1、5.2. Gas Exchange,2018/7/18,1,DefinitionDiffusion of oxygen and carbon dioxide between the alveoli and the blood inside the pulmonary capillaries.What we have to know?Basic principles of gas diffusionFactors that determine the rate of gas diffusion,Partial Pressure of Individual Gases in a Gas Mixtu
2、re,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 concentration of the gas
3、 molecule for air.,2018/7/18,2,Applied Gas Laws,QuestionAt the sea level (barometric pressure is 760 mmHg), how much is the partial pressure of oxygen if we know air consists of 21% oxygen?HintsDaltons law states that the total pressure exerted by a gaseous mixture is equal to the sum of the partial
4、 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.,2018/7/18,3,Answers,PB = PN2 + PO2 + PHO2 + PCO2 (at the sea level, PB = 760 mmHg).WherePB = barometric pres
5、sure 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 .,2018/7/18,4,Partial Pressures of Individual Gases at the Seal Level (PB = 760 mmHg),2018/7/18,6,Water Vapor (水蒸汽） Lowers PO2 once Air Is Bein
6、g 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 mmHgPN2 = (760 47) X 0.79 = 563 mmHg,2018/7/18,7,The Partial Pressure of Oxyge
7、n 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.,2018/7/18,8,The Determinants of Alveolar PO2,2018/7/18,9,Factors that Determine Alveolar PCO2
8、,2018/7/18,10,2018/7/18,11,Diffusion of Gases through the Respiratory Membrane,2018/7/18,12,2018/7/18,12,Alveolar-Capillary Membrane (Respiratory Membrane),2018/7/18,12,Six layers construct a barrierAlveolar fluid layer Alveolar epithelium Alveolar basement membraneInterstitial matrixCapillary basem
9、ent membrane Capillary endothelium,Gases Diffuse according to Their Concentration Gradients,2018/7/18,13,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 arteries should be close
10、 or identical to that in alveolar space.,Why is Arterial PO2 Lower than That of Alveolar Air?,2018/7/18,14,Two contributors: a shunt and a low ventilation/perfusion ratio.The conducting airways have a separate circulation, called bronchial circulation. About half of venous blood from the bronchial c
11、irculation returns to the pulmonary veins, resulting in an admixture of unoxygenated blood and oxygenated blood and a drop in arterial PO2.,Quantitative Description of Diffusion,Ficks law: D = P.T.A.S/d.(MW)1/2Where D = diffusion rate P = partial pressure gradient (driving force)T = temperatureA = m
12、embrane 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).,2018/7/18,15,Between Oxygen and Carbon Dioxide, Which Gas Diffuses Faster?,ConclusionsAt the same
13、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.,2018/7/18,16,Factors that Affect Gas Diffusion Rate across the Respiratory Membrane,1. The partial pressure gradient between two
14、 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, emphysema,2018/7/18,17,Factors That Affect Partial Pressure Gradient,Alveolar ventilationLarger alveolar ventilat
15、ion helps to reduce the buffering power of FRC, enhancing PAO2 and reducing PACO2. Pulmonary blood flowIncreased pulmonary blood flow rises O2 and CO2 partial pressure gradient. The kinetics of hemoglobin binding to oxygen and carbon dioxide.,2018/7/18,18,Diffusing Capacity of the Respiratory Membra
16、ne,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.,2018/7/18,19,
17、The Diffusing Capacity of the Respiratory Membrane is not Fixed,The diffusing capacity for oxygen in young, healthy men is 21 ml/min/mmHg.Exercise increases the diffusing capacity.,2018/7/18,20,Ventilation/perfusion ratio,The ratio of alveolar minute ventilation (VA) to minute pulmonary blood flow (
18、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
19、.,2018/7/18,21,2018/7/18,22,Abnormal VA/Q Ratios Alter Gas Tensions,Gravity Causes Uneven Pulmonary Blood Flow in the Upright Person,2018/7/18,23,Gravity Causes a Mismatch of Regional Ventilation and Blood Flow in the Lungs,2018/7/18,24,Ventilation-Perfusion Ratios Affect Capillary Blood Gas Tension
20、,2018/7/18,25,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 the left heart t
21、hrough an atrial or ventricular septal defect.An anatomic shunt is often called a right-left shunt.,2018/7/18,26,Summary,The 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
22、 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.,2018/7/18,27,Summary-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.,2018/7/18,28,
