Gas exchange occurs over a gas exchange surface - boundary between the outside environment and the internal environment of the organism.Oxygen and carbon dioxide need to quickly diffuse across the surface.Gas exchange surfaces: Have a large surface area. Are thin (one layer of epithelial cells) for a short diffusion pathway. Maintenance of a steep concentration gradient of gases across the exchange surface, increases the rate of diffusion.Diffusion only efficient over small distances - multicellular organisms have evolved with specialised gas exchange surfaces. Insects have trachea - used for gas exchange. Air moves into trachea through pores called spiracles. Oxygen travels down a concentration gradient. Trachea branch off into tracheoles. Tracheoles have thin, permeable walls and link directly to individual cells. Oxygen diffuses directly into respiring cells. Carbon dioxide moves down a concentration gradient towards spiracles. Carbon dioxide released into atmosphere via spiracles. Rhythmic abdominal movements move air in and out of spiracles.
Most insects live on land - water can easily evaporate from surface of body so can easily dehydrate.Problem:Insects need to conserve water but need to maintain a thin, moist, permeable surface with a large area.Solution: Water proof covering (cuticle) over body surface, the cuticle is impermeable to water, carbon dioxide and oxygen. Small surface area to volume ratio to minimise the area over which water is lost. When cells respire, oxygen is used up and its concentration towards the ends of the tracheoles falls. This creates a diffusion gradient. Carbon dioxide produced during respiration - creates a diffusion gradient in opposite gradient. Movement of muscles in insects creates mass movements of air in and out of the trachea, speeding up exchange of gases.For diffusion to be effective, diffusion pathway needs to be short. This limits the size that insects can attain.