Summary of gas exchange in fish gills

1.Fish have developed specialised gas-exchange organs called gills, which are composed of thousands of filaments.

2.The filaments in turn are covered in feathery lamellae which are only a few cells thick and contain blood capillaries.

3.This structure gives a large surface area and a short distance for gas exchange.

4.Water flows over the filaments and lamellae, and oxygen can diffuse down a concentration gradient the short distance between water and blood, whilst CO2 diffuses in the opposite direction, also down its concentration gradient.

5.Each gill is covered by a muscular flap (the operculum) on the side of a fish's head.

6.The gills are so thin that they cannot support themselves without water, so if a fish is taken out of water after a while the gills will collapse, the SA/Vol ratio falls, and the fish suffocates.