Summary OCR A/AS-Level Biology 3.1.5 Gas Exchange in Other Organisms

Gas Exchange in Bony Fish
Counter-current Flow
 Blood flow in the gill lamellae is in the opposite direction to the water flow – maintains
diffusion gradient between blood and water
 As blood flows through capillaries and the oxygen saturation increases, it meets water
of increasing oxygen saturation – concentration of oxygen is always higher in water than
in blood
 Maximises gas exchange compared to a parallel-current system since it maintains the
concentration gradient along the whole length of the gill lamellae
 up to 80% oxygen extraction rate compared to a parallel-current flow with 50%



Ventilation in Gills
 Inspiration
o Opens mouth by lowering the floor of the buccal cavity
o Volume of buccal cavity increases
o Pressure inside buccal cavity decreases, drawing water into it
o Operculum is shut
 Expiration
o Closes mouth by raising the floor of the buccal cavity
o Volume of the buccal cavity decreases
o Pressure inside the buccal cavity increases, forcing water out
o Water is forced across the gill filaments
o Water forces operculum open and flows out