College Answer

This question related to an area of basic respiratory physiology. A good answer necessitated
the precise meaning of venous admixture, being that amount of mixed venous blood which
would have to be added to ideal pulmonary end-capillary blood to explain the observed
pulmonary end-capillary to arterial PO2 difference. Diagnosis required the candidate to
mention, a demonstrated increased in the Aa-DO2, what are normal values. For quantification
mention and description of the shunt equation was required. Candidates lacked a definition
for venous admixture, were inaccurate with their description of the shunt equation and often
overlooked mentioning calculation of arterial and mixed venous blood oxygen content.
Syllabus - B1a
Reference: Nunn’s Applied Respiratory Physiology 6th Ed p122

Discussion:

The college answer here is informative with regards to what was expected, notwithstanding sections which appear to have been generated by Google Translate ("Diagnosis required the candidate to
mention, a demonstrated increased in the Aa-DO2, what are normal values"). As far as one can tell, they expected something like this:

• Shunt is the volume of blood which enters the systemic arterial circulation without participating in gas exchange
• Venous admixture is that amount of mixed venous blood which would have to be added to ideal pulmonary end-capillary blood to explain the observed  difference between pulmonary end-capillary PO₂ and arterial PO₂ 
• Shunt fraction is the calculated ratio of venous admixture to total cardiac output
• The shunt equation, otherwise known as the Berggren equation, is used to calculate the shunt fraction:
  Qs/Qt = (Cc_O2 - Ca_O2) / (Cc_O2 + Cv_O2)
  where
      Qs/Qt = shunt fraction
      Cc_O2 = pulmonary end-capillary O₂ content, same as alveolar O₂ content
      Ca_O2 = arterial O₂ content
      Cv_O2 = mixed venous O₂ content
• Sources of venous admixture include:
  • "True" intrapulmonary shunt, blood which passes through lung regions where V/Q = 0
  • V/Q scatter, blood which passes through lung regions where V/Q < 1.0
  • Thebesian veins, which contribute myocardial venous blood with low oxygen content 
  • Bronchial veins,  which drain the bronchial walls 
  • Intracardiac right-to-left shunts
• Normal shunt fraction in healthy adults is 4-10%