Gas Exchange in Fish- mark scheme answers

Describe how a bony fish ventilates its gills and absorbs oxygen from water.

• Water is forced over the gill filaments by pressure differences by the pumping action of the mouth and the operculum.
  
• Water only flows in one diresction (unidirectionally) over the gills.
• Water flows in the opposite direction to the blood ( a counter-current mechanism).
• This maintains a concentration gradient across the whole gill filament...
• ...because blood always meets water with a higher oxygen concentration ( equilibrium is never reached).

Features of fish gills which allow them to achieve efficient gaseous exchange

• LARGE surface area
• Dense network of capillaries
• Thin epithelium
• Short diffusion pathway