SA:Vol Ratio and Gas Exchange (FISH)

• What property of fick's law do fishes gills have that make them good at gas exchange?

  • They have a large surface area
• How is flow of water through the gills maintained?

  • The fish opened its mouth and the floor lowers, this decreases the pressure in the mouth due to increased volume and water flows in.
  • The fish closes its mouth and the floor is raised, the decreased volume increases the pressure and water is forced out over the gills.
• How many gills does every fish have at each side of its head?

  • 4
• Describe Gill filaments.

  • These are a series of lines on the gill that start at the head and finish towards the body.
  • They have many lamella to provide a large surface area.
• Describe Lamellae

  • Found positioned at right angles on the gill filaments. Lamellae contain capillaries
  • They have a thin epithelium for a short diffusion distance between water and blood.
• Describe how oxygen is taken from water in a fishes gas exchange system.

  • Water carrying oxygen enters through the fish's mouth passing through the Lamellae on the gill filaments, where most of the oxygen is removed.
  • Finally, water containing little oxygen leaves through the gill opening.
• How is a diffusion gradient maintained in a fishes gas exchange system.

  • The gill filament and lamellae are positioned in a way that blood and water flow happens in opposite directions.
  • This massively increases the fishes ability to absorb oxygen, as a diffusion gradient is always maintained.
  • This is known as the counter current flow system.
• Describe the counter current flow system and its effect on oxygen uptake.

  • Blood and water flow in different directions.
  • Equilibrium is never reached because a concentration gradient is maintained across the whole length of the Lamellae.
  • Almost all oxygen in the water diffuses into the blood.

  Examiner's tip:

  • Always refer to blood and water flowing in the opposite directions in the countercurrent system.
  • Describe how this maintains a difference in the oxygen concentration and a diffusion gradient across the whole length of the gill lamellae.
• Describe the countercurrent flow system and its effect on oxygen uptake.

  • Water and blood flow in the same direction.
  • A diffusion gradient is only maintained part of the way across the Lamellae
  • Only about 50% of the oxygen from water is absorbed.
• Gill filaments:

  • Each gill filament has many lamellae positioned at right angles to the filament.
  • Lamellae contain any capillaries close to the lamellae surface for blood circulation.
  • Lamellae and capillaries have thin epithelium.
• The more lamella the faster the fish.

  The colder the water the more oxygen in the water.

  The fresher the water the more oxygen in it.

  The more oxygen in the water the less lamellae needed.