gas exchange in fish & insects

• What is the structure of gills in fish?

  • Series of gills on each side of the head
  • Each gill arch is attached to two stacks of filaments
  • On the surface of each filament, there are rows of lamellae
  • The lamellae surface consists of a single layer of flattened cells that cover a vast network of capillaries
• Ventilation in fish P1?

  • When the fish open their mouth they lower the floor of the buccal cavity. This causes the volume inside the buccal cavity to increase, which causes a decrease in pressure within the cavity
  • The pressure is higher outside the mouth of the fish and so water flows into the buccal cavity
• Ventilation in fish P2?

  • The fish then raises the floor of the buccal cavity to close its mouth, increasing the pressure within the buccal cavity
  • Water flows from the buccal cavity (high pressure) into the gill cavity (low pressure)
  • As water enters pressure begins to build up in the gill cavity and causes the operculum (a flap of tissue covering the gills) to be forced open and water to exit the fish
  • The operculum is pulled shut when the floor of the buccal cavity is lowered at the start of the next cycle
• Counter-current system in fish?

  • The capillary system within the lamellae ensures that the blood flow is in the opposite direction to the flow of water - it is a counter-current system
  • The counter-current system ensures the concentration gradient is maintained along the whole length of the capillary
  • The water with the lowest oxygen concentration is found adjacent to the most deoxygenated blood
• Lamellae: located within the body, supported by arches, along which are multiple projections of gill filaments, which are stacked up in piles
• How does the counter-current exchange system maximise oxygen absorbed by the fish?

  Maintains a steep concentration gradient, as water is always next to blood of a lower oxygen concentration.

  Keeps rate of diffusion constant and enables 80% of available oxygen to be absorbed
• Why do fish swim with their mouth open?

  So that water flows over the gills (the site of gas exchange).
• Ventilation mechanism in fish?

  Fish lower the buccal cavity and open their mouth; this increases the volume of the buccal cavity and so decreases the pressure, so water flows into the buccal cavity to rebalance.

  Simultaneously, the operculum valve will shut and the operculum cavity will expand, increasing the volume of the operculum, causing a decrease in pressure.

  The fish will then raise the floor of the buccal cavity, forcing the water from the buccal cavity over the gills within the operculum cavity.
• Gas exchange in fish?

  across the gills

  fish have 4 layers of gills on both sides of their heads

  the gills are made of gill filaments, every gill filament is covered in gill lamellae (gill plates, semi-circles)
• 3 specialised features in fish gas exchange?

  1)large surface area: many gill filaments, all covered in gill lamellae which are stacked at right angles to each other

  2)short diffusion distance: the gill lamellae and filaments are both thin

  3)maintaining concentration gradient: counter-current mechanism
• Why do fish have a counter-current mechanism?

  to compensate for the fact that water has a lower dissolved oxygen concentration compared to the oxygen concentration in the atmosphere
• What is the counter-current mechanism in fish?

  This is when water flows over the gill lamellae in the opposite direction to the flow of the blood in the capillaries.

  This ensures that a diffusion gradient is maintained across the entire length of the gill lamellae. (There is a constant concentration gradient between the water and blood.)
• • 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 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.
• why do bony fish have special adaptations?


  to get enough oxygen from the water, as water has a lower concentration pf oxygen than air
• what is each gill made of?


  many thin branches called gill filaments, these increase the surface area for the exchange of gases
• what are the gill filaments covered in?


  gill plates / secondary lamellae

  these further increase the surface area for exchange of gases
• how would you explain the counter-current mechanism in the gills?


  blood flows through the gill plates in one direction and water flows over in the opposite direction

  this maintains a steep concentration gradient between water and the blood

  the concentration of oxygen in the water is always higher than that in the blood, so as much oxygen as possible diffuses from the water into the blood
• how are the fish gills ventilated?


  the fish opens its mouth, this lowers the buccal cavity, so the volume of the buccal cavity increases, decreasing the pressure inside the cavity

  this draws water into the cavity

  when the fish closes its mouth, the floor of the buccal cavity raises, decreasing the volume and increasing the pressure, thus forcing water out of the cavity across the gill filaments
• 3rd step of fish gill ventilation?

  each gill is covered by a bony flap: operculum (protects the gill)

  when the pressure increases in the buccal cavity, it forces the operculum on each side of the head to open, allowing water to leave the gills
• how does water enter and leave the gill plate?


  water enters the gill plate with a low oxygen concentration

  water leaves gill plate with a high oxygen concentration
• how do you dissect fish gills?


  wear an apron, lab coat, and gloves

  place the fish on cutting board

  push back the operculum, use scissors to remove the gills

  cut each gill arch through the bone at the top and bottom, you then should be able to see the gill filaments

  draw a biological drawing and label
• what do insects use for ventilation?

  tracheae (microscopic air-filled pipes)

  air moves into the tracheae through pores on the insects surface called spiracles
• what does the tracheae branch off into?


  smaller tracheoles which have thin, permeable walls, and go to individual cells

  the tracheoles contain tracheal fluid, which oxygen dissolves in

  the oxygen then diffuses from the trachea fluid into body cells

  co2 diffuses in the opposite direction
• what are spiracles?


  openings in the exoskeleton that allow air to enter the insect and flow into the system of tracheae