Gas exchange surfacesEdexcel A-Level Biology A: Revision notes

Cells need oxygen for aerobic respiration and must remove carbon dioxide. Gases cross the exchange surface by diffusion.

The surface area to volume ratio (SA:V) falls as organisms get bigger. For a cube of side 2 cm, SA = 6 × 2² = 24 cm² and V = 2³ = 8 cm³, a ratio of 3 : 1. For a cube of side 4 cm, SA = 96 cm² and V = 64 cm³, a ratio of 1.5 : 1.

A single-celled organism has a large SA:V and a short diffusion distance, so its cell surface is enough. A large animal has a small SA:V and a high demand for oxygen, so it needs a specialised gas exchange surface with a large surface area.

Efficient gas exchange surfaces share three properties:

• a large surface area, so more molecules can cross at once
• a thin surface (short diffusion distance), often one cell thick
• a steep concentration gradient, maintained by ventilation (moving air or water) and blood flow (removing the diffusing gas)

Gas exchange surfaces are also moist, so gases dissolve and can cross the membrane.

Fick's law states:

rate of diffusion ∝ (surface area × concentration difference) ÷ thickness of the exchange surface

So the rate increases with a larger area or a larger concentration difference, and decreases with a greater thickness.

Worked example: surface A has area 12 cm², concentration difference 8 and thickness 0.2 mm, so rate ∝ 12 × 8 ÷ 0.2 = 480. Surface B has area 12 cm², difference 6 and thickness 0.1 mm, so rate ∝ 12 × 6 ÷ 0.1 = 720. B is 1.5 times faster.

Doubling the thickness halves the rate (if the other factors are constant), because the rate is proportional to 1/thickness.

Air flows through the trachea, bronchi and bronchioles to the alveoli, which are the gas exchange surfaces. The alveoli are surrounded by a capillary network.

• Many alveoli (about 300 million) give a very large surface area (about 70 m²).
• The alveolar wall is squamous epithelium, one cell thick, and the capillary wall is also one cell thick, so the diffusion distance is very short.
• Ventilation (breathing) keeps the oxygen concentration high and carbon dioxide low in the alveolar air.
• Blood flow removes oxygen and brings carbon dioxide, keeping the gradient steep.
• The moist lining lets gases dissolve.

Lung diseases can be explained by Fick's law.

• Pulmonary fibrosis: scar tissue thickens the alveolar walls, so the diffusion distance increases and the rate falls.
• Emphysema: alveolar walls break down, forming fewer, larger air spaces, so the surface area falls.
• Pneumonia: fluid in the alveoli increases the distance and reduces the area in contact with air.
• High altitude: the inhaled air contains less oxygen, so the concentration difference is smaller.
• Exercise: faster breathing and blood flow keep the concentration gradient steeper, increasing the rate.