Oxygen Transport for MRCEM Primary: Dissociation Curve, P50 and Oxygen Content

A normal oxygen saturation does not tell you how much oxygen is in the blood. Equally, a change in haemoglobin affinity does not necessarily mean that the lungs have become better or worse at oxygenating blood. These distinctions connect respiratory physiology with the reasoning needed for MRCEM Primary SBAs.

This guide separates pressure, saturation, content and delivery before bringing them together in worked examples. Use it alongside your MRCEM Primary revision plan, then test your understanding with the original questions below.

Start by identifying which oxygen variable the question describes

Pressure, saturation, content and delivery are different measurements

PaO₂ is arterial oxygen partial pressure, expressed here in kPa. SaO₂ describes arterial haemoglobin oxygen saturation; SpO₂ is the estimate obtained by pulse oximetry. CaO₂ is the amount of oxygen per volume of arterial blood. DO₂ describes the amount delivered to the systemic circulation per unit time. The oxygen-content and delivery relationships are explained in OpenAnesthesia’s physiology resource.

Why the distinction matters in an SBA

Before choosing an answer, underline the variable being asked about. “Which patient has the highest saturation?” and “Which patient has the greatest oxygen delivery?” are different tasks. In the second, a saturation value alone is insufficient: you also need information about haemoglobin and blood flow.

For example, compare two hypothetical patients who both have SaO₂ of 98%. One has haemoglobin 150 g/L and the other 75 g/L. If you conclude that their oxygen content must be equal, you have treated a percentage as an absolute quantity. We will calculate the difference below.

Read the oxygen–haemoglobin dissociation curve

Check the axes before interpreting the shape

The conventional curve plots oxygen saturation vertically against oxygen partial pressure horizontally. Cooperative binding produces its sigmoid shape: binding of oxygen favours further binding. The plateau supports loading in the lungs; the steeper portion permits substantial unloading as tissue oxygen tension falls. These principles are described in the Faculty of Intensive Care Medicine’s Critical CARE Course.

Do not assume that every graph uses saturation on its vertical axis. A graph of oxygen content also depends on haemoglobin concentration. When answering an image-based question, read both axis labels before interpreting a change in height as a change in affinity.

P50 provides a measure of affinity

P50 is the oxygen partial pressure at which haemoglobin is 50% saturated. For normal adult blood under standard conditions, it is approximately 3.5 kPa, or 26–27 mmHg. A higher P50 indicates lower oxygen affinity; a lower P50 indicates higher affinity. The European Respiratory Society review discusses this relationship.

Use a horizontal line at 50% saturation to reason through a graph. If it intersects the new curve further to the right, more oxygen pressure is needed to achieve the same saturation. That is reduced affinity, regardless of how the question describes the underlying cause.

Predict curve shifts without relying only on a mnemonic

Right shift: lower affinity and a higher P50

Increased carbon dioxide, lower pH and increased temperature shift the curve rightwards. Increased 2,3-BPG also favours a right shift. At the same oxygen tension, saturation is lower and unloading is facilitated. The carbon dioxide and pH effects are central to the Bohr effect. See the ERS discussion of physiological curve shifts.

Left shift: higher affinity and a lower P50

Opposite changes in pH, temperature, carbon dioxide and 2,3-BPG favour a left shift. Fetal haemoglobin also has higher oxygen affinity than adult haemoglobin because it interacts less strongly with 2,3-BPG. Its curve is therefore left-shifted. The fetal haemoglobin mechanism is explained in OpenAnesthesia’s uteroplacental circulation resource.

A shift is different from moving along a curve

Imagine a stem that specifies unchanged pH, temperature, carbon dioxide and haemoglobin characteristics, but a lower PO₂. You should move to a different point on the same curve. Do not infer a shift simply because saturation has fallen. A useful revision exercise is to label each new stem “different position” or “different curve” before reading the options.

Separate the Bohr effect from the Haldane effect

Bohr: carbon dioxide and hydrogen ions influence oxygen binding

The Bohr effect concerns how carbon dioxide and acidity alter haemoglobin’s affinity for oxygen. In an exam stem, the output you are explaining is oxygen binding or release. A question about lower oxygen affinity in an acidic environment belongs in this category.

Haldane: oxygenation influences carbon dioxide carriage

Deoxygenated blood can carry more carbon dioxide at a given PCO₂ than oxygenated blood. Oxygenation in the lungs facilitates carbon dioxide release. This is the Haldane effect, involving changes in carbamino carriage and hydrogen-ion buffering. A physiological modelling study explores the interaction between both effects.

To distinguish them, complete the sentence: “A change in ___ alters the carriage of ___.” For Bohr, start with carbon dioxide/acidity and finish with oxygen. For Haldane, start with oxygenation and finish with carbon dioxide. Both operate together; their names identify different directions of influence.

Calculate arterial oxygen content with consistent units

Use a formula that matches the quantities supplied

For these simplified examples, use CaO₂ = (1.34 × Hb × SaO₂) + (0.0225 × PaO₂). Enter Hb in g/dL, SaO₂ as a fraction and PaO₂ in kPa; the result is mL O₂/dL. The first term represents haemoglobin-bound oxygen and the second dissolved oxygen. The equivalent dissolved coefficient is 0.003 when PaO₂ is in mmHg, as shown in Virginia Tech’s gas-transport teaching.

The coefficient 1.34 mL O₂/g is an approximation used for these exercises; published conventions vary slightly. Follow any coefficient supplied in the examination. Convert Hb from g/L to g/dL by dividing by ten, and saturation from percentage to fraction by dividing by 100. The equation assumes interpretable saturation measurements and does not resolve dyshemoglobinaemia from a conventional pulse-oximeter reading.

Worked comparison: the same saturation with different haemoglobin

Patient A has Hb 150 g/L, SaO₂ 98% and PaO₂ 13 kPa. Convert Hb to 15 g/dL and saturation to 0.98. Bound oxygen is 1.34 × 15 × 0.98 = 19.698 mL/dL. Dissolved oxygen is 0.0225 × 13 = 0.2925 mL/dL. Total content is approximately 19.99 mL/dL.

Patient B has Hb 75 g/L with the same saturation and PaO₂. The calculation becomes (1.34 × 7.5 × 0.98) + 0.2925 = 10.14 mL/dL. Although the percentage saturation is identical, the calculated oxygen content is almost halved. The small dissolved contribution explains why the total is not exactly half.

Extend the calculation to oxygen delivery

Systemic oxygen delivery equals cardiac output multiplied by arterial oxygen content. With output in L/min and content in mL/dL, multiply by ten to reconcile litres and decilitres: DO₂ = cardiac output × CaO₂ × 10. This is a delivery calculation, not oxygen consumption; the latter requires the arterial–venous content difference. See OpenAnesthesia.

If Patient A’s cardiac output is 5 L/min, delivery is approximately 5 × 19.99 × 10 = 1,000 mL/min. At the same output, Patient B’s is approximately 507 mL/min. If Patient B’s output instead rises to 7 L/min, delivery becomes approximately 710 mL/min. These constructed examples show why haemoglobin, oxygenation and flow should be considered together; they do not prescribe a transfusion threshold.

Recognise situations in which saturation can mislead

Anaemia reduces carrying capacity

A person with anaemia may retain a normal PaO₂ and SpO₂ while having reduced oxygen content. Reducing haemoglobin alone does not require an immediate change in oxygen affinity. Longer-term adaptations may coexist, so distinguish the direct effect of reduced haemoglobin from compensatory changes. The BTS oxygen guideline explicitly cautions that normal oximetry does not exclude low oxygen content from anaemia.

Carbon monoxide affects both carriage and interpretation

Carbon monoxide occupies haemoglobin binding sites and increases the oxygen affinity of remaining sites. Oxygen carriage and unloading are impaired. Conventional pulse oximetry can appear reassuring, and PaO₂ does not measure carboxyhaemoglobin. Co-oximetry is needed to assess haemoglobin species. A normal displayed saturation must not exclude suspected carbon monoxide poisoning; see the BTS guideline.

Keep the revision objective clear. This article explains the physiology behind the misleading measurements. For broader poisoning presentations, use the StudyMedical toxicology overview. For pH, compensation and mixed disorders, continue to the ABG interpretation guide.

Five original MRCEM Primary practice SBAs

Questions: identify the variable before selecting an option

These are independently written educational questions, not official or recalled RCEM questions. Assumptions are deliberately stated so that each question has one best answer.

Question 1. In a laboratory comparison, all conditions are held constant except for a change in haemoglobin type. P50 falls from 3.6 to 2.8 kPa. Which conclusion follows most directly?

• A. Oxygen affinity has decreased.

• B. Oxygen affinity has increased.

• C. Haemoglobin concentration has increased.

• D. Cardiac output has fallen.

• E. Arterial oxygen content has doubled.

Question 2. A sample’s PO₂ is reduced while pH, PCO₂, temperature, 2,3-BPG and haemoglobin type remain constant. Which description is most appropriate?

• A. A compulsory rightward curve shift.

• B. A compulsory leftward curve shift.

• C. A rise in P50 caused by reduced PO₂ alone.

• D. Movement along the existing dissociation curve.

• E. A switch from sigmoid to linear haemoglobin binding.

Question 3. Hb is 100 g/L and SaO₂ is 95%. Using 1.34 mL O₂/g and ignoring dissolved oxygen, what is the haemoglobin-bound oxygen content?

• A. 1.27 mL/dL.

• B. 9.50 mL/dL.

• C. 12.73 mL/dL.

• D. 13.40 mL/dL.

• E. 127.30 mL/dL.

Question 4. Two hypothetical patients have equal cardiac output. X has Hb 150 g/L and SaO₂ 90%; Y has Hb 100 g/L and SaO₂ 100%. Ignore dissolved oxygen and assume no abnormal haemoglobin species. Which statement is correct?

• A. X has higher calculated oxygen delivery.

• B. Y has higher calculated oxygen delivery because saturation is higher.

• C. Their calculated oxygen delivery is equal.

• D. Delivery cannot be compared without their exact PaO₂ values.

• E. The comparison depends only on which patient has a left-shifted curve.

Question 5. At a fixed PCO₂, a blood sample releases carbon dioxide when its haemoglobin becomes oxygenated. Which mechanism best describes this observation?

• A. The Bohr effect.

• B. Reduced haemoglobin concentration.

• C. Increased cardiac output.

• D. Increased oxygen affinity caused by fetal haemoglobin.

• E. The Haldane effect.

Answers and explanations

1: B. The lower P50 means that less oxygen pressure achieves 50% saturation: affinity is higher. A reverses that relationship. C and D introduce quantities not determined by P50. E claims a change in content without the haemoglobin and saturation data needed to calculate it. The decisive clue is the change in pressure required for a fixed saturation.

2: D. The question changes PO₂ but fixes the listed determinants of curve position. A and B wrongly turn movement along a curve into a shift. C confuses the sample’s current PO₂ with the curve’s P50. E invents a change in binding behaviour. Read the controlled conditions carefully before reaching for a curve-shift mnemonic.

3: C. First convert 100 g/L to 10 g/dL. Then calculate 1.34 × 10 × 0.95 = 12.73 mL/dL. A is tenfold too small. B omits the oxygen-binding coefficient. D assumes full saturation. E fails to convert g/L to g/dL. Writing the units alongside every value makes the errors easier to detect.

4: A. X has content of 1.34 × 15 × 0.90 = 18.09 mL/dL; Y has 1.34 × 10 × 1.00 = 13.40 mL/dL. Equal flow means delivery follows that comparison. B prioritises saturation while ignoring haemoglobin. C ignores unequal content. D is unnecessary because dissolved oxygen is explicitly excluded. E confuses affinity with the directly supplied measurements. X’s higher calculated delivery does not make its hypoxaemia clinically unimportant.

5: E. The changed variable is oxygenation, and the response is altered carbon dioxide carriage. That identifies Haldane. A reverses the direction of influence. B is not part of the stated experiment. C cannot explain the behaviour of the isolated sample. D concerns oxygen affinity, not the specified carbon dioxide-release phenomenon.

Consolidate the topic for your examination

A short self-test before moving on

Without looking back, explain why equal saturation does not imply equal content. Then explain how P50 identifies affinity, how a shift differs from movement along a curve, and why oxygen delivery needs a flow term. Finally, repeat Question 3 using Hb 120 g/L and SaO₂ 90%; the bound content should be 14.472 mL/dL.

Which RCEM syllabus should you use?

RCEM describes MRCEM Primary as 180 SBAs in three hours. Its announcement dated 3 August 2026 states that the updated Basic Sciences Syllabus applies from the 21 April 2027 examination onwards. Check the official exam information and syllabus announcement for your sitting. This article does not claim a fixed allocation of oxygen-transport questions or replace the applicable syllabus.

Put your physiology knowledge into practice

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