fish

• In the gill lamella of fish, water flows in the opposite direction to blood, this is called a counter current exchange system
• countercurrent exchange increases efficiency as it reduces the amount of oxygen that leaves the blood without being used
• The counter current exchange system maintains a concentration gradient for diffusion
• In a counter current system oxygen always diffuses from water, into the blood
• explain gas exchange in fish:

  • fish opens mouth and water flows in, mouth is then closed to increase pressure
  • water passes over the lamellae, and o2 diffuses into the bloodstream