Fish have a waterproof body and relatively smallSA: V, so cannot absorb oxygen directly from the water into their body tissues and cells so have specialised exchange surfaces with a large surface area to obtain the oxygen
Fish are aquatic animals adapted to extracting oxygen from water: oxygen content in air ~20.9%, oxygen content in water ~0.8% so fish have to pass large water volumes over their systems relative to the air volumes ventilated by land animals
Fish gills are red because of haemoglobin; they can extract as much as 80% of available oxygen passing through its gills, due to: large SA, short diffusion distances and a maintained concentration gradient between blood and water
Fish have developed a specialised internal gas exchange surface; the gills, there are usually 4 gill arches on either side of the fish to support the gills, they lie between the mouth cavity and the opercular flaps
The fish continuously pumps water through its mouth and over gill arches, using coordinated movements of the jaw and operculum (gill cover); a swimming fish opens its mouth and lets water flow past its gills
Learn this keyphrase: "The countercurrent system ensures that maximum oxygen gradient is maintained across the full length of lamellae as the blood and water flow in opposite directions"
Two pumps act in ventilation
A) Suction
B) Pressure
C) extended
D) buccal
E) buccal
F) closed
Short diffusion distance; Gill lamellae is very thin so that blood flowing through them is only a short distance from seawater (about 5m in active fish such as mackerel)
