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Gas Exchange in Humans - Gas exchange surfaces

Grade 11IGCSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Gas exchange in humans occurs in the alveoli, which are tiny air sacs located at the end of the bronchioles in the lungs.

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The process occurs via diffusion, where molecules move from an area of high concentration to an area of low concentration. O2O_2 diffuses from the alveoli into the red blood cells, while CO2CO_2 diffuses from the plasma into the alveoli.

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A large surface area is provided by the millions of alveoli in the lungs, estimated to be between 7070 m2m^2 and 100100 m2m^2 in an adult human, which increases the total volume of gas exchanged.

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A very thin diffusion pathway is maintained because the alveolar walls and the capillary walls are each only one cell thick (approximately 0.50.5 μm\mu m).

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A steep concentration gradient is maintained by continuous ventilation (breathing) and a constant flow of blood through the surrounding capillaries.

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The internal surface of the alveoli is coated with a thin layer of moisture, allowing O2O_2 and CO2CO_2 to dissolve before diffusing across the membrane.

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Inhaled air contains approximately 21%21\% O2O_2 and 0.04%0.04\% CO2CO_2, whereas exhaled air contains approximately 16%16\% O2O_2 and 4%4\% CO2CO_2.

📐Formulae

Rate of Diffusion∝Surface Area×Concentration GradientThickness of Membrane\text{Rate of Diffusion} \propto \frac{\text{Surface Area} \times \text{Concentration Gradient}}{\text{Thickness of Membrane}}

C6H12O6+6O2→6CO2+6H2O+EnergyC_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}

Percentage change=Final value−Initial valueInitial value×100\text{Percentage change} = \frac{\text{Final value} - \text{Initial value}}{\text{Initial value}} \times 100

💡Examples

Problem 1:

Calculate the difference in the percentage of O2O_2 and CO2CO_2 between inhaled and exhaled air based on the standard average values (21%21\% vs 16%16\% for O2O_2 and 0.04%0.04\% vs 4%4\% for CO2CO_2).

Solution:

For O2O_2: 21%−16%=5%21\% - 16\% = 5\%. For CO2CO_2: 4%−0.04%=3.96%4\% - 0.04\% = 3.96\%.

Explanation:

This shows that the body absorbs roughly 5%5\% of the oxygen from the air and adds roughly 4%4\% of carbon dioxide as a waste product of aerobic respiration.

Problem 2:

Explain why a person with emphysema, which causes the breakdown of alveolar walls into larger, fewer sacs, struggles with physical activity.

Solution:

The breakdown of alveolar walls reduces the total Surface Area\text{Surface Area} available for diffusion. According to Fick's Law: Rate∝Surface Area\text{Rate} \propto \text{Surface Area}.

Explanation:

Because the total surface area is reduced, the rate at which O2O_2 diffuses into the blood is significantly lower, leading to insufficient oxygen levels for the high energy demands of exercise.

Gas exchange surfaces - Revision Notes & Key Diagrams | IGCSE Grade 11 Biology