T2 L3: Gas Exchange and Transport - MPa

• What are some other roles of the lung?

  • metabolise compounds
  • filter unwanted material
  • reservoir for blood
• What is Fick's Law?

  The rate of diffusion is:

  1. proportional to surface area
  2. proportional to concentration difference
  3. inversely proportional to thickness of membrane
• What are the characteristics of the blood-gas interface?


  1. Extremely thin (0.3- 0.3 μm)
  2. Large surface area (50-100m2)
  3. Large area due to ~500 million alveoli
• How is the organisation of pulmonary blood vessels?

  initially arteries, veins, bronchi run close together

  towards periphery of lung, veins move away & pass through lobules

  Arteries and bronchi travel closely together

  pulmonary artery -> pulmonary capillaries -> pulmonary vein
• What is the main characteristic of pulmonary capillaries?

  extremely thin, around 7-10 micro m (just large enough for a RBC)

  extreme thinness off blood-gas barrier means capillaries easily damaged
• What are the laws of diffusion?

  Diffusion constant depends on the properties of the tissue & the gas

  The constant is:

  • proportional to the solubility of the gas (Sol)
  • inversely proportional to the molecular weight (MW)

  so CO2 diffuses 20x more rapidly than O2 as it has a much higher Sol but similar MW

  
• What is FiO2?

  fractional inspired oxygen
• What is SaO2?

  haemoglobin saturation
• What is PAO2?

  Partial pressure of oxygen in alveoli
• What is PaO2?

  Partial pressure of oxygen in arterial blood
• What does right shift mean in Hb-O2 dissociation curve?

  decreased affinity

  O2 into tissue

  Causes:

  • increased H+ conc (decrease pH)
  • increased temp
  • increased altitude
  • increase 2-3 DPG
  • HbF (foetal haemoglobin)
  • MetHb
• What does left shift mean in Hb-O2 dissociation curve and what may cause it?

  increased affinity

  less O2 into tissue:

  • decreased H+ ion concentration (increased pH)
  • decreased temp
  • decreased altitude
  • decreased 2-3 DPG
• What is the effect of carbon monoxide on Hb?

  CO moves rapidly across blood-gas barrier

  binds tightly to RBC (COHb)

  large amount of CO can be taken up with minimal increase in ppCO

  as RBC moves through capillary, ppCO doesn't change

  more CO diffuse across blood-gas barrier

  ability of O2 to bind to Hb greatly reduced
• Why is the alveolar gas equation important?

  as there is no easy/convenient way to sample alveolar gas

  as it is a gas, it behaves according to Dalton's law:

  • Total pressure of a mixture of gases is equal to the sum of the partial pressure of the constituent gases
• What is Respiratory Quotient (RQ)?

  volume of CO2 released / volume of O2 absorbed during respiration

  calculated for particular substrates ie carbs, organic acid, fat, protein

  carbs result in equal ratio of CO2 release and O2 consumption, around 0.8
• What is the alveolar gas equation?

  Where

  PAO2: pp oxygen in alveoli

  FiO2: fraction of inspired Oxygen

  PaCO2: pp CO2 in alveoli, from ABG

  RQ: respiratory quotient, usually 0.8

  
• What is the alveolar gas equation used for?

  to calculate A-a gradient
• What is the A-a gradient?

  measure of how effectively oxygen moves from alveoli into pulmonary vasculature

  PAO2 – PaO2
• What does high A-a gradient mean?

  more difficult O2 exchange
• How is the difference in regional ventilation in lungs?

  Ventilation per unit volume is greatest at the at the bottom of the lung and becomes progressively smaller towards the top

  so lower regions ventilate better than upper regions
• When does the regional difference in ventilation disappear?

  when supine
• Why is ventilation better at the bottom than at the top of the lungs?

  weight of fluid in pleural cavity greater at base

  increased intrapleural pressure to less negative value

  alveoli less expanded and have higher compliance at base

  more substantial increase in volume on inspiration

  perfusion also greater at base of lung due to gravity pulling blood down
• How is the ventilation/perfusion ratio different throughout the lung?

  Higher in the base
• What is V/Q mismatch?

  when ventilation and perfusion are not matched at the alveolar site
• What are the classical gas exchange abnormalities in pulmonary embolism?

  hypoxaemia (low PaO2): V/Q mismatch:

  • redistribution of blood from occluded pulmonary arteries to non-occluded vessel
  • reduction in mixed venous oxygen due to reduction in CO

  Hypocapnoea (low PaCO2): due to hyperventilation

  increased A-a gradient