Gaseous Exchange at the Alveoli

Gaseous exchange is the process by which oxygen moves from the air in the alveoli into the blood, and carbon dioxide moves from the blood into the alveoli to be breathed out. This lesson covers the mechanism of gaseous exchange, the six key features of the alveoli that make it efficient, the role of haemoglobin, and how gaseous exchange changes during exercise. This is one of the most frequently examined topics within the AQA GCSE PE specification (3.1.1.2).

---

What Is Gaseous Exchange?

Gaseous exchange is the process by which gases move between the air in the alveoli and the blood in the surrounding capillaries. It occurs by diffusion — the movement of particles from an area of high concentration to an area of low concentration, down the concentration gradient.

Two gases are exchanged:

Gas	Direction of Movement	Reason
Oxygen (O₂)	From the alveoli into the blood	Oxygen concentration is higher in the alveolar air than in the blood arriving from the body
Carbon dioxide (CO₂)	From the blood into the alveoli	Carbon dioxide concentration is higher in the blood (having been produced by cells during respiration) than in the alveolar air

Exam Tip: Always describe gaseous exchange in terms of concentration gradients. Oxygen diffuses from high concentration in the alveoli to low concentration in the blood. Carbon dioxide diffuses from high concentration in the blood to low concentration in the alveoli. Using the term "concentration gradient" will gain you marks.

---

The Process of Gaseous Exchange — Step by Step

1. Air is inhaled and travels through the respiratory pathway (mouth/nose → trachea → bronchi → bronchioles → alveoli).
2. The alveoli are surrounded by a dense network of capillaries carrying deoxygenated blood from the pulmonary artery.
3. Oxygen in the alveolar air is at a higher concentration than in the blood in the capillaries. Oxygen therefore diffuses across the thin alveolar wall and thin capillary wall into the blood.
4. Once in the blood, oxygen binds to haemoglobin in the red blood cells, forming oxyhaemoglobin. This allows large quantities of oxygen to be transported efficiently.
5. At the same time, carbon dioxide in the blood is at a higher concentration than in the alveolar air. Carbon dioxide therefore diffuses from the blood into the alveoli.
6. The carbon dioxide is then expelled from the lungs when we exhale.
7. The now-oxygenated blood travels back to the heart via the pulmonary veins, to be pumped around the body by the left ventricle.

graph LR
    A[Alveolar Air] -->|O₂ diffuses into blood| B[Capillary Blood]
    B -->|CO₂ diffuses into alveoli| A
    B --> C[O₂ binds to haemoglobin]
    C --> D[Oxyhaemoglobin formed]
    D --> E[Carried to body tissues via pulmonary veins and heart]

    style A fill:#27ae60,color:#fff
    style B fill:#e74c3c,color:#fff
    style C fill:#3498db,color:#fff
    style D fill:#2980b9,color:#fff
    style E fill:#f39c12,color:#fff

---

The Six Features of Alveoli That Aid Gaseous Exchange

AQA expects you to know six features of the alveoli that make gaseous exchange efficient. This is one of the most commonly set questions in the exam.

1. Large Surface Area

There are approximately 300–500 million alveoli in the lungs. Their combined surface area is approximately 70 square metres — roughly the size of half a tennis court. This enormous surface area means that a very large volume of gas can be exchanged at any one time.

2. Thin Walls (Short Diffusion Distance)

The walls of the alveoli are only one cell thick (approximately 0.2 micrometres). The adjacent capillary walls are also one cell thick. This means that gases only need to diffuse across two very thin layers of cells — a total distance of less than 1 micrometre. The shorter the diffusion distance, the faster the rate of diffusion.

3. Moist Lining

The inner surface of each alveolus is coated with a thin layer of moisture. Gases must dissolve in this moisture before they can diffuse across the alveolar wall. The moist lining ensures that oxygen can dissolve quickly and diffuse into the blood efficiently.

4. Rich Capillary Network

Each alveolus is wrapped in a dense network of tiny capillaries. This ensures that there is always a large volume of blood flowing past the alveolar surface, ready to pick up oxygen and release carbon dioxide.

5. Good Blood Supply (Maintained Concentration Gradient)

The continuous flow of blood through the capillaries means that deoxygenated blood is constantly being brought to the alveoli, and oxygenated blood is constantly being carried away. This maintains a steep concentration gradient for both oxygen (high in alveoli, low in blood) and carbon dioxide (high in blood, low in alveoli). A steeper gradient means faster diffusion.

6. Concentration Gradient

The concentration gradient is the difference in concentration of a gas between two areas. At the alveoli:

• Oxygen concentration is high in the alveolar air and low in the deoxygenated blood → oxygen diffuses into the blood.
• Carbon dioxide concentration is high in the blood and low in the alveolar air → carbon dioxide diffuses into the alveoli.

The gradient is maintained by continuous ventilation (breathing in fresh air and exhaling stale air) and continuous blood flow past the alveoli.

Feature	How It Aids Gaseous Exchange
Large surface area	More space for diffusion to occur simultaneously
Thin walls	Short diffusion distance = faster diffusion
Moist lining	Gases dissolve in moisture, aiding diffusion across the membrane
Rich capillary network	Large volume of blood exposed to alveolar air
Good blood supply	Maintains steep concentration gradient by constantly refreshing blood
Concentration gradient	Drives the diffusion of O₂ into the blood and CO₂ out of the blood

Exam Tip: For a 6-mark question on alveolar adaptations, you should aim to name all six features and explain how each one specifically assists gaseous exchange. Simply listing the features without linking each one to efficient exchange will not earn full marks.

---

Haemoglobin and Oxyhaemoglobin