Ventilation and gas exchange in bony fish and insects

• Fish have a small surface area to volume ratio for gas exchange, apart from this they have an impermeable membrane so gases can’t diffuse through their skin therefore fish need a specialised exchange surface.
• Bony fish have four pairs of gills, each gill supported by an

  arch
• Along each arch in the fish, there are multiple projections called gill filaments, with lamellae on them which participate in gas exchange. Blood and water flow across the lamellae in a counter current direction meaning they flow in opposite direction
• The Gill fillaments are held apart by water flow. Therefore, in the absence of water they stick together, thus meaning fish cannot survive very long out of water.
• Ventilation is required to maintain a continuous unidirectional flow.
• Ventilation begins with the fish opening its mouth followed by lowering the floor of buccal cavity, thus enabling water to flow into it. Afterwards, fish closes its mouth, causing the buccal cavity floor to raise, thus increasing the pressure
• The water that is breathed in is forced over the gill filaments by the difference in pressure between the mouth cavity and opercular cavity. The operculum

  acts as a valve and pump and lets water out and pumps it in
• Insects do not possess a transport system therefore oxygen needs to be transported directly to tissues undergoing respiration
• This is achieved with the help of spiracles, small

  openings of tubes, either bigger trachea or smaller tracheoles, which run into the body of an insect and supply it with the required gases
• At the end of each tracheole is a small amount of

  tracheal fluid which allows gasses to dissolve and then diffuse into the cells. Spiracles can be opened and closed to avoid excessive water loss.