MRCS PART A · APPLIED BASIC SCIENCES
Cardiovascular physiology: flow, delivery and extraction
Two selected oxygen-transport objectives with two foundation SBAs and two separately scored transfer SBAs. Source checks: 3 October 2026. Fictional steady-state models explain conservation, not transfusion, oxygen or fluid prescriptions. All oxygen contents below use mL O2 per litre of blood, not per decilitre.
2 topics · 4 original SBAs
1 · Distinguish the carrier flow from its oxygen cargo
Your task: Calculate systemic oxygen delivery from flow and arterial content; change only the specified variable.
What does cardiac output measure?
It is a blood-volume flow rate, not an oxygen amount. Multiplying cardiac output in L blood/min by arterial oxygen content in mL O2/L blood gives delivery in mL O2/min.
What if content is supplied per decilitre instead?
Convert the units first: 20 mL O2/dL blood equals 200 mL O2/L blood. Otherwise a L/min flow calculation will be wrong by a factor of ten.
Can unchanged flow coexist with falling oxygen delivery?
Yes. Delivery is flow multiplied by arterial content. Lower content reduces delivery at unchanged flow. Conversely, increased content does not guarantee increased delivery if flow falls enough.
Can two different flow/content pairs give the same delivery?
Yes. A product alone cannot identify its two factors. Equal global delivery does not establish equal regional perfusion, extraction or clinical safety.
Is oxygen delivered the same as oxygen consumed?
No. Delivery counts oxygen arriving in arterial blood per minute; consumption counts oxygen taken up. Venous blood still carries oxygen. Distinguish the total arriving cargo from the amount removed.
Does the calculated number select treatment?
No. These fictional values do not define a resuscitation, transfusion or oxygen target. Real assessment requires patient-specific data, current guidance and supervision.
2 · Balance what enters and leaves
Your task: Use the Fick balance with mixed venous content, consistent units and explicitly steady conditions.
What is the systemic oxygen balance?
At steady state, oxygen consumption equals cardiac output multiplied by arterial minus mixed venous oxygen content: VO2 = CO × (CaO2−CvO2). Therefore CO = VO2/(CaO2−CvO2). Content differences must use units compatible with flow.
Can a single peripheral vein stand in for mixed venous blood?
Not automatically. It reflects its drained territory. The systemic balance needs blood representative of the mixed systemic venous return, not an arbitrary local sample.
What if flow falls but consumption and arterial content stay fixed?
The arterial-minus-venous content difference must increase to preserve the stated consumption. Mixed venous content falls. This follows only while the assumed steady-state balance remains physically possible.
Must consumption stay fixed in a real deteriorating patient?
No. The transfer model explicitly holds it fixed. If delivery cannot sustain uptake, that assumption fails; do not extrapolate the equation to a negative venous content or assume unlimited extraction.
What does extraction fraction mean?
It is oxygen consumption divided by oxygen delivery; equivalently (CaO2−CvO2)/CaO2 in this steady-state model. It is a fraction of the incoming oxygen, not the venous content itself.
Does a global fraction prove that every organ is adequately supplied?
No. A whole-body balance can conceal regional differences. A model result is not proof of adequate organ perfusion or a universal safe threshold.