An effective gas exchange system maximises the rate of diffusion described by Fick's law. Whatever the organism, efficient gas exchange surfaces share the same features.1. A large surface area. Folding, branching or flattening gives a large surface area in a small space, such as the millions of alveoli in mammalian lungs, the filaments and lamellae of fish gills, and the network of tracheoles in insects.2. A thin surface. The barrier between the medium and the blood is very thin, often one flat cell thick, giving a short diffusion distance.3. A steep concentration gradient. This is kept up in two ways:
• ventilation brings fresh air or water to the surface, keeping the oxygen concentration high and the carbon dioxide concentration low there;
• a good blood supply constantly carries oxygenated blood away and brings deoxygenated blood.4. Moist. The surface is moist so that gases can dissolve before they diffuse across the cells.5. Permeable to oxygen and carbon dioxide.Fish gills show how a steep gradient can be kept along the whole surface. Water and blood flow in opposite directions over the lamellae, called countercurrent flow. Blood always meets water with a higher oxygen concentration than itself, so oxygen diffuses into the blood along the entire length of the lamella.If ventilation or blood flow stopped, the oxygen concentrations on the two sides of the surface would become equal and the concentration gradient would decrease, so gas exchange would slow down.
• ventilation brings fresh air or water to the surface, keeping the oxygen concentration high and the carbon dioxide concentration low there;
• a good blood supply constantly carries oxygenated blood away and brings deoxygenated blood.4. Moist. The surface is moist so that gases can dissolve before they diffuse across the cells.5. Permeable to oxygen and carbon dioxide.Fish gills show how a steep gradient can be kept along the whole surface. Water and blood flow in opposite directions over the lamellae, called countercurrent flow. Blood always meets water with a higher oxygen concentration than itself, so oxygen diffuses into the blood along the entire length of the lamella.If ventilation or blood flow stopped, the oxygen concentrations on the two sides of the surface would become equal and the concentration gradient would decrease, so gas exchange would slow down.