1. Addis Ababa universityCollege of Health Science Department of Medical Physiology Presentation on blood gas exchange 8/30/2011 girmay f 1

2. Presentation outline 1. Objectives 2. Introduction 3. Diffusion 3.1.Determinants of diffusion 3.2.Diffusion capacity 3.3.mesurement of diffusion capacity 4. Partial pressure 4.1 partial pressure of O2 and CO2 in the body 5. Blood gas exchange 6. References girmay f 2 8/30/2011

3. 1.Objectives At the end of this presentation students will able to:- Define what diffusion and diffusion capacity is? Explain the role of diffusion in gas exchange. List the factors that affect diffusion. Explain the mechanism of blood gas exchange. girmay f 3 8/30/2011

4. 2.Introduction Oxygen in the inspired air enters the alveoli and then diffuses across the alveolo capillary membrane in to the pulmonary capillary blood. The respiratory membrane is the actual site for diffusion of alveolar gases and gases present in the blood in dissolved form. The respiratory membrane consists of six layers. The gases have to diffuse through these. girmay f 4 8/30/2011

5. Introduction cont’d The layers are:- A layer of fluid lining the alveolus. A layer of epithelial cells. The basement membrane of the alveolar epithelial cells. The interstitial space between the epithelial and endothelial cells. The basement membrane of the capillary endothelium. capillary endothelial cells. girmay f 5 8/30/2011

7. Diffusion and chemical reaction within blood

8. Diffusion with terminal respiratory unitgirmay f 6 8/30/2011

10. the surface area of the membrane

12. the partial pressure difference between the two sides.

13. the fluid temperature.

15. thickness of the tissue and square root of molecular wt of the gas .girmay f 8 8/30/2011

16. Determinants of diffusion cont’d The thickness of the respiratory membrane increase in conditions like pulmonary edema, fibrosis. The surface area of the respiratory membrane can be greatly decreased by many conditions. e.g. removal of an entire lung decreases the total surface area to one-half normal. emphysema ,many of the alveoli coalesce, with dissolution of many alveoli walls. girmay f 9 8/30/2011

18. Determinants of diffusion cont’d Diffusion co efficient or diffusivity it depends on two physical properties of gas. Solubility in membrane and molecular weight. According to graham’s law Diffusion coefficient is directly propertional to solubility and inversely propertional to square root of molecular weight. Diffusivity of CO2 is much more about 20 times than O2 . girmay f 11 8/30/2011

19. Determinants of diffusion cont’d Diffusion -limited and perfusion –limited transfer of gases:- transfer of gas will be diffusion-limited or perfusion limited depends on the rate of equilibration of the gas with blood. Transfer of gases which quickly equilibrate with blood, will be perfusion-limited. 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. carbon mono oxide, which almost never equilibrate with blood, because carbon mono oxide rapidly taken up by hemoglobin. girmay f 12 8/30/2011

20. 3.2 Diffusion Capacity volume of gas that will diffuse through the membrane each minute for a pressure difference of 1 mmHg. Different for different blood gases. Diffusion capacity of O2 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. Strenuous exercise and factors which increase pulmonary blood flow and alveolar ventilation can increase the diffusion capacity to 65ml/min/mmHg. girmay f 13 8/30/2011

21. Diffusion capacity cont’d This increase is caused by several factors 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. 2. A better match of Ventilation perfusion ratio 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. girmay f 14 8/30/2011

22. Diffusion capacitycont’d Diffusion capacity of carbon dioxide 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. Diffusion capacity 400 ml/min/mm Hg * gradient < 1 mmHg. girmay f 15 8/30/2011

24. Measurement of diffusing capacity cont’d The carbon monoxide method The principle of the CO method is the following A small amount of CO is breathed into the alveoli The PCO in the alveoli is measured from appropriate alveolar air samples. The carbon monoxide pressure in the blood is essentially zero because hemoglobin combines with this gas so rapidly. The pressure difference of CO across the respiratory membrane is equal to its partial pressure in the alveolar sample. Measuring the volume of CO absorbed in short period of time. girmay f 17 8/30/2011

25. Measurement of diffusing capacity cont’d DLCO=VCO/PA-PC=VCO/PA To convert CO-diffusing capacity to oxygen-diffusing capacity 1.DLCO*1.23 the diffusion coefficient for oxygen is 1.23 times that for carbon monoxide. DLCO is 17ml/min/mmHg, DLO2 =1.23*17ml/min/mmHg=21ml/min/mmHg girmay f 18 8/30/2011

26. 4. Partial Pressure Dalton’s Law the total pressure of a gas mixture is equal to the sum of the pressures that each gas in the mixture would exert independently. Partial pressure The pressure exerted by each type of gas in a mixture. PAN2+PAO2+PACO2+PAH2O= PB PAO2PVO2loading of blood with O2 PACO2PVCO2unloading of excess CO2 girmay f 19 8/30/2011

27. Partial Pressures of Gases in Inhaled Air girmay f 20 8/30/2011

28. girmay f 21 8/30/2011

30. 5.Blood gas exchange 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. Leaving the alveolar capillaries PO2 = 104 mm Hg PCO2 = 40 mm Hg Blood leaving the alveolar capillaries returns to the left atrium & is pumped by the left ventricle into the systemic circulation. This blood travels through arteries & arterioles and into the systemic, or body, capillaries. As blood travels through arteries & arterioles, no gas exchange occurs. girmay f 23 8/30/2011

31. Blood gas exchange Entering the systemic capillaries PO2 = 95 mm Hg PCO2 = 40 mm Hg Body cells (resting conditions) PO2 = 40 mm Hg PCO2 = 45 mm Hg 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. Leaving the systemic capillaries. PO2 = 40 mm Hg PCO2 = 45 mm Hg girmay f 24 8/30/2011

32. Blood gas exchange 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). This blood is then pumped to the lungs (and the alveolar capillaries) by the right ventricle. Remember in a normal person alveolar PO2 = arterial PO2, and alveolar PCO2 = arterial PCO2 . girmay f 25 8/30/2011

33. Overview of Gas Exchange in the Lungs girmay f 26 8/30/2011 Adapted from: Costanzo, LS. Physiology, 1st ed. 1998.

34. 6.References Guyton and hall: Text of medical physiology 11th edition. Berne and levy physiology 6th edition. Internet websites. girmay f 27 8/30/2011

35. Thank you! girmay f 28 8/30/2011