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  1. 1. DiffusionD.A. Asir John Samuel, BSc (Psy), MPT (Neuro Paed),MAc, DYScEd, C/BLS, FAGELecturer, Alva’s college of Physiotherapy,MoodbidriDr.Asir John Samuel (PT)
  2. 2. Transport of O2 and CO2• O2 from alveoli into pulmonary blood incombination with Hb• Presence of Hb in RBC allows blood totransport 30 to 100 times as much O2transported in dissolved form• Increases CO2transport 15-20 foldDr.Asir John Samuel (PT)
  3. 3. Diffusion• Gases moves from one point to another bypressure difference from first point to next• O2 diffuses from alveoli into pulmonary capillaryblood because PO2in alveoli is greater than inpulmonary blood• Higher PO2 in capillary blood than tissues causesO2 to diffuse into surrounding cellsDr.Asir John Samuel (PT)
  4. 4. Diffusion• When O2 is metabolised in cells, intracellularCO2 rises to a high value• CO2 diffuses into tissue capillaries• Similarly diffuses out of blood into alveolibecause CO2 in pulmonary capillary blood isgreater than in alveoliDr.Asir John Samuel (PT)
  5. 5. Uptake of O2• PO2 of gaseous oxygen in alveolus averages104 mm Hg• PO2 of venous blood entering pulmonarycapillary at arterial end averages 40 mm Hg• Initial pressure difference (104 – 40 = 64)causes O2to diffuse into pulmonary capillaryDr.Asir John Samuel (PT)
  6. 6. Transport of O2• About 98% of blood enters left atrium isoxygenated upto PO2 of about 104 mm Hg• Another 2% of deoxygenated blood of PO2 ofabout 40 mm Hg enters directly from bronchialcirculation – shunt flow• Both blood combines – venous admixture of blood• Blood pumped from Lt side of heart fall to about95 mm Hg Dr.Asir John Samuel (PT)
  7. 7. Diffusion from peripheral capillaries• PO2 when arterial blood reaches peripheraltissues is still 95 mm Hg• PO2 in interstitial fluid that surrounds tissuesaverages only 40 mm Hg• This tremendous pressure difference causesO2 to diffuse rapidly (95 – 40 = 55)mm HgDr.Asir John Samuel (PT)
  8. 8. Tissue capillaries to tissue cells• Intracellular PO2 ranges about 23 mm Hg• Tissue capillaries and tissue cells pressuredifference (40 – 23 = 17) mm Hg causes O2 todiffuse rapidly• 1 to 3 mm Hg of O2 pressure is normallyrequired for all support of chemical processes• 23 mm Hg is more adequateDr.Asir John Samuel (PT)
  9. 9. Transport in blood• About 97% of O2 transported from lungs totissues is carried in chemical combination withhaemoglobin (Hb) in RBC• Remaining 3% is transported in dissolved statein water of plasma and cells• Under normal conditions, O2 is carried totissues almost entirely by HbDr.Asir John Samuel (PT)
  10. 10. Oxygen with Hb• O2 combines loosely and reversibly withhaeme portion of Hb• Each Hb molecule contains 4 Hb chaincontaining 1 atoms of iron each• Each atom binds with 1 molecule of O2• Each Hb molecule carries 4 molecules of O2(8O2 atoms) Dr.Asir John Samuel (PT)
  11. 11. Oxygen with Hb• When PO2 is high, as in pulmonary capillaries,O2 binds with Hb• But, when PO2 is low as in tissue capillaries, O2is released from Hb• This is basis for O2 transport from lungs totissuesDr.Asir John Samuel (PT)
  12. 12. Amount of O2 combine with Hb• Blood of normal person contains about 15 g ofHb/100 ml of blood• Each gram of Hb combines with 1.34 ml of O2• 15 x 1.34 = 20.1 gram• Hb in 100 ml blood caries about 20 ml of O2when 100 % saturatedDr.Asir John Samuel (PT)
  13. 13. Amount of O2 released• At 97% saturated blood, 19.4 ml/100 ml• Reduced to 14.4 at PO2 40 mm Hg• Ultimately tissue receives 5 ml/100 ml bloodas is PO2 23 mmHgDr.Asir John Samuel (PT)
  14. 14. O2 –Hb dissociation curve• oxygen–hemoglobin dissociation curverelates percentage saturation of the O2carrying power of hemoglobin to the PO2• Sigmoid shape• Demonstrates progressive increase in % of Hbbound with O2 as blood PO2 increases, is calledper cent saturation of HbDr.Asir John Samuel (PT)
  15. 15. O2 –Hb dissociation curveDr.Asir John Samuel (PT)
  16. 16. O2 –Hb dissociation curve• Blood leaving lungs and entering systemicarteries usually has PO2 of about 95 mm Hg• Usual O2 saturation of systemic arterial bloodis about 97%• In normal venous blood, PO2 is about 40 mmHg and saturation is about 75%Dr.Asir John Samuel (PT)
  17. 17. Combing O2 with Heme• Combination of the first heme in the Hbmolecule with O2 increases the affinity of thesecond heme for O2• Oxygenation of the second increases theaffinity of the third, and so on• So that the affinity of Hb for the fourth O2molecule is many times that for the firstDr.Asir John Samuel (PT)
  18. 18. Factors affecting it• pH• Temperature• Concentration of 2,3-biphosphoglycerate(BPG; 2,3-BPG)Dr.Asir John Samuel (PT)
  19. 19. Effect on pH• Rise in temperature or a fall in pH shifts thecurve to the right• When the curve is shifted in this direction, ahigher PO2 is required for hemoglobin to bind agiven amount of O2Dr.Asir John Samuel (PT)
  20. 20. Effect on temperature• A fall in temperature or a rise in pH shifts thecurve to the left• Lower PO2 is required to bind a given amountof O2Dr.Asir John Samuel (PT)
  21. 21. P50• A convenient index for comparison of suchshifts is the P50, the PO2 at which hemoglobinis half saturated with O2• The higher the P50, the lower the affinity ofhemoglobin for O2Dr.Asir John Samuel (PT)
  22. 22. Bohr effect• Decrease in O2 affinity of hemoglobin when thepH of blood falls is called the Bohr effect• Deoxygenated hemoglobin (deoxyhemoglobin)binds H+ more actively than does oxygenatedhemoglobin (oxyhemoglobin)Dr.Asir John Samuel (PT)
  23. 23. Bohr effect• pH of blood falls as its CO2 content increases,so that when the PCO2 rises, the curve shiftsto the right and the P50 rises• Hemoglobins oxygen binding affinity isinversely related both to acidity and to theconcentration of carbon dioxideDr.Asir John Samuel (PT)
  24. 24. Effect on 2,3-BPG• 2,3-BPG is very plentiful in red cells• Formed from 3-phosphoglyceraldehyde, aproduct of glycolysis via the Embden–Meyerhof pathway• HbO2 + 2,3-BPG ↔ Hb- 2,3-BPG + O2Dr.Asir John Samuel (PT)
  25. 25. Dr.Asir John Samuel (PT)
  26. 26. Effect on 2,3-BPG• Increase in the concentration of 2,3-BPG shiftsthe reaction to the right, causing more O2 tobe liberated• Acidosis inhibits red cell glycolysis• 2,3-BPG concentration falls when the pH islowDr.Asir John Samuel (PT)
  27. 27. Myoglobin• Myoglobin is an iron-containing pigmentfound in skeletal muscle• Resembles hemoglobin but binds 1 ratherthan 4 mol of O2 per mole• Rectangular hyperbola rather than a sigmoid• Because its curve is to the left of thehemoglobin curve, it takes up O2 from Hb inthe blood Dr.Asir John Samuel (PT)
  28. 28. CO2 transport• Solubility of CO2 in blood is about 20 timesthat of O2• More CO2 than O2 is present in simple solutionat equal partial pressures• CO2 that diffuses into red blood cells is rapidlyhydrated to H2CO3 because of the presence ofcarbonic anhydraseDr.Asir John Samuel (PT)
  29. 29. CO2 transport• H2CO3 dissociates to H+ and HCO3–• Some of the CO2 in the red cells reacts withthe amino groups of hemoglobin and otherproteins (R), forms carbamino compoundsDr.Asir John Samuel (PT)
  30. 30. Haldane effect• Deoxyhemoglobin binds more H+ thanoxyhemoglobin does and forms carbaminocompounds more readily• Binding of O2 to hemoglobin reduces its affinityfor CO2• Deoxygenation of the blood increases its ability tocarry carbon dioxide while oxygenated blood hasa reduced capacity for carbon dioxideDr.Asir John Samuel (PT)
  31. 31. Chloride shift• HCO3– content of red cells is much greaterthan that in plasma• As the blood passes through the capillaries,about 70% of the HCO3– formed in the redcells enters the plasma• Excess HCO3– leaves the red cells in exchangefor Cl–Dr.Asir John Samuel (PT)
  32. 32. Chloride shiftDr.Asir John Samuel (PT)
  33. 33. Chloride shift• Process is mediated by anion exchanger 1• It is a major membrane protein in RBC• Because of this chloride shift, the Cl– contentof the red cells in venous blood is significantlygreater than that in arterial blood• Chloride shift occurs rapidly and is essentiallycomplete within 1 sDr.Asir John Samuel (PT)