Adaptations for gas exchange in fish

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  • Adaptations for gas exchange in fish
    • Fish are more active than invertebrates so have a higher demand for oxygen
    • Fish use water instead of air as their gas exchange medium
      • However, water contains less oxygen than air and is more dense so the rate of diffusion in water is slower
    • To meet their oxygen demand, fish have developed specialised gas exchange surfaces in the form of gills on gill lamellae
      • To increase  gas exchange efficiency, water is forces over the gills by pressure changes in the body which maintain a continuous, unidirectional flow
        • Gills are made up of many folds to increase the surface area over which water can flow and gases can be exchanged
          • The density of the water stops the gills from collapsing and lying on top of each other which would reduce surface area
    • There are two types of fish, catagorised according to the material that makes up their skeleton;
      • Cartilaginous Fish
        • e.g. Sharks and Rays
        • Skeleton made from cartilage
        • Most live in the sea
        • Just behind the head on each side they have 5 gill clefs which open at gill slits
          • Water enters the mouth and is forced through the gill slits when the roof of the mouth is raised
            • Blood flows through the gill capillaries in the same direction as the sea water (parallel flow)
        • Parallel flow
          • Only 50% of oxygen diffusion occurs
            • Only half the length of the lamella is used for gas exchange
              • Inefficient
                • Blood - As distance across lamellae increases, so oxygen saturation increases up to about 50%
                  • Water - As distance across lamellae increases, so oxygen saturation decreases up to about 50%
                    • At 50%, equilibrium is reached, so maximum saturation is 50%
      • Bony Fish
        • Counter Current Flow
          • Maximum concentration of oxygen in blood achieved
            • Diffusion occurs across the entire length of the lamellae
              • Equilibrium is never reached, concentration gradient is maintained
                • Efficient
                  • Maximum saturation reached at 75%
                    • Blood - As distance across the lamellae increases so oxygen saturation increases
                      • Water - As distance across the lamellae increases, so oxygen saturation increases
        • Skeleton made of bone
        • Live in freshwater and the sea an exist in greater numbers than cartilaginous fish
        • Gills covered with a flap called the operculum
          • Bllod flows through the gill capillaries in the opposite direction to the water
        • Adaptations for gas exchange
          • Gills are found just behind the head in the pharynx
            • There are four pairs of gills
              • On each side of the fish, a flap covers and protects the gills
                • Each gill is supported by a gill arch and along each gill arch are many pairs gill filaments and on these are the gas exchange surfaces - the lamellae
                  • The lamellae are formed by numerous thin folds lying on top of each other
          • Out of the water, the gill collapses as the gill filaments lie on top of each other and stick together
            • In water, the gill filaments are supported and the lamellae provide a large surface area
              • The lamellae have lots of blood capillaries, these take up oxygen from the water and carbon dioxide passes out
                • Fish blood also contains haemoglobin, which increases the efficiency of transportation in the bloo
        • Ventilation Mechanism
          • Fish ventilation is unidirectional because water is too dense to move in two directions
            • Water is forced over the gill filaments by pressure differences which maintain a continuous, unidirectional flow
              • The fish opens it's mouth. The muscles in the buccal cavity floor contract and the buccal cavity floor is lowered
                • There is an increase in the volume of the buccal cavity and a decrease in the pressure. The pressure is lower than outside the body, so water flows in from an area of high pressure to low pressure
                  • Furthur pressure changes then force water from the buccal cavity over the gills and the deoxygenated water then flows out of the fish


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