What happens to the V/Q ratio from top to bottom of the upright lung?

PROMPT – What happens to the relative values of ventilation and perfusion?

  • Both ventilation and perfusion increase with blood flow (perfusion (Q) increasing more than ventilation (V) and this results in V/Q ratio DECREASING down the lung

Pass Criteria:

  • 3 of 3 bold to pass (be able to explain the concept)

Explain the reasons for the normal Alveolar-arterial O2 difference.

  • Normally 5-10mmHg. A-a Gradient = measure of the difference between alveolar and arterial concentration of O2
    • Even though P Alv O2 at apex 40mmHg above base, most of blood flow (Q) comes from base where P Alv O2 is low -> decrease in P Art O2
    • Shunt: Bronchial blood & coronary blood
  • Also non-linear shape of O2 dissociation curve means that addition of small amount of shunted blood with low O2 concentration greatly decreases PO2 of arterial blood and units with high PO2 have little effect on O2  concentration because curve is flat at high O2 concentration.

Pass Criteria:

  • Bold

What is the formula for A-a gradient?

  • PAO2 = PIO2 – (PACO2)/R + F


Draw a diagram that demonstrates the components of total lung volume.


  • Should correctly include TLC, VC, FRC, TV, RV, IRV, ERV

Pass Criteria:

  • Bold to pass
  • TLC, VC, FRC, TV, RV correct

In an adult, what are the typical volumes of these components?

  • TLV: ~7000mL
  • VC: ~4500 – 5000mL
  • RV: ~1200mL
  • FRC: ~2400mL
  • TV: ~500mL

Pass Criteria:

  • 2/4 (reasonable approximations)

Which lung volumes can be measured in the ED?

  • Spirometer for FEV1 and FVC. TV on ventilator.
  • Helium dilution or body plethysmography for TLC, FRC and RV.

Pass Criteria:

  • 1/2 spirometer


What are the receptors involved in the control of ventilation?

  • Central chemoreceptors
  • Peripheral chemoreceptors
  • Pulmonary stretch receptors
  • Irritant receptors
  • J receptors
  • Bronchial C fibres
  • Nose and upper airway receptors
  • Joint and muscle receptors
  • Gamma system
  • Arterial baroreceptors
  • Pain and temperature receptors

Pass Criteria:

  • Bold + 3 others

Where are the central chemoreceptors located?

  • 200-400 um below ventral surface of medulla

Pass Criteria:

  • Medulla must be stated

How do these receptors function?
Prompt: How do H+ ions affect their function?

  • BBB permeable to CO2; relatively impermeable to HCO3
  • Increase in blood pCO2 –> increased CSF pCO2 –> increased H+ in CSF
  • Increased H+ in CSF stimulated ventilation
  • Decreased H+ in CSF inhibits ventilation; causes cerebral vasodilation –> enhanced diffusion of  pCO2 into CSF
  • CSF pH 7.32 – less buffering than blood, CSF pH chenges more for given pCO2
  • Prolonged pH changes compensated by HCO3 transport across BBB
    • Chronic CO2 retention has near normal CSF H+

Pass Criteria:

  • Bold concepts to pass


In an alveolus, what factors affect oxygenation?

  • Ventilation
  • Perfusion
  • Diffusion across the blood gas barrier
  • Alveolar-pulmonary capillary pO2 gradient

Pass Criteria:

  • Bold

Describe the oxygen uptake along a pulmonary capillary.

  • Alveolar pulmonary capillary O2 gradient
    • Alveolar pO2 = 100 mmHg
    • Pulmonary capillary pO2 = 40 mmHg
  • Blood gas barrier thickness 0.3 microns
  • RBC transit time = 0.75s
  • Under normal circumstances O2 uptake is perfusion limited (complete in 0.25s) & alveolar end capillary O2 difference is minimal
  • Rate of rise of end capillary pO2 is steep
    • O2 -Hb dissociation curve

Pass Criteria:

  • Must have knowledge of 3 of 4 concepts in bold
  • Numbers not required to pass

How does hypoxia affect oxygenation?

  • Alveolar pulmonary capillary O2 gradient is decreased
  • O2 diffusion is decreased & rate of rise of pO2 for given O2 concentration in blood is less

Pass Criteria:

  • Can draw graph to explain



Draw a diagram that demonstrates the components of total lung volumes.

respiratory 5

Pass Criteria:

  • 3 of 6 to pass

What are the typical volumes?

  • TLC ~ 7000 mL
  • VC ~ 4500-5000 mL
  • FRC ~2400 mL
  • TV ~500 mL
  • RV ~ 1200 mL

Pass Criteria:

  • 2 of 4 reasonable approximations

Optional: Which of these volumes can be measured in the ED?

  • FEV1, FVC, or TV


What are the physiological causes of hypoxaemia?

  • Hypoventilation
  • Diffusion
  • Shunt
  • Ventilation/Perfusion inequality

Pass Criteria:

  • 3 to pass

How does ventilation/perfusion inequality result in hypoxaemia?

  • Lung units with low V/Q ratio have effluent blood with low pO2 (close to mixed venous). Units with high V/Q ratio have relatively high pO2 but because of non-linear O2 dissociation curve add little to O2 concentration (compared to the decrement caused by the low V/Q areas). Overall mixed return has lower O2.

Pass Criteria:

  • Bold plus demonstrate understanding of concepts to pass.

How can ventilation/perfusion inequality be measured?
Prompt: Is there a formula used to quantify V/Q inequality?

  • Using the alveolar-arterial pO2 difference (the A-a gradient) – subtracting the (measured) arterial pO2 from the ‘ideal’ alveolar pO2 as given by the alveolar gas equation.

Pass Criteria:

  • Bold plus demonstrate understanding of concepts to pass.


What is lung compliance?

  • Change in lung volume per unit change in airway pressure – measure if lung ‘distensibility
  • Normally 200 mLs/cm H20
  • It occurs because of the opposing inward elastic recoil of the lungs and outward recoil of the chest wall
  • It is represented by the slope of the nonlinear lung pressure-volume curve.

Pass Criteria:

  • Concept to pass

What physiologic factors affect lung compliance?

  • Age
  • Volume of the lung
  • Phase of respiration (lower in deflation/expiration than inflation/inspiration)
  • Surfactant

Pass Criteria:

  • 3 to pass

How is lung compliance affected in emphysema?

  • Compliance is increased because of loss of lung elasticity and destruction of the lung connective tissue and elastin
  • The lungs are easy to inflate but have a reduced capacity to recoil
  • Patients have to force their expiration to expel air from the lungs
  • Resultant increase in the FRC.

Pass Criteria:

  • Bold to pass

What are the physiologic effects of pulmonary surfactant?

  • Lowers alveolar surface tension
  • Increases lung compliance
  • Reduces work of breathing
  • Improves the stability of alveoli
  • Keeps the alveoli ‘dry

Pass Criteria:

  • 3 to pass


What is 'dead space'?

  • Portion of the tidal volume that does not participate in gas exchange
  • VT = VD + VA

Pass Criteria:

  • Demonstrate principle in bold to pass

What types of daed space are there?
Prompt: Explain the difference between the two types

  • Anatomical
    • Volume of conducting airways (without alveoli) – trachea, bronchi, terminal bronchi
    • About 150 mL of 500 mL tidal volume
    • Determined by the increased diameter of airways during inspiration and size/posture of the individual
  • Physiological
    • Volume of gas that does not eliminate CO2 and does not equilibrate with blood
    • Same as anatomical dead space in normal individuals
    • Increased in lung disease because of inequality of blood flow and ventilation within the lung

Pass Criteria:

  • Two types of dead space and describe what it is.
  • May mention VQ mismatch.

(Bonus) How is dead space measured?

  • Anatomical – Fowler’s method
  • Physiological – Bohr method

Pass Criteria:

  • Will accept either.