Why do large animals need a gas exchange surface?

Their surface area to volume ratio is small, the diffusion distance is long and their oxygen demand is high.

Rate of diffusion is proportional to (surface area × concentration difference) / thickness of the exchange surface.

How does doubling the thickness of an exchange surface affect the rate of diffusion?

It halves the rate, because rate is inversely proportional to thickness.

Name three features of an efficient exchange surface.

Large surface area, thin (short diffusion distance), steep concentration gradient.

How are alveoli adapted to give a large surface area?

There are millions of them, and each is surrounded by a dense network of capillaries.

How is the alveolar wall adapted for a short diffusion distance?

The alveolar and capillary walls are each only one cell thick (squamous cells).

How does ventilation maintain the concentration gradient?

It replaces the alveolar air, keeping oxygen concentration high and carbon dioxide concentration low.

How does blood flow maintain the concentration gradient?

It carries oxygenated blood away and brings in blood with low oxygen and high carbon dioxide concentrations.

Why does emphysema reduce the rate of gas exchange?

Alveolar walls break down, so the surface area for diffusion falls.

Why does fibrosis reduce the rate of gas exchange?

Scar tissue thickens the alveolar wall, so the diffusion distance increases.

Why does a smaller alveolar oxygen concentration at altitude slow diffusion?

The concentration difference between alveolar air and blood is smaller, so the gradient is less steep.

Gas exchange surfaces and the lungEdexcel International A Level Biology: Flashcards

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Why can a flatworm exchange gases across its body surface?

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Why can a flatworm exchange gases across its body surface?: It has a large surface area to volume ratio and a very short diffusion distance.
Why do large animals need a gas exchange surface?: Their surface area to volume ratio is small, the diffusion distance is long and their oxygen demand is high.
State Fick's law.: Rate of diffusion is proportional to (surface area × concentration difference) / thickness of the exchange surface.
How does doubling the thickness of an exchange surface affect the rate of diffusion?: It halves the rate, because rate is inversely proportional to thickness.
Name three features of an efficient exchange surface.: Large surface area, thin (short diffusion distance), steep concentration gradient.
How are alveoli adapted to give a large surface area?: There are millions of them, and each is surrounded by a dense network of capillaries.
How is the alveolar wall adapted for a short diffusion distance?: The alveolar and capillary walls are each only one cell thick (squamous cells).
How does ventilation maintain the concentration gradient?: It replaces the alveolar air, keeping oxygen concentration high and carbon dioxide concentration low.
How does blood flow maintain the concentration gradient?: It carries oxygenated blood away and brings in blood with low oxygen and high carbon dioxide concentrations.
Why does emphysema reduce the rate of gas exchange?: Alveolar walls break down, so the surface area for diffusion falls.
Why does fibrosis reduce the rate of gas exchange?: Scar tissue thickens the alveolar wall, so the diffusion distance increases.
Why does a smaller alveolar oxygen concentration at altitude slow diffusion?: The concentration difference between alveolar air and blood is smaller, so the gradient is less steep.

Exam questions on Gas exchange surfaces and the lung

Flatworms are flat, leaf-shaped animals less than 1 mm thick. They have no lungs or gills and take in oxygen directly across their body surface. A much larger animal, such as a rabbit, is made of the same types of cell but cannot obtain enough oxygen in this way.
Explain why the flatworm does not need a specialised gas exchange system.2 marks
A patient has a lung disease in which the walls of the alveoli have become thickened by scar tissue. Doctors measure the rate at which oxygen diffuses from the alveoli into the blood and find that it is lower than normal.
Use Fick's law to explain why the rate of diffusion of oxygen into the blood is reduced in the first patient.2 marks
The wall of each alveolus in a healthy lung is only one cell thick, and each alveolus is surrounded by a dense network of capillaries. Blood entering these capillaries has a low oxygen concentration and a high carbon dioxide concentration, while the air in the alveoli is repeatedly replaced by breathing.
Describe how the structure of the alveoli and the surrounding capillaries is adapted for rapid gas exchange.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).