Gaseous Exchange

This lesson covers gaseous exchange at the alveoli — how oxygen enters the blood and carbon dioxide is removed — as required by the Edexcel GCSE PE specification (1PE0 — Topic 1). You need to understand the process of diffusion at the alveoli, the adaptations that make gas exchange efficient, and the crucial role of red blood cells and haemoglobin.

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What Is Gaseous Exchange?

Gaseous exchange is the process by which oxygen passes from the air in the alveoli into the blood, and carbon dioxide passes from the blood into the alveoli to be breathed out. This exchange occurs by diffusion — the movement of molecules from an area of high concentration to an area of low concentration (down the concentration gradient).

graph LR
    A["Air in alveolus<br>(high O₂, low CO₂)"] --> B["Alveolar wall<br>(one cell thick)"]
    B --> C["Capillary wall<br>(one cell thick)"]
    C --> D["Blood in capillary<br>(low O₂, high CO₂)"]

    E["O₂ diffuses →<br>into blood"] -.-> D
    D -.-> F["CO₂ diffuses →<br>into alveolus"]

    style A fill:#27ae60,color:#fff
    style D fill:#c0392b,color:#fff

The Process

1. Air rich in oxygen fills the alveoli after being breathed in.
2. Blood arriving at the alveolar capillaries from the pulmonary artery is deoxygenated — it has low oxygen and high carbon dioxide.
3. Because the concentration of oxygen is higher in the alveoli than in the blood, oxygen diffuses across the alveolar wall and the capillary wall into the blood.
4. Because the concentration of carbon dioxide is higher in the blood than in the alveoli, carbon dioxide diffuses from the blood into the alveoli.
5. The oxygen-rich blood then travels back to the heart via the pulmonary veins.
6. The carbon dioxide in the alveoli is exhaled during the next breath out.

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Adaptations of the Alveoli for Efficient Gas Exchange

The alveoli have several key adaptations that make gaseous exchange extremely efficient:

Adaptation	How It Helps Gas Exchange
Huge number (~300 million per lung)	Provides a massive total surface area (~70 m²) for gas exchange
Walls are one cell thick	Minimises the diffusion distance — gases can pass through very quickly
Surrounded by a dense network of capillaries	Ensures a large blood supply is always in close contact with the alveoli
Capillary walls are one cell thick	Further reduces the diffusion distance
Moist inner surface	Gases dissolve in the moisture, making diffusion easier
Rich blood supply	Maintains a steep concentration gradient — fresh deoxygenated blood constantly arrives
Constant ventilation	Breathing constantly refreshes the air in the alveoli, maintaining a high O₂ and low CO₂ concentration

Exam Tip: When explaining why gaseous exchange is efficient, use the phrase "short diffusion distance" — this refers to the alveolar and capillary walls both being one cell thick. Also mention the "large surface area" and "steep concentration gradient." These three factors together explain the efficiency.

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The Role of Red Blood Cells

Red blood cells (erythrocytes) are the cells in the blood responsible for carrying oxygen from the lungs to the body's tissues. They have several adaptations:

Adaptation	How It Helps
Biconcave disc shape	Increases the surface area for oxygen absorption
No nucleus	More space inside the cell for haemoglobin, allowing more oxygen to be carried
Contain haemoglobin	Haemoglobin is the protein that binds to oxygen
Small and flexible	Can squeeze through narrow capillaries, bringing oxygen close to tissues
Produced in bone marrow	Continuously produced to replace old cells (lifespan ~120 days)

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Haemoglobin and Oxyhaemoglobin

Haemoglobin is an iron-containing protein found inside red blood cells. It is responsible for carrying oxygen in the blood.

The Process

1. At the alveoli (in the lungs), where oxygen concentration is high, haemoglobin combines with oxygen to form oxyhaemoglobin:

   Haemoglobin + Oxygen → Oxyhaemoglobin

2. The oxyhaemoglobin is transported in the red blood cells through the arteries and capillaries to the working muscles and tissues.

3. At the tissues (e.g., working muscles), where oxygen concentration is low and carbon dioxide concentration is high, the oxyhaemoglobin releases the oxygen for the cells to use:

   Oxyhaemoglobin → Haemoglobin + Oxygen

4. The haemoglobin then picks up some of the carbon dioxide produced by the cells and carries it back to the lungs for removal (although most CO₂ is carried dissolved in the plasma or as bicarbonate ions).

graph LR
    A["At the LUNGS<br>(high O₂)"] --> B["Haemoglobin + O₂<br>→ Oxyhaemoglobin"]
    B --> C["Transported in<br>red blood cells<br>to muscles"]
    C --> D["At the MUSCLES<br>(low O₂, high CO₂)"]
    D --> E["Oxyhaemoglobin<br>→ Haemoglobin + O₂"]
    E --> F["O₂ used by muscle<br>for energy"]

    style A fill:#27ae60,color:#fff
    style D fill:#c0392b,color:#fff
    style F fill:#e67e22,color:#fff

Exam Tip: Edexcel specifically requires you to know the terms haemoglobin and oxyhaemoglobin and to understand the reversible reaction. At the lungs, haemoglobin binds with oxygen (forming oxyhaemoglobin). At the tissues, oxyhaemoglobin releases oxygen (reverting to haemoglobin). Make sure you can explain both directions.

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Gaseous Exchange During Exercise

During exercise, the rate of gaseous exchange increases significantly:

Change During Exercise	Effect on Gas Exchange
Increased breathing rate and depth	More fresh air reaches the alveoli, maintaining a high O₂ concentration
Increased heart rate and cardiac output	More blood flows through the pulmonary capillaries per minute
Increased CO₂ production by muscles	Steeper concentration gradient for CO₂ diffusion out of the blood
Increased O₂ demand by muscles	Steeper concentration gradient for O₂ diffusion into the blood
Vasodilation of pulmonary capillaries	More blood exposed to the alveoli for exchange

The result is that both the delivery of oxygen to the blood and the removal of carbon dioxide from the blood are greatly accelerated during exercise.

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Gas Exchange at the Muscles (Tissue-Level Gas Exchange)

Gas exchange does not only occur at the lungs — it also occurs at the muscles (and all other body tissues). The process is the same (diffusion), but in reverse:

• Oxygen diffuses from the blood in the capillaries (high O₂) into the muscle cells (low O₂ — because the muscles are using it up).
• Carbon dioxide diffuses from the muscle cells (high CO₂ — because the muscles are producing it) into the blood in the capillaries (low CO₂).

During exercise, this exchange is faster because:

• The muscles are using more oxygen (lower O₂ in the cells = steeper gradient).
• The muscles are producing more carbon dioxide (higher CO₂ in the cells = steeper gradient).
• Capillarisation (from long-term training) increases the surface area for exchange.

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Common Exam Mistakes