Gas exchange surfaces and the lungEdexcel International A Level Biology: Revision notes

Cells need oxygen for aerobic respiration and must remove carbon dioxide. Small organisms such as a flatworm have a large surface area to volume ratio and a short diffusion distance, so diffusion across the body surface supplies all their cells.

As organisms get larger, volume increases faster than surface area, so the surface area to volume ratio falls. The distance from the surface to the inner cells also rises, and active animals have a high demand for oxygen. Large animals therefore need a specialised gas exchange surface such as the lungs.

An efficient exchange surface has:

• a large surface area, to give many places for diffusion
• a thin surface (often one cell thick), so the diffusion distance is short
• a steep concentration gradient, maintained by ventilation and blood flow, so net diffusion continues

Diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration, down a concentration gradient, and needs no energy from the cell.

Fick's law links the factors above:

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

So the rate doubles if the surface area doubles or the concentration difference doubles, and it halves if the thickness doubles.

Use it to explain disease: in emphysema alveolar walls break down, so surface area falls; in fibrosis scarring thickens the alveolar wall, so the diffusion distance rises. Both reduce the rate of oxygen diffusion into the blood.

Air passes down the trachea and bronchi to the alveoli, the exchange surface. Their adaptations:

• millions of alveoli, giving a very large surface area
• walls of alveoli and capillaries only one cell thick (squamous cells), so the diffusion distance is very short
• a dense capillary network around each alveolus, giving a large area of blood close to the air
• a short distance between the air in an alveolus and the blood in a capillary

These adaptations give a large surface area and a short diffusion distance.

Ventilation (breathing in and out) replaces the alveolar air, so the oxygen concentration in the alveoli stays high and the carbon dioxide concentration stays low.

The circulation carries oxygenated blood away from the capillaries and brings in blood with a low oxygen concentration and a high carbon dioxide concentration.

Together these keep oxygen diffusing into the blood and carbon dioxide diffusing out, down their concentration gradients. Faster breathing and a faster heart rate during exercise keep the gradients steep.