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VENTILATION/PERFUSION & GAS EXCHANGE

VENTILATION/PERFUSION & GAS EXCHANGE. AIMS. What factors are important in gas exchange?. What factors affect gas transfer?. What is the ventilation/perfusion ratio?. GAS EXCHANGE. ADEQUATE ALVEOLAR VENTILATION (V A ) TRANSFER OF GAS ACROSS ALVEOLAR-CAPILLARY MEMBRANE

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VENTILATION/PERFUSION & GAS EXCHANGE

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  1. VENTILATION/PERFUSION & GAS EXCHANGE AIMS • What factors are important in gas exchange? • What factors affect gas transfer? • What is the ventilation/perfusion ratio?

  2. GAS EXCHANGE • ADEQUATE ALVEOLAR VENTILATION(VA) • TRANSFER OF GAS ACROSS ALVEOLAR-CAPILLARY MEMBRANE • ADEQUATE PERFUSION • MATCHING OF VENTILATION AND PERFUSION

  3. Pulmonary blood flow • PULMONARY CIRCULATION • Small, supply- deoxygenated blood by pulmonary artery, drained by pulmonary veins to left atrium • low arterial pressures (mean=15mmHg, SP=25,DP=8), • capillaries -vulnerable to stretch and collapse, • pressures 7-12 mmHg • radial traction opens blood vessels as lung expands • low resistance- tenth of systemic • large surface area, • cardiac output- 5 L/min

  4. Distribution of blood flow in lung • Unequal • upright lung - from base to apex • supine lung- higher blood flow in apex, base unchanged • blood flow in dependent region is greater • differences due to changes in pressures in pulmonary artery i.e P=hqg, i.e P increases by 1cmH20 for every cm down the lung

  5. TRANSFER OF GASES • FICK’S LAW • GAS FLOW=PAS/ TMw Where, P= pressure gradient A= surface area S= solubility T= thickness Mw= molecular weight • Diffusion- THIN MEMBRANE(0.2-1m); LARGE SURFACE AREA (50-70 m2) • Measurement of gas transfer- transfer factor, use CO (0.3%)-(KCO)

  6. VENTILATION/PERFUSION • VENTILATION- VT & f; PERFUSION- cardiac output • RATIO OF VENTILATION TO PERFUSION • efficiency of gas transfer by alveoli • ratios= 0.5-3.0, mean = 0.8 • extreme situations • diffusion limitation • perfusion limitation • limitations in both • Posture (gravity) - affects ratios • P= hg - pressure 1cmH2O /cm of lung • standing BF from base to apex, VAfrom base to apex, ratios from base to apex • top of lung not very important

  7. CONCLUSIONS • 1. Factors important for gas exchange - VA, gas transfer, perfusion and matching. • 2. • 3. Gas transfer is directly proportional to pressure difference, surface area, & inversely to thickness- Fick’s law. Efficiency of gas exchange is increased if ventilation and perfusion are matched.

  8. O2 TRANSPORT • Solubility- small (0.3 ml O2/100 ml blood),bound to Haemoglobin (Hb) • Hb (heme +globin) • each Hb can bind 4 O2 • 15g of Hb/100 ml blood • 1.34ml O2/g Hb • SaO2= 97% • Oxygen dissociation curve • sigmoid, plateau at 100 mmHg • steep at 50 mmHg (in the tissues),thus unloading of O2, • curve shift to the right (Bohr shift) CO2, T, 2,3 DPG, pH • shift to left, Hb more saturated

  9. CO2 TRANSPORT • 1/ PHYSICALLY DISSOLVED- (5% ) • 20x >soluble than O2 • 2/ BICARBONATE (HCO3-)- 70% • Small amount produced in plasma • Maj. prod. in RBC • Requires carbonic anhydrase, • CO2+H2O H2CO3 H+ + HCO3- • Chloride shift • 3/Carbamino compounds- 5-10%; amine groups in Hb combine with CO2 • CO2 dissociation curve • Almost linear in physiological range • Haldane effect, shift to the right, easier to lose CO2 at lungs, and release O2 to tissues

  10. ACID-BASE BALANCE • AIMS: • What are acids and bases? • What is pH? • Why does pH vary? • How do you regulate pH?

  11. ACIDS & BASES • Acids- release H+ ions • Bases- release OH- ions • In body fluids at low concentrations • thus pH scale (1-14) to represent [H+] and [OH-] • pH scale • used to represent [H+] in body, • = - log10 [H+] • blood= 7.40.02 • gastric acid= 1, urine= 6 • pH>7 is basic, pH<7 is acidic

  12. VARIATION OF pH • IMPORTANCE • Death if pH remains < 6.8 or >8 for less than a day • Metabolic enzymes stop working, depression of CNS • Changes in pH • due mainly to [ H+] produced by the body • Acid production- H2CO3, H2PO4, H2SO4, Lactic, Pyruvic acids • Vomiting, diarrhoea, dehydration

  13. REGULATION OF ACID BASE • DIRECT BUFFERING IN BLOOD • seconds • proteins, phosphates, Hb, HCO3- • INDIRECT BY LUNGS • minutes • increase or decrease breathing • INDIRECT BY KIDNEYS • slow, hours & days • excreting excess base or acid • HENDERSON-HASSELBALCH EQUATION • defines the relationship between pH and the concentration of a salt and its corresponding acid

  14. CONCLUSIONS • 1. Acids are substances release H+ ions, Bases release OH- ions • 2.pH scale (1-14) is a measure of the acidity and alkalinity • 3. pH varies due to internal & external changes of acids & bases (range=7.35-7.45) • 4. pH regulated by, blood buffering, lungs & kidney

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