呼吸科呼吸系统2课件

5.2. Gas Exchange *1 lDefinition lDiffusion of oxygen and carbon dioxide between the alveoli and the blood inside the pulmonary capillaries. lWhat we have to know? lBasic principles of gas diffusion lFactors that determine the rate of gas diffusion Partial Pressure of Individual Gases 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 concentration of the gas molecule for air. *2 Applied Gas Laws Question nAt 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 nDaltons 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. *3 Answers PB = PN2 + PO2 + PHO2 + PCO2 (at the sea level, PB = 760 mmHg). Where 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 . *4 Partial Pressures of Individual Gases at the Seal Level (PB = 760 mmHg) *6 Ambient Dry air (mm Hg) Moist Tracheal Air (mmHg) Alveolar Air (mmHg) Systemic Arterial Blood (mmHg) Mixed Venous Blood (mmHg) O21601501029540 CO200404046 Water Vapor 047474747 N2600563571571571 Total760760760760704 Water Vapor (水蒸汽） Lowers 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, nPO2= (760 47) X 0.21 = 150 mmHg nPN2 = (760 47) X 0.79 = 563 mmHg *7 The 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. *8 The Determinants of Alveolar PO2 *9 Factors that Determine Alveolar PCO2 *10 *11 Diffusion of Gases through the Respiratory Membrane *12 *12 Alveolar-Capillary Membrane (Respiratory Membrane) *12 Six layers construct a barrier 1.Alveolar fluid layer 2.Alveolar epithelium 3.Alveolar basement membrane 4.Interstitial matrix 5.Capillary basement membrane 6.Capillary endothelium Gases Diffuse according to Their Concentration Gradients *13 1. 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? *14 nTwo 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 Diffusion Ficks law: D = P.T.A.S/d.(MW)1/2 Where D = diffusion rate P = partial pressure gradient (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). *15 Between Oxygen and Carbon Dioxide, Which Gas Diffuses Faster? Conclusions lAt 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. * 16 GasSolubility coefficient at 37C (ml/dl/mmHg) Partial pressure gradient (mmHg) CO20.592X63.552 O20.0244X621.5128 CO2/O224.3X0.0972.35 Factors that Affect Gas Diffusion Rate across the Respiratory Membrane 1. 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 *17 Factors That Affect Partial Pressure Gradient Alveolar ventilation nLarger alveolar ventilation helps to reduce the buffering power of FRC, enhancing PAO2 and reducing PACO2. Pulmonary blood flow nIncreased pulmonary blood flow rises O2 and CO2 partial pressure gradient. The kinetics of hemoglobin binding to oxygen and carbon dioxide. *18 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. *19 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. *20 Ventilation/perfusion ratio 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. 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. *21 *22 VA/QExplanationSignificance Hypoventilation and/or pulmonary blood flow A-V shunt (Hypoxia with or without CO2 accumulation) Hyperventilation and/or pulmonary blood flow Increase in alveolar dead space(Hypoxia with or without CO2 accumulation) Abnormal VA/Q Ratios Alter Gas Tensions Gravity Causes Uneven Pulmonary Blood Flow in the Upright Person *23 Gravity Causes a Mismatch of Regional Ventilation and Blood Flow in the Lungs *24 Ventilation-Perfusion Ratios Affect Capillary Blood Gas Tension *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 through an atrial or ventricular septal defect. An anatomic shunt is often called