Fish Gills

• Water contains far less O₂ than air and rate of diffusion is slower
• 1 litre water: 5 ml 0₂, 1 litre air: 200 ml O
• Water is dense, this prevents gills collapsing and lying on top of each other-which would reduce surface area
• Water is 800x as dense as air
• Water is viscous and does not flow as freely as air
• Water is 50x as viscous
• Fish are divided into 2 main groups according to the material that makes up their skeleton

  • Cartilaginous fish
  • Bony fish
• Cartilaginous fish


  • Skeleton made of cartilage
  • Nearly all live in sea
• Bony fish


  • Skeleton made of bone
  • Gills covered by the operculum
  • Live in both fresh and sea water
  • Most numerous aquatic vertebrate (7 more species than cartilaginous fish)
• Bony fish gills


  1. 4 pairs of gills in the pharynx, supported by a gill arch

  2. Along each gill arch are thin plates called gill lamellae these are the gas exchange surfaces

  3. The curved lamellae form a basket-like sieve through which the water flows
• Gill lamellae


  • Permeable and have a thin epithelium, short diffusion distance
  • An extensive network of blood capillaries allowing efficient diffusion
  • High affinity haemoglobin for oxygen carriage
• How fish speed up diffusion

  1. Ventilation

  2. At low speed, opercular pumping is used

  3. At higher speed, ventilation is more efficient
• Countercurrent flow- In bony fish


  1. Blood in the gill plates flows in the opposite direction to the water at the gill lamellae
  2. Maintaining the concentration gradient
  3. Therefore oygen can diffuse into the blood along their entire length
• Parallel flow- In cartilaginous fish


  1. When the two fluids (blood and water) travel in the same direction at the gill lamellae
  2. Maintaining a concentration gradient
  3. For oxygen to diffuse into the blood only up to the point where its concentration in the blood is equal to water
• Countercurrent Flow is more efficient as it maximises:

  1. Oxygen removal in water
  2. Blood oxygen content
• Gills work well in water, but not in air, out of water the gill lamellae lie on top of each other and stick together
• Countercurrent flow

  ❖ Water & blood flow opposite directions.

  ❖ This maximizes the concentration gradient over the whole surface & speeds up diffusion.

  ❖ Blood always meets water with a higher oxygen content.

  ❖ 80% of oxygen removed from the water. (3x better than human lungs)

  ❖ Equilibrium is never reached.
• Parallel Flow

  • Water is taken into mouth and forced out through gill slits when floor of mouth raised.

  • Blood travels through gill capillaries in same direction as sea water (parallel flow) = inefficient gas exchange.

  
• Buccal-Opercular Pump when water enters the buccal cavity

  Mouth opens

  1. water enters
  2. Opercular valves closed
  3. Buccal cavity expands
  4. opercular cavity expands, pressure drops
• Buccal-opercular cavity when water enters opercular cavity

  Mouth is closed

  1. Opercular valve closed
  2. Buccal cavity compressed - floor of mouth is rasied
  3. Opercular cavity expands
• Buccal-Opercular pump when water flows out of opercular cavity

  Mouth closed

  1. Opercular valve open
  2. Buccal cavity compressed
  3. Opercular cavity compressing
• Buccal-opercular pump when getting reading to take in water

  Mouth open

  1. Opercular valve open
  2. Buccal cavity expands
  3. Opercular cavity compressed
• Gills


  Specialised internal gas exchange surface in fish
• Gills


  • Made up of numerous gill filaments containing gill lamellae at right angles to the filaments
  • Greatly increase the surface area for the exchange of oxygen and carbon dioxide
• Fish ventilation


  1. Cartilaginous fish (e.g. shark): Blood and water flow in the same direction over the gill (parallel flow)

  2. Bony fish (e.g. salmon): Blood and water flow in opposite directions (counter-current flow)
• Parallel flow


  Gas exchange is only possible over part of the gill filament surface as an equilibrium is reached which prevents further diffusion and reduces the oxygen that can be absorbed into the blood
• Cartilaginous fish ventilation mechanism is basic: as they swim, they open their mouth allowing water to pass over the gills
• Counter-current flow


  More efficient system because diffusion is maintained along the entire length of the gill filament as there is always a higher concentration of oxygen in the water than in the blood it meets, resulting in higher oxygen absorption
• Bony fish have a more advanced ventilation mechanism than cartilaginous fish