Blood gas exchange 2


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Blood gas exchange 2

  1. 1. Addis Ababa universityCollege of Health Science Department of Medical Physiology<br />Presentation on blood gas exchange <br />8/30/2011<br />girmay f<br />1<br />
  2. 2. Presentation outline<br />1. Objectives<br />2. Introduction<br />3. Diffusion<br /> 3.1.Determinants of diffusion <br /> 3.2.Diffusion capacity<br /> 3.3.mesurement of diffusion capacity<br />4. Partial pressure<br /> 4.1 partial pressure of O2 and CO2 in the body<br />5. Blood gas exchange<br />6. References<br />girmay f<br />2<br />8/30/2011<br />
  3. 3. 1.Objectives <br />At the end of this presentation students will able to:- <br />Define what diffusion and diffusion capacity is?<br />Explain the role of diffusion in gas exchange.<br /> List the factors that affect diffusion.<br />Explain the mechanism of blood gas exchange.<br />girmay f<br />3<br />8/30/2011<br />
  4. 4. 2.Introduction <br />Oxygen in the inspired air enters the alveoli and then diffuses across the alveolo capillary membrane in to the pulmonary capillary blood.<br />The respiratory membrane is the actual site for diffusion of alveolar gases and gases present in the blood in dissolved form.<br /> The respiratory membrane consists of six layers. <br />The gases have to diffuse through these.<br />girmay f<br />4<br />8/30/2011<br />
  5. 5. Introduction cont’d <br />The layers are:- <br /> A layer of fluid lining the alveolus.<br />A layer of epithelial cells.<br />The basement membrane of the alveolar epithelial cells. <br />The interstitial space between the epithelial and endothelial cells.<br />The basement membrane of the capillary endothelium.<br /> capillary endothelial cells.<br />girmay f<br />5<br />8/30/2011<br />
  6. 6. 3.Diffusion <br />The process of diffusion is simply random molecular motion of molecules down a concentration gradient .<br />Steps of diffusion in respiratory system<br /><ul><li>Diffusion across air-blood barrier
  7. 7. Diffusion and chemical reaction within blood
  8. 8. Diffusion with terminal respiratory unit</li></ul>girmay f<br />6<br />8/30/2011<br />
  9. 9. 3.1 Determinants of Diffusion<br />8/30/2011<br />girmay f<br />7<br />Diffusion = (P1-P2 ) * Area * Solubility*T<br />Ficks Law<br /> L * <br />MW<br />D  ∆P*S* A/d*√MW<br />Factors affecting diffusion: <br /><ul><li> the thickness of the membrane
  10. 10. the surface area of the membrane
  11. 11. the diffusion coefficient of the gas</li></ul> in the substance of the membrane.<br /><ul><li>Pressure difference of the gas.</li></li></ul><li>Determinants of Diffusion cont’d<br />According toFick’s law,the rate of transfer of a gas through a sheet of tissue is directly proportional:-<br /><ul><li> to the tissue area
  12. 12. the partial pressure difference between the two sides.
  13. 13. the fluid temperature.
  14. 14. solubility coefficient of the gas. </li></ul> inversely proportional <br /><ul><li> to fluid viscocity.
  15. 15. thickness of the tissue and square root of molecular wt of the gas .</li></ul>girmay f<br />8<br />8/30/2011<br />
  16. 16. Determinants of diffusion cont’d<br />The thickness of the respiratory membrane increase in conditions like pulmonary edema, fibrosis.<br />The surface area of the respiratory membrane can be greatly decreased by many conditions.<br /> e.g. removal of an entire lung decreases the total surface area to one-half normal.<br /> emphysema ,many of the alveoli coalesce, with dissolution of many alveoli walls.<br />girmay f<br />9<br />8/30/2011<br />
  17. 17. Determinants of diffusion cont’d<br />Pressure difference across the alveolo-capillary membrane<br /><ul><li>Important factor determining the rate of diffusion.</li></ul>- in case of oxygen, the pressure gradient is about 60 mmHg but it is only 6mmhg in case of CO2.however,because of the much greater diffusion coefficient of CO2 both gases take the same time (about 0.25 second ) to be equilibrated between alveoli and pulmonary capillary blood.<br />girmay f<br />10<br />8/30/2011<br />
  18. 18. Determinants of diffusion cont’d<br />Diffusion co efficient or diffusivity<br />it depends on two physical properties of gas. Solubility in membrane and molecular weight.<br />According to graham’s law<br /> Diffusion coefficient is directly propertional to solubility<br /> and inversely propertional to square root of molecular weight.<br />Diffusivity of CO2 is much more about 20 times than O2 .<br />girmay f<br />11<br />8/30/2011<br />
  19. 19. Determinants of diffusion cont’d<br />Diffusion -limited and perfusion –limited transfer of gases:-<br /> transfer of gas will be diffusion-limited or perfusion limited depends on the rate of equilibration of the gas with blood. <br />Transfer of gases which quickly equilibrate with blood, will be perfusion-limited.<br /> e.g. N2O, quickly equilibrates with blood(in 0.1 second which is much lower than pulmonary circulation time of 0.75 sec).so, transfer of this gas will be limited by perfusion.<br />carbon mono oxide, which almost never equilibrate with blood, because carbon mono oxide rapidly taken up by hemoglobin.<br />girmay f<br />12<br />8/30/2011<br />
  20. 20. 3.2 Diffusion Capacity<br /> volume of gas that will diffuse through the membrane each minute for a pressure difference of 1 mmHg.<br />Different for different blood gases.<br />Diffusion capacity of O2 <br /> 21 ml /min/ mmHg under resting conditions and the mean pressure gradient across the respiratory membrane is 11 mmHg so that oxygen diffusion is 230 ml/min.<br />Strenuous exercise and factors which increase pulmonary blood flow and alveolar ventilation can increase the diffusion capacity to 65ml/min/mmHg.<br />girmay f<br />13<br />8/30/2011<br />
  21. 21. Diffusion capacity cont’d <br />This increase is caused by several factors<br />1.Opening up of a number of previously dormant pulmonary capillaries or extra dilatation of already open capillaries, thereby increasing the surface area of the blood into which the oxygen can diffuse.<br />2. A better match of Ventilation perfusion ratio<br /> In general during exercise, the oxygenation of the blood is increased by alveolar ventilation and greater diffusing capacity of the respiratory membrane for transporting oxygen into blood.<br />girmay f<br />14<br />8/30/2011<br />
  22. 22. Diffusion capacitycont’d <br />Diffusion capacity of carbon dioxide<br />CO2 diffuse through the respiratory membrane so rapidly that the average PCO2 in pulmonary blood is not far different from pco2 in the alveoli the average difference is less than 1 mmHg.<br />Diffusion capacity 400 ml/min/mm Hg * gradient < 1 mmHg.<br />girmay f<br />15<br />8/30/2011<br />
  23. 23. 3.3 Measurement of diffusing capacity<br />The oxygen-diffusing capacity can be calculated from measurement <br />1.Alveolar PO2<br />2.PO2 in pulmonary capillary blood<br />3.The rate of oxygen uptake by the blood.<br /><ul><li>Measuring the PO2 in the pulmonary capillary blood is so difficult and so imprecise that it is not practical to measure oxygen diffusing capacity by such a direct procedure except on experimental basis. </li></ul>girmay f<br />16<br />8/30/2011<br />
  24. 24. Measurement of diffusing capacity cont’d<br />The carbon monoxide method<br />The principle of the CO method is the following<br />A small amount of CO is breathed into the alveoli<br />The PCO in the alveoli is measured from appropriate alveolar air samples. <br />The carbon monoxide pressure in the blood is essentially zero because hemoglobin combines with this gas so rapidly.<br />The pressure difference of CO across the respiratory membrane is equal to its partial pressure in the alveolar sample. <br />Measuring the volume of CO absorbed in short period of time. <br />girmay f<br />17<br />8/30/2011<br />
  25. 25. Measurement of diffusing capacity cont’d<br />DLCO=VCO/PA-PC=VCO/PA<br />To convert CO-diffusing capacity to oxygen-diffusing capacity <br />1.DLCO*1.23<br /> the diffusion coefficient for oxygen is 1.23 times that for carbon monoxide.<br />DLCO is 17ml/min/mmHg,<br />DLO2 =1.23*17ml/min/mmHg=21ml/min/mmHg<br />girmay f<br />18<br />8/30/2011<br />
  26. 26. 4. Partial Pressure<br />Dalton’s Law<br /> the total pressure of a gas mixture is equal to the sum of the pressures that each gas in the mixture would exert independently.<br />Partial pressure<br />The pressure exerted by each type of gas in a mixture.<br />PAN2+PAO2+PACO2+PAH2O= PB<br />PAO2PVO2loading of blood with O2<br />PACO2PVCO2unloading of excess CO2<br />girmay f<br />19<br />8/30/2011<br />
  27. 27. Partial Pressures of Gases in Inhaled Air<br />girmay f<br />20<br />8/30/2011<br />
  28. 28. girmay f<br />21<br />8/30/2011<br />
  29. 29. 4.1 Partial pressures of O2 and CO2 in the body<br />Alveoli<br />PO2 = 104 mm Hg <br />PCO2 = 40 mm Hg<br />Alveolar capillaries<br /><ul><li>Entering the alveolar capillaries.</li></ul>PO2 =40mmHg,relatively low because this blood has just returned from systemic circulation & has lost much of its oxygen.<br />PCO2 = 45 mm Hg (relatively high because the blood returning from the systemic circulation has pick up CO2 from the tissues<br />girmay f<br />22<br />8/30/2011<br />
  30. 30. 5.Blood gas exchange <br /> in the alveolar capillaries, the diffusion of gasses occurs: oxygen diffuses from the alveoli into the blood & carbon dioxide from the blood into the alveoli.<br />Leaving the alveolar capillaries<br />PO2 = 104 mm Hg <br /> PCO2 = 40 mm Hg<br />Blood leaving the alveolar capillaries returns to the left atrium & is pumped by the left ventricle into the systemic circulation. <br />This blood travels through arteries & arterioles and into the systemic, or body, capillaries. <br />As blood travels through arteries & arterioles, no gas exchange occurs.<br />girmay f<br />23<br />8/30/2011<br />
  31. 31. Blood gas exchange<br />Entering the systemic capillaries<br />PO2 = 95 mm Hg <br />PCO2 = 40 mm Hg<br />Body cells (resting conditions)<br />PO2 = 40 mm Hg <br />PCO2 = 45 mm Hg<br />Because of the differences in partial pressures of oxygen & carbon dioxide in the systemic capillaries & the body cells, oxygen diffuses from the blood & into the cells, while carbon dioxide diffuses from the cells into the blood.<br />Leaving the systemic capillaries.<br />PO2 = 40 mm Hg <br /> PCO2 = 45 mm Hg<br />girmay f<br />24<br />8/30/2011<br />
  32. 32. Blood gas exchange<br />Blood leaving the systemic capillaries returns to the heart (right atrium) via venules & veins (and no gas exchange occurs while blood is in venules & veins). <br />This blood is then pumped to the lungs (and the alveolar capillaries) by the right ventricle.<br />Remember in a normal person alveolar PO2 = arterial PO2, and alveolar PCO2 = arterial PCO2 .<br />girmay f<br />25<br />8/30/2011<br />
  33. 33. Overview of Gas Exchange in the Lungs<br />girmay f<br />26<br />8/30/2011<br />Adapted from: Costanzo, LS. Physiology, 1st ed. 1998.<br />
  34. 34. 6.References <br />Guyton and hall: Text of medical physiology 11th edition.<br />Berne and levy physiology 6th edition. <br />Internet websites.<br />girmay f<br />27<br />8/30/2011<br />
  35. 35. Thank you!<br />girmay f<br />28<br />8/30/2011<br />