Gas exchange

Cells respire continuously. Aerobic respiration consumes oxygen and produces carbon dioxide, and both gases move into or out of cells by diffusion. In a single-celled organism, the whole plasma membrane is enough to serve as the exchange surface.

Growth creates two problems. The surface area-to-volume ratio falls, so there is less surface for each unit of respiring tissue. The distance from the centre of the body to the outside also rises, and diffusion is only fast over very short distances. A tough, protective skin makes things worse.

So large, active animals have a specialized exchange organ, and usually a transport system as well.

Net diffusion needs a concentration difference, and diffusion itself tends to remove that difference. Three features stop this from happening:

• Dense networks of blood vessels bring a large area of blood very close to the surface.
• Continuous blood flow carries oxygen away as soon as it arrives, and delivers more carbon dioxide.
• Ventilation replaces the medium outside: air for lungs, water for gills.

A fish must keep pumping water over its gills, because water that stops moving soon comes to equilibrium with the blood.

• High surface area: millions of tiny alveoli. The size and number give the area, not the round shape.
• Branched bronchioles: these distribute air evenly to every part of the lung.
• Extensive capillary beds: these add area and keep gradients steep. Blood low in oxygen arrives from the pulmonary artery and leaves through the pulmonary vein.
• Surfactant: a phospholipid-rich secretion that lowers the surface tension of the water lining. Without it, the moist walls would stick together and alveoli would collapse during exhalation.

The alveolar wall is one layer of flattened cells, so the diffusion path is short, and oxygen dissolves in the moist lining before it diffuses into the blood.