼吸系统2.ppt

呼吸科呼吸系统呼吸科呼吸系统2 2Partial Pressure of Individual Gases in a Gas MixtureAbove 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 concentration of the gas molecule for air.2024/5/24 周五2Applied 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 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.2024/5/24 周五3AnswersPB=PN2+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.2024/5/24 周五4Partial Pressures of Individual Gases at the Seal Level(PB=760 mmHg)2024/5/24 周五6Ambient Dry air(mm Hg)Moist Tracheal Air(mmHg)Alveolar Air(mmHg)Systemic Arterial Blood(mmHg)Mixed Venous Blood(mmHg)O21601501029540CO200404046Water Vapor047474747N2600563571571571Total760760760760704Water Vapor(水蒸汽）水蒸汽）Lowers PO2 once Air Is 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 mmHgnPN2=(760 47)X 0.79=563 mmHg2024/5/24 周五7The 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.2024/5/24 周五8The Determinants of Alveolar PO22024/5/24 周五9Factors that Determine Alveolar PCO2 2024/5/24 周五102024/5/24 周五11Diffusion of Gases through the Respiratory Membrane2024/5/24 周五122024/5/24 周五12Alveolar-Capillary Membrane(Respiratory Membrane)2024/5/24 周五12Six layers construct a barrier1.Alveolar fluid layer 2.Alveolar epithelium 3.Alveolar basement membrane4.Interstitial matrix5.Capillary basement membrane 6.Capillary endotheliumGases Diffuse according to Their Concentration Gradients2024/5/24 周五131.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 be close or identical to that in alveolar space.Why is Arterial PO2 Lower than That of Alveolar Air?2024/5/24 周五14nTwo contributors:a shunt and a low ventilation/perfusion ratio.nThe conducting airways have a separate circulation,called bronchial circulation.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 DiffusionFicks law:D=P.T.A.S/d.(MW)1/2Where 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).2024/5/24 周五15Between Oxygen and Carbon Dioxide,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.2024/5/24 周五16GasSolubility 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 Respiratory Membrane 1.The partial pressure gradient 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,emphysema2024/5/24 周五17Factors That Affect Partial Pressure 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.2024/5/24 周五18Diffusing 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.DefinitionThe volume of a gas that will diffuse through the respiratory membrane each minute for a partial pressure difference of 1 mm Hg.2024/5/24 周五19The Diffusing Capacity of the Respiratory Membrane is not FixedThe diffusing capacity for oxygen in young,healthy men is 21 ml/min/mmHg.Exercise increases the diffusing capacity.2024/5/24 周五20Ventilation/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 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 0.8.2024/5/24 周五212024/5/24 周五22VA/QExplanationSignificanceHypoventilation and/or pulmonary blood flowA-V shunt(Hypoxia with or without CO2 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 Flow in the Upright Person2024/5/24 周五23Gravity Causes a Mismatch of Regional Ventilation and Blood Flow in the Lungs2024/5/24 周五24Ventilation-Perfusion Ratios Affect Capillary Blood Gas Tension2024/5/24 周五25Venous 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 the left heart through an atrial or ventricular septal defect.An anatomic shunt is often called a right-left shunt.2024/5/24 周五26SummaryThe uptake of oxygen is determined by diffusion properties of the alveolar-capillary membrane,the O2 partial pressure gradient,and pulmonary blood flow.The transfer 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.2024/5/24 周五27Summary-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.2024/5/24 周五28