In larger organisms Diffusion alone cannot meet the needs of the inner-most cells of larger organisms. Larger organisms must have some form of internal transport system for gases. The tracheal system of insects Insects have a system of small, highly branching air tubes that carry gases in and out of their bodies. Tracheal system of insects Insects have a gas transport system made of tracheal tubes and spiracles. Spiracles close to reduce loss of moisture.
Larger organisms use a blood circulatory system to transport gases.
40 times the surface area of the body Alveoli Air sacs surrounded by capillaries. If flows are in the same direction. See drawing on board. Countercurrent flow in gills extracts more of the oxygen from the water. Metabolic rate of fish is lower.