gas exchange in fish

• fish are waterproof
• fish have a small surface area : volume ratio so they require a gas exchange surface which is why they have gills
• fish obtain oxygen from water but water contains 30 times less oxygen than in air so they have a special adaptation to maintain the concentration gradient to enable to occur
• 3 key features of a gas exchange surface

  1. large surface area : volume ratio
  2. short diffusion distance
  3. maintained concentration gradient
• fish gill anatomy

  • four layers of gills on both side of the head
  • gills are made of stacks of gill filament
  • each gill filament is covered in gill lamellae positioned at right angles
  • when fish open their mouth water rushes in and over the gills through a hole on the sides of their heads
• adaptation for efficient gas exchange:

  1. large surface area : volume ratio - due to having many gill filaments covered in gill lamellae
• adaptation for efficient gas exchange:

  2. short diffusion distance - due to the capillary network in every lamellae and having very thin lamellae
• adaptation for efficient gas exchange:

  3. maintaining the concentration gradient - due to the water flowing in the opposite direction of blood flow in the capilliaries this is called counter current flow
• counter current exchange principle:

  • water flowing over the gills is opposite to the direction of blood flow in the capillaries
  • counter current flow ensures that equilibrium will never be reached
  • diffusion can occur across the entire length of the lamellae
• Describe how the structure of an individual lamella in fish gills creates a short diffusion pathway for oxygen and carbon dioxide.

  • The capillaries in each lamella bring the blood very close to the surface. In addition, the surface of each lamella is made up of just a single layer of flattened cells.