FISH -GAS EXCHANGE

==Adaptations for Gas Exchange== [image:http://i.imgur.com/e8ZWZ8L.png?1] ===Mouth & Opercula=== [image:http://i.imgur.com/rsMJwTM.png?2] Alternate opening of the mouth and two flaps of skin that cover the gills called the opercula (singular: operculum) helps to force water across the gill surface = '''ventilate''' the gas exchange surface. This ensures that a '''concentration gradient is maintained''' allowing oxygen dissolved in the water to diffuse into the gills and carbon dioxide to diffuse out. Water is more dense than air and it takes more force to move it across the gas exchange surface, this is why fish require relatively large structures to ventilate their gas exchange surface. During '''inspiration''' the mouth opens, muscles lower the floor of the buccal (mouth) cavity and the opercula bulge outward. This increases the internal volume and lowers the internal water pressure within the mouth. The Opercula remain closed ensuring water enters through the mouth. During expiration the mouth closes and muscles raise the floor of the buccal cavity and push in the walls of the operculum. This reduces the internal volume and increases the internal water pressure within the mouth. The opercula open and water is pushed out over the gill surface. Alternatively some larger fish such as Tuna swim continuously with their mouth open to keep water flowing across their gill surface. ===Gill Arch & Gill Rakers=== [image:http://i.imgur.com/6RuZbf5.png?3] The gill arches (brachial arches) are a series of bony loops that support the gill filaments. The feeding behaviour of fish means that sand, shell fragments and other debris all enter the mouth. Because fish also use their buccal (mouth) cavity to ventilate their gills there is an increased risk of damage to the gill filaments from such debris. Gill rakers help to filter out this debris that would otherwise damage or stick to the gill filaments, reducing their surface area. ===Primary & Secondary Lamellae=== The primary lamellae (gill filaments) are long thin filaments that extend in two alternating rows from the gill arch. Their shape and staggered arrangement gives them a large surface area. These filaments are the site of gas exchange and they contain many tiny blood vessels called capillaries (this is what gives them a dark red appearance). Their staggered arrangement and the continuous flow of water across the filaments ensures that they do not stick together, maximising the surface area that is exposed for gas exchange. While fish are still subjected to the forces of gravity, they do not need extensive support structures because this flow of water (the gas exchange medium itself) helps keep the filaments in an open conformation (supports the gas exchange surface). The primary lamellae (gill filaments) are covered in a large number of tiny folds known as secondary lamellae. These further increase the surface area available for gas exchange. ===Blood Vessels=== Blood is used to transport respiratory gases (oxygen / carbon dioxide) between the gills and respiring cells around the body. In the lamellae, blood moves though tiny capillaries in the opposite direction to the flow of water. This is known as a '''counter-current''' system. This transport system helps to ensure a concentration gradient is maintained across the gas exchange surface by replacing oxygen rich blood with oxygen poor blood. This is an adaptation to the oxygen poor, aquatic, environment in which fish are found. ==Advantages== *An aquatic lifestyle ensures that the gas exchange surface (gills) remain moist. Lamellae (primary and secondary) give the gill a large surface area increasing the rte of diffusion and therefore gas exchange. *The counter-current exchange system makes gas exchange extremely efficient. Fish are able to extract a much higher proportion of oxygen from water than most animals can form air. This is essential for an aquatic lifestyle as water has a much lower oxygen content than air. *Water helps to support the gills. Water flowing across the lamellae keeps them apart maximising the surface area available for gas exchange. *Unidirectional ventilation is more efficient as the gas exchange medium is replaced fully, creating a greater concentration gradient which increases the rate of diffusion across the gas exchange surface. ==Disadvantages== *This system is only efficient in water as the buoyancy of the water helps keep lamellae apart. On land they would stick together drastically reducing the surface area available for gas exchange. The gills would also dry out as fish have no need for systems to keep the gas exchange system moist. *Most aquatic animals are cold-blooded as the limited oxygen content of water would not support the high metabolic demands of larger warm-blooded animals (those that are warm blooded tend to be air breathers).