Gaseous Exchange at the Alveoli

This lesson covers gaseous exchange at the alveoli as required by the OCR GCSE PE specification (J587). You need to understand the process by which oxygen enters the blood and carbon dioxide leaves the blood, the features of the alveoli that make this process efficient, and the role of red blood cells in transporting oxygen.

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What Is Gaseous Exchange?

Gaseous exchange is the process by which oxygen (O₂) passes from the air in the alveoli into the blood, and carbon dioxide (CO₂) passes from the blood into the air in the alveoli. This exchange occurs by diffusion — the movement of molecules from an area of high concentration to an area of low concentration.

graph LR
    A["Air in alveolus<br>(high O₂, low CO₂)"] -->|"O₂ diffuses into blood"| B["Blood in capillary"]
    B -->|"CO₂ diffuses into alveolus"| A

    style A fill:#27ae60,color:#fff
    style B fill:#e74c3c,color:#fff

The Process Step by Step

1. Air enters the alveoli through the bronchioles (as described in the previous lesson).
2. The air in the alveoli has a high concentration of oxygen and a low concentration of carbon dioxide.
3. The blood arriving at the alveoli in the pulmonary capillaries is deoxygenated — it has a low concentration of oxygen and a high concentration of carbon dioxide (because the body's cells have used up the oxygen and produced CO₂ as waste).
4. Because of the concentration gradient, oxygen diffuses from the alveolus (high O₂) into the blood (low O₂).
5. At the same time, carbon dioxide diffuses from the blood (high CO₂) into the alveolus (low CO₂).
6. The blood in the capillary is now oxygenated and returns to the heart via the pulmonary veins.
7. The CO₂ in the alveoli is breathed out during expiration.

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Features of the Alveoli That Make Gaseous Exchange Efficient

The alveoli have several features that maximise the efficiency of gaseous exchange:

Feature	How It Helps
Enormous surface area	~300 million alveoli provide a surface area of ~70 m², allowing a huge amount of exchange to occur simultaneously
Thin walls (one cell thick)	Short diffusion distance — gases can pass through very quickly
Rich blood supply (dense capillary network)	A constant flow of blood maintains the concentration gradient — deoxygenated blood is always arriving, and oxygenated blood is always leaving
Moist lining	Gases dissolve in the moisture, making diffusion easier
Good ventilation	Continuous breathing brings fresh air (high O₂) and removes stale air (high CO₂), maintaining the concentration gradient

Exam Tip: When asked to explain why gaseous exchange is efficient, always mention: (1) large surface area, (2) thin walls (short diffusion distance), (3) rich blood supply (maintains concentration gradient), and (4) moist lining. These four features are the key to a full-marks answer.

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The Role of Red Blood Cells

Red blood cells (erythrocytes) are essential for transporting oxygen from the lungs to the working muscles.

How Red Blood Cells Transport Oxygen