Respiration in Water

• Respiration


  Exchange of gases (mainly O2 and CO2) between ambient medium & body fluids

  
• Diffusion


  • Adequate for small unicellular animals
  • Cytoplasmic streaming distributes gases around cell
  • Rate of diffusion (Fick's law)
  • Efficient respiratory surfaces facilitate diffusion
  • Moist
  • Vascularized

  
• Breathing


  • Diffusion
  • Bulk transport
• Cutaneous respiration


  • Gas exchange occurs across the skin
  • Requires moist surface
  • Increase surface area by elongated cylindrical or flat shape

  
• Cold, fast-flowing streams


  • Lungs disadvantage buoyancy
  • Decrease metabolic rate
  • Increase oxygen
• Lungless vertebrates


  • Greatly enlarged surfaces & thin to facilitate gas exchange
  • Specialized respiratory organs: Tracheae, External gills, Lungs, Internal gills

  
• Gill


  Evaginated extension of body surface
• Lung


  Invaginated internal surface
• Exception: Respiratory trees of sea cucumber are invaginated structures
• Large aquatic animals usually have gills to increase surface area for gas exchange
• External filamentous gills


  • Hairy frog

  
• External filamentous gills


  • Present during periods of increased activity and O2 demand
  • Forceful fluttering ensures convective movement of water
  • Branchial filaments exposed to environment

  
• Internal lamellar gills


  • Axolotl
• Internal lamellar gills


  • Countercurrent exchange
• Opercular gills (teleost)


  • Secondary gill lamella are principal site of gas exchange
• Gill ventilation


  • Water is forced over gills by a favourable buccal-opercular pressure gradient throughout most of the respiratory cycle
  • Suction pump
  • Force pump
  • Dual pump mechanism drives one-way flow of water
• Septal gills (elasmobranch)


  • Interbranchial septum seen externally as gill flap
  • Internal lamellar gills
• Ram ventilation


  • Water being forced over gills by swimming movement
  • Active fast-swimming sharks & teleost fish (eg tuna) suffocate if forced to stop swimming
• • Surface area depends on no. gill arches, filaments & lamella
  • Generally correlated with body mass & metabolic demand