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Structure of the Gills

Fish have a waterproof covering. They are relatively large and so have a small surface area to volume ratio.In order to supply and remove respiratory gases they have evolved a specialised internal gas exchange surface called the gills.Gills are located in the body behind the head.They are made of stacks of very thin gill filaments. At right angles to the gill filaments are the gill lamellae which increase the surface area of the gills.The flow of water over the gill lamellae and the flow of blood within them are in opposite directions.This is called a countercurrent flow.

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Specification: OCR Biology A (H020), AS-level, 3.1.1 Exchange surfaces. Topic: Exchange Surfaces and Breathing.

Key terms in this lesson

surface area to volume ratio
The ratio of an organism's outer surface area to its volume; it decreases as size increases, which limits the rate of exchange achievable by diffusion alone.

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