BIOLOGY - GAS EXCHANGE IN FISH

Skeleton made of bone, freshwater and marine, scales and gills covered by a 'flap' (known as a operculum)

Skeleton made of cartilage, marine only, no scales and gills not covered (slits exposed)

Opens mouth (Buccal cavity), floor of mouth lowers (buccal cavity floor), operculum opens, volume of cavity increases, pressure of mouth cavity decreases and water flows in

The gills are made up of bony gill rakers, gill arches and fleshy gill filaments made of lamellae (the gas exchange surface across which the water flows)

Many thin gill filaments supported by a bony arch, Gill filaments covered by smaller lamellae (very thin walls), this increases the surface area of the exchange surface (large surface area). They have an extensive capillary network (permeable and contain haemoglobin for transport of oxygen). Blood flows through the gill arches into the filaments and then out again into the gill lamallae before making the return journey to the gill arch (good blood supply). Water flows over the gill lamallae as it passes through the gills and this is where gas exchange takes place. The density of water prevents the gills from collapsing.

If the blood and water flowed in the same direction (parallel flow), diffusion would only occur until equilibrium was reached. This is less efficient as the blood would only be able to reach a maximum of 50%-60% O2 saturation

In the gills, the flow of blood is in the opposite direction to the flow of water across the gills. This means that blood is continually meeting fresh water with a higher percentage saturation of oxygen. It maintains a steep concentration gradient across the whole gill filament and approx. 90% O2 saturation can be reached

Water is needed for diffusion in fish. Gills dry out when out of water, causing filaments to stick together and reduces surface area for gas exchange