| Front | Describe and explain how fish lungs are adapted to allow rapid exchange of oxygen between air in the alveoli and blood in the capillaries around them (6 marks) |
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| Back | The large surface area provided by lamellae/filaments increases diffusion / makes diffusion efficient; Candidates are required to refer to lamellae or filaments. Do not penalise for confusion between two Thin epithelium/distance between water and blood; Water and blood flow in opposite directions/countercurrent; (Point 4) maintains concentration gradient (along gill) / equilibrium not reached / as water always next to blood with a lower concentration of oxygen; Circulation replaces blood saturated with oxygen; Ventilation replaces water (as oxygen removed); |
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