Fish

• water goes in through the mouth, past the gills and out
• the gills are the exchange surface
• gills are thin and have a high blood supply
• filaments increase the surface area of the gills
• fish's circulatory system is a single circuit
• gills need to be efficient as oxygen is not soluble in water, there is less oxygen in water compared to air and water is denser than air so it is harder to move over the gills
• gill filaments are stacked to increase surface area
• gill lamellae stand at right angles to increase the surface area
• lamellae are only a few cells thick and contain lots of capillaries which increase rate of diffusion
• constant blood flow in the gills maintains a concentration gradient
• there always needs to be a higher concentration of oxygen in the water compared to the blood in order to maintain a concentration gradient
• counter-current flow ensures there is a constant concentration gradient
• counter-current flow refers to the water flowing in the opposite direction to blood
• counter-current flow maintains a steep concentration gradient as the blood encounters 'fresh' water which has a higher concentration of oxygen
• counter-current flow prevents the concentration of oxygen in the water and blood from becoming equal, it ensures there is constant diffusion
• ram ventilation is where fish swim through water and open their mouth. It is simple but it means fish must be constantly swimming to breathe which isn't simple
• normal ventilation is where fish take water in through the mouth, the mouth closes and water is pushed over the gil filaments and out the gil slits