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Very small organisms can obtain sufficient oxygen through the skin (e.g. flatworms) but larger organisms must have special structures, such as gills, and must have ways of increasing water flow over those structures.

Fish have developed gills for respiration which have:

• large surface area
• High blood flow
• small/short diffusion distances
• contain four gill arch es (bony fishes) on each side of the fish head
• each gill arch has 2 rows of gill filament s
• each gill filament has many lamella e

The operculum in fish is a long bony cover for the gill that can be used for pushing water. Some fishes pump water using the operculum. Without an operculum, other methods are required, such as ventilation. Sharks use this system. When they swim, water flows into the mouth and across the gills. Relying on this technique means that a shark can never stop swimming.

Fish use a type of countercurent flow to maximize the intake of oxygen that diffuse through the gill. Countercurrent flow is when deoxygenated blood moves through gill in one direction while oxygenated water moves in the gill in the opposite direction.