MRCS PART A · APPLIED BASIC SCIENCES
Respiratory physiology: shunt, dead space and blood mixing
Two selected gas-exchange objectives, two foundation SBAs and two separately scored transfer SBAs. Read each transfer afresh: only its stated change applies. Source checks: 3 October 2026. These fictional compartment experiments are not oxygen-prescribing instructions or a complete respiratory course.
2 topics · 4 original SBAs
1 · Ask what reaches this unit
Your task: Classify zero ventilation and zero perfusion without reversing the numerator and denominator.
What does V/Q compare?
Use alveolar ventilation (fresh gas reaching the exchange unit) divided by its blood flow. Both are rates. A whole-lung average can conceal very different regional units.
Can total minute ventilation replace alveolar ventilation here?
Not without accounting for ventilation that does not reach the gas-exchanging units. Specify the compartment and the kind of ventilation before dividing.
What if blood flows through an unventilated unit?
With positive blood flow and zero alveolar ventilation, V/Q is zero: a shunt compartment. It receives blood without fresh alveolar gas.
What if fresh gas reaches a unit with no blood flow instead?
That is alveolar dead space. For positive ventilation and zero perfusion, V/Q tends toward infinity; it is not zero.
What changes when perfusion returns?
If ventilation stays positive and blood flow becomes positive, the ratio becomes finite. Calculate it from the new facts; the old dead-space label does not persist automatically.
What if both rates are zero?
Zero divided by zero is undefined. An inactive unit is not classified by pretending that this ratio is zero or infinite.
2 · Mix oxygen amounts, not pressures
Your task: Weight blood oxygen content by blood-flow fraction in a stated two-compartment model.
How do two blood streams combine?
Conserve oxygen amount. If the streams have contents C1 and C2 and fractions f and (1−f) of the total blood flow, the mixed content is f×C1 + (1−f)×C2. All contents must use the same units.
Are these fractions ventilation fractions?
No. This calculation weights blood streams by their blood flow, not the gas supplied to their alveoli.
Can I average the two oxygen partial pressures instead?
Not as a substitute for content mixing. Haemoglobin binding makes the relationship between oxygen pressure and blood oxygen content nonlinear. A mixed content alone does not supply enough information for a unique arterial oxygen pressure.
Do mL O2/dL and mmHg describe the same thing?
No. The first measures oxygen amount per blood volume; the second is a pressure. Keeping the units visible prevents treating them as interchangeable.
What must stay fixed in a transfer question?
Use the stated end-stream contents and change only the specified flow fraction. Real patients may change ventilation, perfusion, haemoglobin, venous content and cardiac output together; the model deliberately holds those outside the question.
Does one model result prescribe oxygen or ventilator treatment?
No. It explains a mechanism. Clinical oxygen targets, assessment and treatment require current guidance and patient-specific review, not extrapolation from these fictional numbers.