Diffusion of Gases PDF

CO2 O2 O2 O2 DIFFUSION OF GASES CO2 O2 O2 CO2 Ventilation haldanes effect well po2 is high co2 is release from hb and o is bound to hb 1...

CO2 O2 O2 O2 DIFFUSION OF GASES CO2 O2 O2 CO2 Ventilation haldanes effect well po2 is high co2 is release from hb and o is bound to hb 1 2 3  Objectives: Define respiratory membrane and diffusion principles Define diffusing capacities Compare the pressures in the pulmonary circulation with those in systemic circulation List the zones in the lungs Comprehend the ventilation/ perfusion ratio Explain the uptake of oxygen and carbon dioxide 4 5 Respiratory Unit a respiratory bronchiole, alveolar ducts, atria, and alveoli The alveolar walls are thin, and have a network of interconnecting capillaries Gas exchange between alveolar air & pulmonary blood – terminal portions of the lungs respiratory membrane or pulmonary membrane  Layers of the respiratory membrane which one of the below is not a layer of respiratpry memebrane ? * a layer of fluid lining the alveolus containing surfactant * the alveolar epithelium composed of thin epithelial cells * an epithelial basement membrane * a thin interstitial space * a capillary basement membrane (in many places fuses with the epithelial basement membrane) * the capillary endothelial membrane 8  Factors that affect the rate of gas diffusion through the respiratory membrane (1) the thickness of the respiratory membrane; (2) the surface area of the membrane; (3) the diffusion coefficient of the gas; (4) the pressure difference between the two sides of the membrane. 9  Diffusion rate = diff. coefficient x area x conc. difference distance (Fick’s principle) 10  The diffusion coefficient for the transfer of each gas through the respiratory membrane: = S (solubility in the membrane)  molecular weight Diff. coeff. O2= 1 CO2=20; CO=0.8; N2= 0.5; He=0.95 11  Charles’ Law Gases expand to fill the volume available to them volume occupied by a given number of gas molecules at a given temperature and pressure, is same regardless of the composition of the gas. PV = n RT n/V   = P ; P= pressure, V=volume, RT n=no. of moles; T=temperature 12 13  The composition of air: (%) Partial pressures: O2 20.93 20.93 / 100 x 760 = 159 mm Hg PO2 N2 79.04 79.04/ 100 x 760= 600.7 mm Hg PN2 CO2 0.03 0.03/ 100 x 760 = 0.23 mm Hg PCO2 When the air is moist: at 37 oC, 3.7 % PH2O vapor = 47 mm Hg for oxygen: 20.93 / 100 x (760 - 47) = 149 mm Hg 14 15 16 Diffusing capacity of the respiratory membrane: It is the volume of a gas that diffuses through the membrane each minute for a pressure difference of 1 mm Hg. ml/min/mm Hg 17 Diffusing Capacity for oxygen 21 ml/min/mm Hg mean oxygen  P during quiet breathing: 11 mmHg 21 x 11 = 230 ml of O2 diffusing through the respiratory membrane each minute; equal to the rate of which the body uses oxygen 18 19  we see incerase in pulomary blood flow and alvelor ventaltion in strenous exercise pulomary blood flow increase During strenuous exercise or in conditions that greatly increase pulmonary blood flow and alveolar ventilation, the diffusing capacity increases to 65 ml/min/mm Hg opening up of dormant pulmonary capillaries extradilation of already open capillaries 20 21 Diffusing capacity for CO2: diff. coeff. of CO2 is 20 times that of O2 under resting conditions: 400 to 450 ml/min/mm Hg during exercise: 1200 to 1300 ml/min/mm Hg. 22  Gravity affects venous return (cardiac output) and the distribution of systemic blood volume gravity effects pulomary circulaion more that systemic due to lower vasculature presusure The effects of gravity are greater in the pulmonary circulation than in the systemic circuit because the vascular pressures are much lower. The pressure differences resulting from gravity affect the distribution of blood flow up and down the lung. 23 There are 3 zones, depending on the pulmonary arterial, venous & alveolar pressures. 24 zone 1: no blood flow; pulmonary arterial pressure < alveolar pressure (capillaries compressed). zone 2: alveolar pressure > venous outflow pressure, capillaries or venules exposed to alveolar pressure are compressed at the outflow end of the alveolar compartment. zone 3: flow is high, all vessels are distended. 25 Ventilation / perfusion Alveolar ventilation Vo= ? Perfusion Q= ? 26  V/Q ratio The two extreme examples of ventilation perfusion mismatching are: wasted ventilation, V/Q =  venous admixture, V/Q = 0 The V/Q distribution throughout the normal lung is not homogenous, true or flase Some lung units are overventilated and some are underventilated. 27 wasted ventilation V/Q=  V/Q = 0 venous admixture 28 Uptake of O2 by the Pulmonary blood PO2 rises to equal the alveolar pressure by the time blood has moved 1/3rd of the distance through the capillary; safety factor 29 In exercise, even with the shortened time of exposure in the capillaries, the blood still can become fully oxygenated. which onE of the below is correct of capilleries in exersice ? 1. Diffusion capacity for O2 increases almost 3-folds during exercise; * increased surface area of capillaries participating in the diffusion * nearly ideal V/Q ratio in the upper lungs. 2. During normal pulmonary blood flow, the blood normally stays in lung capillaries about three times as long as necessary for full oxygenation. 30 Transport of O2 in the Arterial Blood About 98 % of the blood that enters the left atrium has become oxygenated 104 mm Hg Another 2% of the blood- bronchial circulation, supplying the deep tissues of the lungs, “shunt” flow, blood bypasses gas exchange areas  40 mm Hg (like venous blood) This combines in the pulmonary veins with the oxygenated blood, called venous admixture of blood PO2 of blood pumped into aorta falls to 95 mm Hg 31 32  Diffusion of O2 from the peripheral capillaries into tissue fluid 1-3 mmHg of pressure is enough for full support of the metabolism of the cell; 23 mm Hg is more than adequate: a large safety factor 33 Diffusion of CO2 from the tissue fluid into peripheral capillaries 34 Diffusion of CO2 from the pulmonary blood into the alveolus 35  Control of ventilation & perfusion O2 content  blood flow CO2 content  airflow 36 VQ V/Q =  38