Haemoglobin and Oxygen Transport

By the end of this lesson you should be able to explain and apply each part of this topic — Structure of Haemoglobin, The Oxygen Dissociation Curve, The Bohr Effect and Fetal Haemoglobin (HbF) — and use these ideas accurately in exam-style questions.

Spec Mapping — OCR H420 Module 3.1.2 — Transport in animals, content statements covering the structure of haemoglobin as a quaternary protein with four haem groups, the sigmoidal oxygen dissociation curve and its physiological interpretation, the Bohr effect, and the adaptations of foetal and high-altitude haemoglobins (refer to the official OCR H420 specification document for exact wording). This lesson is a beautiful example of structure-function reasoning in protein biology.

Oxygen is only sparingly soluble in water, so plasma alone can carry just 3 cm³ of O₂ per dm³ of blood — far too little to support the metabolism of a mammal. The solution is haemoglobin, a protein packed into red blood cells that binds oxygen reversibly and raises the blood's oxygen-carrying capacity by a factor of about 70. This lesson examines the structure of haemoglobin, the characteristic sigmoidal oxygen dissociation curve, the effect of carbon dioxide on haemoglobin (the Bohr effect), and the adaptations of different types of haemoglobin.

The history is a tour of physical chemistry applied to biology. Felix Hoppe-Seyler isolated haemoglobin in 1862. Max Perutz (1959) solved its crystal structure by X-ray diffraction at the Medical Research Council Laboratory of Molecular Biology in Cambridge — the first protein structure ever determined, work that won him and John Kendrew the 1962 Nobel Prize. Christian Bohr (father of the physicist Niels Bohr) showed in 1904 that rising CO₂ reduces haemoglobin's oxygen affinity — the Bohr effect. Archibald Hill (1910) wrote down the cooperative-binding equation that bears his name. Together these threads make haemoglobin perhaps the best-understood protein in all of biology.

Key Definitions:

Haemoglobin — a globular conjugated protein consisting of four polypeptide subunits, each containing a haem group with a central iron(II) ion.

Oxyhaemoglobin — haemoglobin with oxygen bound to it.

Partial pressure of oxygen (pO₂) — the contribution of oxygen to the total pressure of a gas mixture, measured in kPa. Around 21 kPa in inhaled air.

Cooperative binding — when the binding of one ligand molecule makes it easier for the next to bind.

Bohr effect — the shift in the haemoglobin dissociation curve to the right caused by increased CO₂ / decreased pH, reducing haemoglobin's affinity for oxygen.

Structure of Haemoglobin

Adult human haemoglobin (HbA) has quaternary structure: four polypeptide chains held together non-covalently. There are:

Two α (alpha) chains, each 141 amino acids long.
Two β (beta) chains, each 146 amino acids long.

Each of the four subunits folds into a compact globular shape and contains a haem group — a porphyrin ring with a central iron (Fe²⁺) ion. Each iron can bind one oxygen molecule. One molecule of haemoglobin can therefore carry a maximum of four O₂ molecules:

$\text{Hb} + 4\,\text{O}_2 \rightleftharpoons \text{Hb(O}_2)_4$

Inside a red blood cell there are about 270 million haemoglobin molecules. Since there are roughly 5 × 10⁹ RBCs per cm³ of blood, each cubic millimetre of blood can carry an enormous amount of oxygen.

The Oxygen Dissociation Curve

Plotting the percentage saturation of haemoglobin against the partial pressure of oxygen gives a characteristic S-shaped (sigmoidal) curve:

flowchart LR
    A[Low pO2 in tissues: about 5 kPa] -->|Hb is about 25 percent saturated| UNLOAD[O2 unloaded to tissues]
    B[High pO2 in lungs: about 13 kPa] -->|Hb is about 98 percent saturated| LOAD[O2 loaded]

Shape of the Curve

At low pO₂ (e.g., respiring tissues, ~5 kPa), the curve rises only slowly. Binding the first O₂ molecule is difficult because haemoglobin is in its tense (T) state, with a lower affinity for oxygen.
Once the first O₂ binds, it causes a conformational change in the protein that brings the other haem groups into a higher-affinity relaxed (R) state. The next few oxygens bind much more easily — the curve rises steeply.
At high pO₂ (e.g., lungs, ~13 kPa), haemoglobin is almost fully saturated (~98%). Adding more oxygen has little additional effect — the curve plateaus.

This is cooperative binding, and it explains the sigmoidal shape.

Why the Shape Matters

In the lungs, pO₂ is high, so haemoglobin loads nearly to saturation. Small decreases in pO₂ (e.g., at altitude) have little effect because the curve is on its plateau.
In the tissues, pO₂ is low and falling because cells are respiring. The steep middle portion of the curve means that a small fall in pO₂ causes a large drop in saturation — in other words, a lot of oxygen is released per unit pO₂ change. This is ideal for efficient unloading.

% Hb saturation pO₂ (kPa)

100 50 0 0 2 5 10 14

Adult Hb (HbA) Bohr shift → (high CO₂ / low pH) Foetal Hb (HbF) ← Myoglobin ←

Reading the curves left-to-right: at the tissue pO₂ of about 5 kPa, adult HbA is ~25% saturated (releasing ~75% of its load), foetal HbF is ~55% saturated (releasing only ~45%, because it has higher affinity), and myoglobin is ~95% saturated — myoglobin only releases its oxygen at very low pO₂, acting as a tissue reserve.

Worked example — calculating how much oxygen the blood actually delivers

Reading percentages off the dissociation curve is only the first step; the skill examiners really prize is turning those percentages into an actual quantity of oxygen delivered, and this requires one further piece of information. Fully saturated blood carries a known amount of oxygen bound to haemoglobin, and a widely used value is that each cubic decimetre of blood, when the haemoglobin is completely saturated, holds about two hundred cubic centimetres of oxygen. This figure is the bridge between saturation and delivery.

Now trace a cubic decimetre of blood on its journey. In the lungs the partial pressure of oxygen is high, around thirteen kilopascals, and the curve shows the haemoglobin is about ninety-eight per cent saturated. The oxygen carried at this point is therefore ninety-eight per cent of two hundred cubic centimetres, which is about one hundred and ninety-six cubic centimetres. The blood then travels to a resting tissue where the partial pressure of oxygen has fallen to about five kilopascals. Reading the curve again, the saturation here is only about twenty-five per cent, so the oxygen still carried is twenty-five per cent of two hundred, which is fifty cubic centimetres. The oxygen actually unloaded to the tissue is the difference between what the blood arrived with and what it leaves with, which is one hundred and ninety-six minus fifty, giving one hundred and forty-six cubic centimetres of oxygen delivered per cubic decimetre of blood.

The calculation becomes even more revealing when we consider exercising tissue, because two effects now act together. In hard-working muscle the partial pressure of oxygen may fall further, to around two kilopascals, and at the same time the high carbon dioxide output triggers the Bohr shift, moving the curve to the right so that saturation at any given partial pressure is lower still. Suppose the combined result is that saturation in the muscle drops to about ten per cent. The oxygen still carried is now only ten per cent of two hundred, which is twenty cubic centimetres, so the oxygen unloaded is one hundred and ninety-six minus twenty, giving one hundred and seventy-six cubic centimetres per cubic decimetre. Comparing this with the resting figure of one hundred and forty-six cubic centimetres shows that the same litre of blood delivers substantially more oxygen to exercising muscle than to resting tissue, without the lungs having to do anything different. This is the dissociation curve and the Bohr effect working in partnership, and being able to quantify the extra delivery, rather than merely assert that it happens, is a genuine top-band skill.

A useful way to compare different haemoglobins numerically, which strong candidates can bring in, is to read off the partial pressure at which the haemoglobin is exactly half saturated. This value is a single-number summary of affinity: a haemoglobin that reaches half saturation at a low partial pressure has a high affinity, because it holds on to oxygen even when little is available, whereas one that only reaches half saturation at a higher partial pressure has a lower affinity. On the curves shown, foetal haemoglobin and the llama's high-altitude haemoglobin reach half saturation further to the left than adult human haemoglobin, confirming their higher affinity, while the mouse's right-shifted curve reaches half saturation further to the right, confirming its lower affinity and readiness to unload. Anchoring the comparison to this half-saturation point, rather than describing the curves only in vague terms of left and right, gives an answer a precision that examiners reward.

The Bohr Effect

In tissues that are respiring rapidly, carbon dioxide concentration is high. This has two key effects:

CO₂ dissolves in the plasma and diffuses into red blood cells, where carbonic anhydrase catalyses its conversion to carbonic acid, which dissociates to H⁺ and hydrogencarbonate (see next lesson). The pH therefore falls.
The resulting H⁺ ions bind to haemoglobin at sites distinct from the O₂-binding sites, causing a conformational change that reduces oxygen affinity.

The net effect is that the dissociation curve shifts to the right: at any given pO₂, the haemoglobin is less saturated. More oxygen is therefore released in exactly those tissues that are working hardest and need it most.

flowchart LR
    C[High CO2 or low pH] --> H[Increased H plus]
    H --> B[Hb affinity for O2 decreases]
    B --> R[Curve shifts right]
    R --> U[More O2 unloaded at same pO2]

This is the Bohr effect, and it is an elegant example of how a single molecular mechanism adjusts oxygen delivery to match tissue demand.

Exam Tip: When describing the Bohr effect, always state explicitly: "Increased CO₂ → lower pH → reduced Hb affinity → curve shifts right → more O₂ unloaded at the tissues." OCR mark schemes reward this causal chain.

Fetal Haemoglobin (HbF)

A fetus receives oxygen across the placenta from its mother's blood. In the placental capillaries, maternal blood has already given up some oxygen to other tissues, so the pO₂ is relatively low. If fetal haemoglobin had the same affinity as maternal haemoglobin, the fetus would not be able to take up enough oxygen. Instead, fetal haemoglobin (HbF) has a structure containing two α chains and two γ (gamma) chains instead of β chains. This small change gives HbF a higher affinity for oxygen than adult haemoglobin at any given pO₂ — its dissociation curve is shifted to the left of HbA.

As a result, at the relatively low pO₂ in the placenta, fetal haemoglobin can still become saturated while maternal haemoglobin releases its oxygen. Soon after birth, production switches to the adult form.

flowchart LR
    MUM[Maternal Hb at placental pO2: partially saturated] --> O2[O2 released]
    O2 --> FET[Fetal Hb: higher affinity, picks up O2]

Myoglobin

Muscle tissue contains myoglobin, a single-subunit relative of haemoglobin. It has only one polypeptide chain and one haem group, so it binds just one O₂ molecule. Because it has no cooperative binding, its dissociation curve is hyperbolic (not sigmoidal) and is shifted far to the left of haemoglobin. This means myoglobin only releases its oxygen at very low pO₂ — in practice, when muscle cells are severely oxygen-starved during intense exercise. Myoglobin therefore acts as an oxygen store in muscle.

Haemoglobin and Oxygen Transport

Haemoglobin and Oxygen Transport

Structure of Haemoglobin

The Oxygen Dissociation Curve

Shape of the Curve

Why the Shape Matters

Worked example — calculating how much oxygen the blood actually delivers

The Bohr Effect

Fetal Haemoglobin (HbF)

Myoglobin

Comparison: O₂ transport vs CO₂ transport mechanisms

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