Oxygen-Haemoglobin Curve & Bohr Effect

Quick answer: The oxygen–haemoglobin dissociation curve is the S-shaped graph of haemoglobin's oxygen saturation against the partial pressure of oxygen (PO₂). It is sigmoid because of cooperative binding among haemoglobin's four subunits. The curve's shifts match oxygen delivery to demand: the Bohr effect (high CO₂, low pH) shifts it right to unload O₂ in active tissues, while fetal haemoglobin's left-shifted curve loads O₂ across the placenta.

Key Facts: Oxygen-Haemoglobin Dissociation Curve

• The curve plots % haemoglobin saturation (y-axis) against partial pressure of oxygen, PO₂ (x-axis). It is sigmoid (S-shaped) because of cooperative binding among haemoglobin's four subunits.
• At arterial PO₂ (~95–100 mmHg) haemoglobin is ~97% saturated; at resting tissue PO₂ (~40 mmHg) it is ~75% saturated, so about 25% of the oxygen is unloaded per circuit.
• P₅₀ (~27 mmHg for adult HbA) is the PO₂ giving 50% saturation: the standard index of O₂ affinity. Higher P₅₀ = lower affinity.
• Bohr effect: rising CO₂ / falling pH shifts the curve right, reducing affinity so O₂ is released exactly where tissues are working hardest.
• Right shift (favours unloading): ↑CO₂, ↓pH, ↑temperature, ↑2,3-BPG. Left shift (favours loading): the reverse, plus fetal haemoglobin.
• Fetal haemoglobin (HbF) binds 2,3-BPG poorly, giving it higher O₂ affinity than adult HbA, so the fetus draws O₂ from the mother's blood across the placenta.
• Myoglobin, the single-subunit O₂ store in muscle, gives a hyperbolic (not sigmoid) curve, loading O₂ even at very low PO₂.

Why is this curve S-shaped instead of a straight line? That single question is the doorway to the whole topic. The S-shape exists because haemoglobin's four subunits cooperate: the first oxygen molecule has to convince a reluctant haemoglobin to open up, but once it binds, the next three slip in easily. Let us decode the curve the way examiners want to see it.

What Is the Oxygen–Haemoglobin Dissociation Curve?

When the percentage saturation of haemoglobin with O₂ (y-axis) is plotted against the partial pressure of oxygen, PO₂ (x-axis, in mmHg), the result is a sigmoid (S-shaped) curve. Reading it correctly is the first exam skill:

• X-axis: PO₂, the driving pressure of oxygen in the blood, measured in mmHg.
• Y-axis: percentage of haemoglobin's O₂-binding sites that are occupied (0–100%).
• Shape: flat at the top, steep in the middle, flat at the bottom, an S lying on its side.

The oxygen–haemoglobin dissociation curve: saturation rises steeply with PO₂ in the middle range, then plateaus near 100% at lung PO₂. Diagram: Ratznium, Public domain, via Wikimedia Commons.

Why Is the Curve Sigmoid? Cooperative Binding

Each haemoglobin molecule is built from four subunits (two alpha and two beta chains), each carrying one haem group that binds one O₂ molecule, so one haemoglobin carries a maximum of four O₂ molecules. The subunits are not independent; they communicate:

• First O₂: binds with difficulty. Deoxyhaemoglobin sits in the tense T (taut) state, with low O₂ affinity.
• Binding flips the shape: the first O₂ pulls haemoglobin toward the relaxed R state, raising the affinity of the remaining sites.
• O₂ molecules 2–4: bind progressively more easily, producing the steep middle of the curve.
• Unloading works in reverse: in the tissues, releasing the first O₂ makes the rest let go more readily, so oxygen pours off where PO₂ is low.

This is cooperative binding, and it is the whole reason for the S-shape: a slow start, a steep middle, and a plateau where there is nothing left to bind.

Reading the Key Points on the Curve

Two standard reference points define the working range of the curve:

Location	PO2	Hb saturation	What it means
Lungs (arterial blood)	~95–100 mmHg	~97%	Nearly complete loading of O2; the flat plateau means small dips in lung PO2 (e.g. at moderate altitude) barely reduce saturation
Resting tissues (venous blood)	~40 mmHg	~75%	Partial unloading; about 25% of the carried O2 is delivered per circuit
P50	~27 mmHg	50% (by definition)	The standard affinity index: higher P50 = lower affinity (right shift); lower P50 = higher affinity (left shift)

During heavy exercise, tissue PO₂ can fall to ~20 mmHg, where saturation drops to roughly 35%, meaning haemoglobin can surrender most of its oxygen when demand is extreme. The sigmoid shape is physiologically ideal: the plateau guarantees reliable loading in the lungs, while the steep middle guarantees generous unloading for a modest PO₂ fall in the tissues.

The Bohr Effect: Shifting the Curve Right

In 1904 Christian Bohr discovered that rising CO₂ and H⁺ (falling pH) reduce haemoglobin's oxygen affinity. The physiological logic is beautiful: hard-working tissues produce the most CO₂, so the blood arriving there is the most acidic, and its haemoglobin is automatically the most willing to release O₂. On a graph, this appears as a right shift of the whole curve: at any given PO₂, saturation is lower.

The effect reverses in the lungs, where CO₂ is breathed out (see the mechanism of breathing in man) and blood pH rises: the curve shifts back left, and haemoglobin eagerly loads O₂. The Bohr effect thus couples oxygen delivery to metabolic activity automatically, with no extra control needed. At the alveoli and in the tissue capillaries, the actual movement of O₂ and CO₂ across the thin capillary walls happens by simple diffusion down partial-pressure gradients, the passive process explained in our article on transport across the cell membrane.

The Bohr effect: rising CO₂ (and acidity) shifts the saturation curve to the right, lowering oxygen affinity so haemoglobin releases O₂ more readily in active tissues. Diagram: Michał Komorniczak, CC BY 3.0, via Wikimedia Commons.

Factors That Shift the Curve: Right vs Left

Factor	Right shift (↓ affinity → O2 unloaded)	Left shift (↑ affinity → O2 loaded)
CO2 (PCO2)	High PCO2 (Bohr effect)	Low PCO2
pH / H+	Low pH (acidosis, more H+)	High pH (alkalosis)
Temperature	High temperature (working muscles)	Low temperature
2,3-BPG	High 2,3-BPG (e.g. at altitude, in anaemia); stabilises deoxyhaemoglobin	Low 2,3-BPG
Haemoglobin type	Adult HbA (the baseline curve)	Fetal HbF: binds 2,3-BPG poorly, so affinity is higher
P50	Rises (above ~27 mmHg)	Falls (below ~27 mmHg)
Physiological setting	Active tissues, exercise, altitude acclimatisation	Lungs, placenta

Learn the right-shift quartet with the mnemonic "CADET, face Right!": CO₂, Acid (low pH), DPG (2,3-BPG), Exercise, Temperature. Every one of them makes haemoglobin release oxygen more easily.

Fetal Haemoglobin: A Left Shift Across the Placenta

Fetal haemoglobin (HbF) has two gamma chains in place of the adult beta chains, and this small change makes it bind 2,3-BPG poorly. Since 2,3-BPG normally lowers O₂ affinity, HbF ends up with a higher oxygen affinity than the mother's adult HbA, and its whole curve sits to the left of the mother's. Across the placenta, oxygen therefore moves from the mother's blood to the fetal blood down the affinity gradient: the fetus effectively steals O₂ from maternal haemoglobin. After birth, HbF is gradually replaced by HbA over the first months of life.

Myoglobin vs Haemoglobin

Myoglobin is haemoglobin's single-subunit cousin, stored in muscle cells as an oxygen reserve. Because it has only one haem group, it shows no cooperativity, and its dissociation curve is hyperbolic, not sigmoid:

Feature	Haemoglobin	Myoglobin
Subunits	4 (tetramer)	1 (monomer)
Cooperativity	Yes	No
Curve shape	Sigmoid	Hyperbolic
Affinity	Moderate (releases O2 readily)	Very high (loads O2 even at very low PO2)
Role	Transport: load in lungs, unload in tissues	Storage: holds O2 in muscle, releases it only when tissue PO2 falls very low

Myoglobin's high affinity means it only surrenders its oxygen when the muscle is working so hard that tissue PO₂ collapses, exactly when the reserve is needed most. Diving mammals such as whales carry far more myoglobin in their muscles than land mammals, which is part of how they stay submerged so long.

Examiner tip

The classic 5-mark question: "Explain the Bohr effect with reference to the oxygen dissociation curve." Structure it as: (1) define the curve and its S-shape from cooperative binding, (2) state Bohr's observation (CO₂/H⁺ lower affinity), (3) describe the right shift, (4) give the physiological payoff: O₂ is released precisely in hard-working tissues. Sketch the curve with labeled axes and mark P₅₀; a surprising number of students lose marks by drawing the axes backwards. And never forget: fetal haemoglobin shifts left.

Frequently Asked Questions

Why is the curve S-shaped (sigmoid)?

Because of cooperative binding: haemoglobin's four subunits influence each other. The first O₂ binds with difficulty to the low-affinity T state, but its binding flips haemoglobin toward the high-affinity R state, so O₂ molecules 2–4 bind progressively more easily. Hence the slow start, steep middle, and plateau.

What is P50? Is it just a number to memorize?

P₅₀ is the PO₂ at which haemoglobin is 50% saturated, about 27 mmHg for adult HbA. It is the standard index of O₂ affinity: a higher P₅₀ means lower affinity (right shift); a lower P₅₀ means higher affinity (left shift).

Right shift vs left shift: I always mix them up.

Use the mnemonic: right = release (both start with R). A right shift means haemoglobin releases O₂ more readily (low affinity): good for active tissues. A left shift means it holds on (high affinity): good for loading O₂ in the lungs or placenta.

How does the fetus get oxygen from the mother?

Fetal haemoglobin (HbF) has a left-shifted curve because it binds 2,3-BPG weakly, giving it higher O₂ affinity than the mother's HbA. Across the placenta, O₂ therefore moves from maternal blood to fetal blood down the affinity gradient.

How does myoglobin's curve differ from haemoglobin's?

Haemoglobin (4 subunits, cooperative) gives a sigmoid curve suited to transport: load in the lungs, unload in the tissues. Myoglobin (1 subunit, no cooperativity) gives a hyperbolic curve: it loads O₂ even at very low PO₂, acting as an oxygen store for muscles.

How does exercise change oxygen delivery?

Exercise raises tissue CO₂, H⁺ and temperature, all shifting the curve right (Bohr effect), and tissue PO₂ can fall to ~20 mmHg. Together, these changes let haemoglobin unload far more of its oxygen, up to about 65% of what it carries, to the working muscles.

Practice MCQs

Q1. The oxygen–haemoglobin dissociation curve is sigmoid because of:

1. Bohr effect
2. Cooperative binding of O₂
3. High 2,3-BPG
4. Low pH

Answer: B — Each bound O₂ raises affinity for the next: the basis of the S-shape.

Q2. A right shift of the curve is caused by:

1. Decreased CO₂
2. Increased pH
3. Increased CO₂ and decreased pH
4. Decreased temperature

Answer: C — The Bohr effect: CO₂/H⁺ lower O₂ affinity, shifting the curve right.

Q3. Fetal haemoglobin's curve is left-shifted because it:

1. Binds 2,3-BPG strongly
2. Has only two subunits
3. Binds 2,3-BPG poorly, giving higher O₂ affinity
4. Carries CO₂ instead of O₂

Answer: C — Weak 2,3-BPG binding raises HbF's O₂ affinity above the mother's HbA.

Q4. P₅₀ is defined as the PO₂ at which haemoglobin is:

1. 25% saturated
2. 50% saturated
3. 75% saturated
4. 100% saturated

Answer: B — P₅₀ (~27 mmHg) is the standard index of O₂ affinity.

Q5. Myoglobin's dissociation curve is:

1. Sigmoid, like haemoglobin
2. Hyperbolic, suited to O₂ storage
3. Linear
4. Shifted right of haemoglobin

Answer: B — Single-subunit myoglobin shows no cooperativity; its hyperbolic curve loads O₂ at very low PO₂.

Q6. Each haemoglobin molecule can carry a maximum of:

1. One O₂ molecule
2. Two O₂ molecules
3. Four O₂ molecules
4. Eight O₂ molecules

Answer: C — Haemoglobin's four haem groups bind four O₂ molecules cooperatively.

Q7. The sigmoid shape is physiologically ideal because haemoglobin:

1. Never releases its oxygen
2. Loads fully at the high PO₂ of the lungs yet unloads readily at the low PO₂ of tissues
3. Prefers binding carbon monoxide
4. Works only at a single PO₂

Answer: B — The plateau gives full loading in the lungs; the steep middle gives generous unloading in tissues.

Q8. Increased 2,3-BPG in red blood cells shifts the curve:

1. To the right, lowering oxygen affinity
2. To the left, raising oxygen affinity
3. Not at all
4. Vertically upward

Answer: A — 2,3-BPG stabilises deoxyhaemoglobin, so high levels (e.g. at altitude, in anaemia) favour O₂ unloading.

Q9. At the lungs haemoglobin is about ___ saturated; in resting tissues about ___:

1. 50%; 50%
2. 97%; 75%
3. 75%; 97%
4. 100%; 0%

Answer: B — Near-full loading (~97%) in the lungs and partial unloading (~75% remaining) in tissues.

Q10. Which of the following does NOT shift the curve to the right?

1. High CO₂
2. High temperature
3. Increased 2,3-BPG
4. High pH

Answer: D — High pH (low CO₂) shifts the curve left, raising affinity; the other three shift it right.

Q11. During heavy exercise, tissue PO₂ can fall to ~20 mmHg. Haemoglobin then unloads approximately:

1. 5% of its oxygen
2. 25% of its oxygen
3. 65% of its oxygen
4. 100% of its oxygen

Answer: C — At ~20 mmHg saturation is roughly 35%, so about 65% of the carried O₂ is delivered to working muscles.

Q12. A blood sample shows a P₅₀ of 32 mmHg (normal ~27 mmHg). This indicates:

1. A left shift and increased O₂ affinity
2. A right shift and decreased O₂ affinity
3. No change in the dissociation curve
4. Conversion of haemoglobin to myoglobin

Answer: B — A higher P₅₀ means 50% saturation needs more PO₂, i.e. lower affinity: a right shift.

Short Questions

Q1. What is the oxygen–haemoglobin dissociation curve, and what shape is it? See answer

The oxygen–haemoglobin dissociation curve plots the percentage saturation of haemoglobin with O₂ (y-axis) against the partial pressure of oxygen, PO₂ (x-axis, in mmHg). It is sigmoid (S-shaped): flat at the top, steep in the middle, and flat at the bottom.

The S-shape comes from cooperative binding. Each haemoglobin molecule carries four O₂ molecules on four haem groups, and binding the first O₂ flips haemoglobin from the low-affinity T (tense) state toward the high-affinity R (relaxed) state, so the remaining O₂ molecules bind progressively more easily.

The shape is physiologically ideal: the plateau at high PO₂ guarantees nearly full loading in the lungs even if alveolar oxygen fluctuates, while the steep middle between ~20–60 mmHg guarantees generous unloading in tissues for only a small fall in PO₂. The air supplying this oxygen is moved by the mechanism of breathing in man.

Q2. Define the Bohr effect and explain its physiological importance. See answer

The Bohr effect, described by Christian Bohr in 1904, states that haemoglobin's affinity for oxygen decreases as CO₂ concentration rises and pH falls (and increases when the reverse happens). On the graph this appears as a right shift of the dissociation curve: at any given PO₂, haemoglobin holds less oxygen.

Its physiological importance is automatic targeting of delivery. Hard-working tissues produce the most CO₂, so the blood reaching them is the most acidic and its haemoglobin the most willing to release O₂. Oxygen is thus delivered precisely where metabolic demand is highest, without any separate control signal.

In the lungs the effect reverses: CO₂ is breathed out and blood pH rises, so the curve shifts back left and haemoglobin eagerly loads O₂. The actual movement of gases across the alveolar and capillary walls happens by diffusion, the passive process covered in our article on transport across the cell membrane.

Q3. List the factors that shift the dissociation curve right and left. See answer

Right shift (decreased affinity, O₂ unloaded): increased CO₂ (the Bohr effect), decreased pH (acidosis, more H⁺), increased temperature (working muscles), and increased 2,3-BPG, which stabilises deoxyhaemoglobin. The mnemonic is "CADET, face Right!": CO₂, Acid, DPG, Exercise, Temperature.

Left shift (increased affinity, O₂ loaded): the reverse conditions, decreased CO₂, increased pH, decreased temperature, decreased 2,3-BPG, plus fetal haemoglobin (HbF), whose poor 2,3-BPG binding gives it permanently higher affinity than adult HbA.

A classic assertion-reason trap pairs these: "A rise in PCO₂ shifts the curve right because it decreases haemoglobin's oxygen affinity." Both the assertion and the reason are true, and the reason correctly explains the assertion, so the answer is that both are true with correct explanation.

Q4. What is P50, and what does a change in its value mean? See answer

P₅₀ is the partial pressure of oxygen at which haemoglobin is 50% saturated, about 27 mmHg in a normal adult. It is a single number that summarises the whole curve's position and therefore haemoglobin's oxygen affinity.

A higher P₅₀ (e.g. 32 mmHg) means 50% saturation now requires a higher PO₂, so affinity is lower: the curve has shifted right. A lower P₅₀ means affinity is higher: the curve has shifted left.

Clinically, P₅₀ rises at high altitude (more 2,3-BPG is produced to aid unloading) and falls in stored blood (2,3-BPG degrades), which is one reason stored blood unloads oxygen less readily. Examiners love asking students to mark P₅₀ on a sketched curve, so practise drawing and labelling the axes first.

Q5. Why does fetal haemoglobin have a left-shifted curve? What is the advantage? See answer

Fetal haemoglobin (HbF) replaces the adult beta chains with gamma chains, and this structural change makes it bind 2,3-BPG poorly. Because 2,3-BPG normally lowers oxygen affinity by stabilising deoxyhaemoglobin, HbF ends up with a higher oxygen affinity than adult HbA, and its dissociation curve sits to the left of the mother's curve.

The advantage is placental transfer. Across the placenta, maternal and fetal blood come close but never mix; oxygen moves from the mother's haemoglobin to the fetus's down the affinity gradient. The fetus effectively draws oxygen out of the mother's blood, which is essential because placental blood never reaches the high PO₂ of adult lungs.

After birth, as the baby starts breathing air, HbF is gradually replaced by adult HbA over the first few months of life, and the curve settles to its adult position. This is also why newborns cope differently with blood transfusions than adults do.

Q6. Compare the dissociation curves of haemoglobin and myoglobin. See answer

Haemoglobin is a tetramer of four subunits whose cooperative binding produces a sigmoid curve, ideal for transport: nearly full loading at the high PO₂ of the lungs (~97% at 100 mmHg) and easy unloading at the low PO₂ of tissues (~75% at 40 mmHg).

Myoglobin is a monomer with a single haem group and no cooperativity, so its curve is hyperbolic. It has very high oxygen affinity, loading O₂ even at very low PO₂, which suits its role as an oxygen store in muscle cells.

The two work as a relay: haemoglobin delivers oxygen down to a PO₂ of about 40 mmHg, and myoglobin picks it up from there, releasing its stored oxygen only when the muscle is working so hard that tissue PO₂ collapses. Diving mammals such as whales carry exceptionally high muscle myoglobin, which is part of how they remain submerged for so long.

References

1. OpenStax, Anatomy & Physiology 2e, §22.5 "Transport of Gases": OpenStax A&P 2e — transport of gases: the oxygen–haemoglobin saturation curve.
2. Hickman, C.P., Roberts, L.S., Keen, S.L. et al. Integrated Principles of Zoology. McGraw-Hill Education. (Gas transport; oxygen dissociation.)
3. Schmidt-Nielsen, K. Animal Physiology: Adaptation and Environment. Cambridge University Press. (Oxygen transport; Bohr effect; myoglobin.)
4. NCBI Bookshelf, StatPearls: "Physiology, Oxygen Transport": NCBI Bookshelf — physiology of oxygen transport.

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