Question 1

Outline the determinants of oxygen delivery to the tissues. 

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College Answer

An opening statement such as oxygen delivery = cardiac output x oxygen content then allowed 
a more detailed description of the determinants of both oxygen content and cardiac output. It 
was expected candidates could detail the formula for Oxygen Content = Sat O2 % x 1.34 x Hb 
x 10 (depending on units) + PaO2 x .003, or x .03 (units) and accurately describe each 
element. Determinant of cardiac output completed the answer, often starting with CO = HR x 
SV while discussing the importance of preload, afterload and contractility.
Many candidates spent little time on cardiac output which cost valuable marks. Some 
candidates provided a lot of detail on how oxygen is managed within the lung, the majority of 
which was not required as part of the answer. Some candidates answered the question 
describing the oxygen cascade only which was not sufficient to score well.


Total DO2 = Cardiac output (L/min) × Oxygen content (ml/L)

Cardiac output:

= HR × SV

Where SV is influenced by preload, afterload and contractility.

Oxygen content: 

= (sO2 × ceHb × BO) + (PaO2 × 0.003)


  • ceHb = the effective haemoglobin concentration
  • PaO2 = the partial pressure of oxygen in arterial gas
  • 0.003 = the content, in ml/L/mmHg, of dissolved oxygen in blood
  • BO the maximum amount of Hb-bound O2 per unit volume of blood (normally 1.39)
  • sO2 = oxygen saturation


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