Fish

• Gills are made up of gill filaments
• Gill filaments are stacked up in a pile, at right angles to gill filaments are gill lamellae
• Gill lamellae increase surface area of the gills
• The counter-current exchange principle ensure that a high concentration gradient is maintained, this is achieved as the blood and water flow in opposite directions
• In the counter-current exchange principle the arrangement means that: blood that is already well loaded with oxygen meets the water (with the maximum concentration of oxygen), therefore diffusion can happen
• Counter-current flow: Blood flows in one direction, with the oxygenated blood moving against the flow of the blood with less oxygen

  
• Ionocytes control the concentration of ions in the blood, maintaining the electrolytes in the fish
• Osmoregulation is also responsible for maintaining the concentration of ions in the fish