VENTILATIONPERFUSION

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VENTILATION/PERFUSION GAS EXCHANGE

• AIMS

• What factors are important in gas exchange?

• What factors affect gas transfer?

• What is the ventilation/perfusion ratio?

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GAS EXCHANGE

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

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Pulmonary blood flow

• PULMONARY CIRCULATION
• Small, supply- deoxygenated blood by pulmonary
  artery, drained by pulmonary veins to left atrium
• low arterial pressures (mean15mmHg,
  SP25,DP8),
• 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

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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 Phqg, i.e P increases by
  1cmH20 for every cm down the lung

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TRANSFER OF GASES

• FICKS 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)

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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

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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- Ficks law.
Efficiency of gas exchange is increased if
ventilation and perfusion are matched.
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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

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CO2 TRANSPORT

• 1/ PHYSICALLY DISSOLVED- (5 )
• 20x gtsoluble than O2
• 2/ BICARBONATE (HCO3-)- 70
• Small amount produced in plasma
• Maj. prod. in RBC
• Requires carbonic anhydrase,
• CO2H2O 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

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ACID-BASE BALANCE

• AIMS
• What are acids and bases?
• What is pH?
• Why does pH vary?
• How do you regulate pH?

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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
• pHgt7 is basic, pHlt7 is acidic

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VARIATION OF pH

• IMPORTANCE
• Death if pH remains lt 6.8 or gt8 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

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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

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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 (range7.35-7.45)
• 4. pH regulated by, blood buffering, lungs
  kidney