Haemoglobin and Oxygen Transport: GCSE Biology

What is haemoglobin and how is it structured?

Haemoglobin is a large protein found in red blood cells (erythrocytes). It consists of four polypeptide chains, each folded around a haem group — a ring-shaped molecule containing an iron(II) ion (Fe²⁺). Each iron ion can bind reversibly with one oxygen molecule (O₂), so one haemoglobin molecule can carry up to four O₂ molecules.

Haemoglobin + 4O₂ ⇌ Oxyhaemoglobin

This is a reversible reaction: haemoglobin binds oxygen when oxygen concentration is high (in the lungs) and releases it when oxygen concentration is low (in the tissues). The direction depends on the partial pressure of oxygen (pO₂) — the concentration of oxygen in the surrounding environment.

The iron in haemoglobin gives blood its red colour. Oxygenated blood (with oxyhaemoglobin) is bright red; deoxygenated blood (with haemoglobin) is darker red (not blue, despite the common myth — veins look bluish through skin because blue light penetrates skin more easily than red light).

How does haemoglobin load and unload oxygen?

At the lungs (pulmonary capillaries):

• Air in the alveoli contains oxygen at a high partial pressure (~13 kPa).
• This high pO₂ favours binding — haemoglobin picks up oxygen and becomes oxyhaemoglobin.
• Blood leaving the lungs is approximately 97–98% saturated with oxygen.

At the body tissues (systemic capillaries):

• Respiring cells consume oxygen continuously — pO₂ in tissues is lower (approximately 2–5 kPa at rest, lower during exercise).
• This low pO₂ favours release — oxyhaemoglobin releases oxygen, which diffuses into cells.
• Blood leaving a resting tissue retains roughly 75% of its oxygen (so ~25% is delivered per pass); during vigorous exercise, much more is released.

What is the oxygen-haemoglobin dissociation curve?

The relationship between oxygen saturation of haemoglobin and partial pressure of oxygen is shown by the oxygen-haemoglobin dissociation curve — an S-shaped (sigmoidal) graph.

Key features of the curve:

• Flat region (right side at high pO₂): even if pO₂ falls slightly below the lung maximum, haemoglobin remains near 100% saturated — this is the "loading plateau" in the alveolar capillaries.
• Steep region (middle): small changes in pO₂ in the tissue range cause large changes in saturation — haemoglobin rapidly releases oxygen to tissues as pO₂ falls.
• Low-pO₂ tail (left side): at very low pO₂ (exercising muscles), saturation falls to very low values — maximum O₂ delivery to the hardest-working tissues.

The S-shape results from cooperative binding: once one O₂ molecule binds to haemoglobin, it slightly changes the protein's shape (conformational change), making it easier for the next O₂ to bind. Unloading works in reverse — releasing one O₂ makes the others easier to release.

What is the Bohr effect?

The Bohr effect describes how increased carbon dioxide concentration shifts the dissociation curve to the right — at any given pO₂, haemoglobin holds less oxygen and releases more.

Mechanism: CO₂ from active cells dissolves in the blood and combines with water to form carbonic acid (H₂CO₃), which dissociates to give H⁺ ions (lowering pH). These H⁺ ions bind to haemoglobin, slightly altering its shape and reducing its affinity for oxygen.

Adaptive significance: tissues that are most metabolically active (exercising muscles, active organs) produce the most CO₂, creating the most acidic local conditions. The Bohr effect ensures that haemoglobin unloads the most oxygen precisely where and when it is most needed — an elegant self-regulating mechanism.

Comparison table: loading and unloading conditions

What is foetal haemoglobin and why is it different?

A foetus in the womb must obtain oxygen from the mother's blood across the placenta — the mother's blood has already been partially deoxygenated by her own tissues. To extract oxygen from blood that is only partially oxygenated, foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin — its dissociation curve is shifted to the left.

This means that at the same pO₂, foetal haemoglobin picks up more oxygen than adult haemoglobin. In the placenta, where maternal pO₂ is relatively low, foetal haemoglobin can still become well saturated — effectively "stealing" oxygen from the mother's blood. After birth, the foetal haemoglobin is gradually replaced by adult haemoglobin over the first few months of life.

Frequently asked questions

Why is the dissociation curve S-shaped rather than a simple straight line?

The S-shape (sigmoidal curve) results from cooperative binding — the four haem groups do not bind oxygen independently. When the first O₂ binds, it causes a slight conformational change in the haemoglobin molecule that makes it easier for the second O₂ to bind, and so on. This cooperative mechanism means that haemoglobin transitions sharply from mostly unloaded to mostly loaded over a relatively small range of pO₂, making it highly effective at both loading in the lungs and unloading in the tissues. A non-cooperative molecule (like myoglobin, a muscle oxygen store with only one haem group) has a simple hyperbolic curve with no steep unloading region.

Why does carbon monoxide poisoning prevent oxygen transport?

Carbon monoxide (CO) binds to the iron(II) in haemoglobin's haem groups at the same site as oxygen, but approximately 200 times more strongly than O₂. Once CO binds, it does not release — haemoglobin is permanently occupied and cannot carry oxygen. Even a low concentration of CO in inhaled air (e.g. from a faulty gas appliance or car exhaust) rapidly ties up a large fraction of the body's haemoglobin, causing oxygen deprivation (tissue hypoxia), which causes headache, dizziness, unconsciousness, and at high exposures, death. The bright red colour of carboxyhaemoglobin means CO-poisoned individuals may not look pale or blue (cyanosed) as expected.

How does altitude affect haemoglobin and oxygen transport?

At high altitude, atmospheric pressure and therefore the partial pressure of oxygen are lower. Less O₂ is available per breath. In response, the kidneys release the hormone erythropoietin (EPO), which stimulates the bone marrow to produce more red blood cells and therefore more haemoglobin. Over weeks at altitude, a person acclimatises: the higher red cell count increases the oxygen-carrying capacity of the blood, partially compensating for the reduced pO₂. This is why athletes sometimes train at altitude — the acclimatisation-induced increase in red cell count persists for some weeks after returning to sea level, temporarily boosting oxygen delivery to muscles.

What is the difference between haemoglobin and myoglobin?

Haemoglobin is the oxygen transport protein in red blood cells — it has four subunits, cooperatively loads oxygen in the lungs, and unloads it in tissues. Myoglobin is an oxygen storage protein found in muscle cells — it has a single polypeptide chain with one haem group. Myoglobin has a higher affinity for oxygen than haemoglobin (its curve is hyperbolic and to the left of haemoglobin's), so it picks up oxygen from haemoglobin as it is released in the muscle. Myoglobin serves as an emergency reserve of oxygen — it only releases O₂ when muscle pO₂ falls very low (during intense or prolonged exercise). Myoglobin gives red (aerobic) muscle its characteristic dark colour.

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