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

1. Red blood cells contain a protein called haemoglobin.
2. In the lungs, where oxygen concentration is high, haemoglobin binds with oxygen to form oxyhaemoglobin.
3. The oxygenated red blood cells travel through the bloodstream to the body's tissues.
4. At the muscles (where oxygen concentration is low because the muscles are using it for energy), the oxyhaemoglobin releases the oxygen — it dissociates back into haemoglobin and oxygen.
5. The released oxygen diffuses into the muscle cells for use in aerobic respiration.

flowchart LR
    A["At the lungs:<br>Haemoglobin + O₂<br>→ Oxyhaemoglobin"] --> B["Blood travels<br>to the muscles"]
    B --> C["At the muscles:<br>Oxyhaemoglobin<br>→ Haemoglobin + O₂"]
    C --> D["O₂ used for<br>aerobic respiration"]

    style A fill:#27ae60,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#4a90d9,color:#fff

Adaptations of Red Blood Cells

Feature	How It Helps
Biconcave disc shape	Increased surface area for oxygen absorption
No nucleus	More space inside the cell for haemoglobin
Contains haemoglobin	Haemoglobin binds with O₂ to form oxyhaemoglobin for efficient transport
Flexible	Can squeeze through narrow capillaries

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Gaseous Exchange During Exercise

During exercise, the rate and efficiency of gaseous exchange increase because:

• Breathing rate and depth increase — more air is moved into and out of the lungs, bringing more oxygen to the alveoli and removing more carbon dioxide.
• Heart rate and cardiac output increase — blood flows through the pulmonary capillaries more quickly, maintaining a steep concentration gradient.
• More blood is directed to the muscles (via redistribution) — more oxygen is delivered where it is needed.
• The concentration gradient is steeper at the muscles — exercising muscles use oxygen rapidly, so the difference in oxygen concentration between the blood and muscle cells is greater, increasing the rate of diffusion.

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Gaseous Exchange at the Muscles

As well as exchange at the lungs, gaseous exchange also occurs at the muscle capillaries:

At the Alveoli (Lungs)	At the Muscles
O₂ diffuses INTO the blood	O₂ diffuses OUT OF the blood into muscle cells
CO₂ diffuses OUT OF the blood	CO₂ diffuses INTO the blood from muscle cells
Blood changes from deoxygenated to oxygenated	Blood changes from oxygenated to deoxygenated

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Common Exam Mistakes

• Saying oxygen is "absorbed" or "sucked" into the blood — the correct term is diffusion. Oxygen moves from high concentration to low concentration by diffusion.
• Forgetting to mention the concentration gradient — diffusion only occurs because there is a difference in concentration. Always mention this.
• Not explaining how the features of alveoli help — do not just list the features. Explain HOW each one makes exchange more efficient (e.g., "thin walls mean a short diffusion distance, so gases cross quickly").
• Confusing what happens at the lungs with what happens at the muscles — at the lungs, O₂ enters the blood; at the muscles, O₂ leaves the blood. They are mirror images.
• Forgetting oxyhaemoglobin — always mention that oxygen binds with haemoglobin to form oxyhaemoglobin for transport.

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Summary