fish

• fish absorb oxygen from the water into their blood
• 1. water containing oxygen enters the fish through its mouth and passes out through the gills
• 2. each gill is made of thin plates called gill filaments, which give a large surface area for exchange of gases
• 3. the gill filaments are covered in tiny structures called lamellae which increase the surface area even more
• 4. the lamellae have lots of blood capillaries and a thin surface layer of cells to speed up diffusion
• 5. blood flows through the lamellae in one direction and water flows over in the opposite direction - counter-current system. maintains large concentration gradient between the water and the blood. concentration of oxygen in water is always higher than the blood, so as much oxygen as possible diffuses into the blood
• blood and water flow in opposite directions, meaning a concentration gradient of oxygen is maintained along the entire gill, so diffusion of oxygen can always occur