Gills are a characteristic feature of fish that function as respiratory organs that extract dissolved oxygen from water and exhale carbon dioxide. The structure is located on the sides of the head and has a dense network of capillaries and blood vessels. Two flaps covering alternately open mouth and operculum (singular: Lid) Helps to push water into the gill surface = ventilation of the gas exchange surface. This maintains a concentration gradient, allowing oxygen dissolved in the water to diffuse into the gills and carbon dioxide to diffuse. Water is denser than air and requires greater force to move across gas exchange surfaces. As such, fish require relatively large structures to vent gas exchange surfaces. During inspiration, the mouth opens, muscles lower the floor of the oral cavity, and the palate expands outward. This increases the internal volume and reduces the internal water pressure in the mouth. The lid remains closed to ensure that water enters through the mouth. During exhalation, the mouth closes and muscles lift the floor of the oral cavity and dent the walls of the lid. This reduces the internal volume and increases the internal water pressure in the mouth. The lid opens and water is squeezed out from the surface of the gills. Alternatively, some large fish, such as tuna, swim continuously with their mouths open, allowing water to flow over the surface of their gills. The gill arch (humeral arch) is a series of loops of bone that support the gill filaments. The feeding behavior of fish means that foreign objects such as sand and shells enter the mouth. Since fish also use the oral cavity for gill ventilation, there is a high risk of damage to the gill filaments by such debris. Gill rakes help remove this dirt that would otherwise damage or adhere to the gill filaments and reduce their surface area. The primary lamellae (gill filaments) extend from the gill arches in two alternating rows. Due to their shape and staggered arrangement, they have a large surface. These filaments are the site of gas exchange and contain many small blood vessels called capillaries (giving them a dark red appearance). The staggered arrangement and continuous flow of water over the filaments keep the filaments from sticking together, maximizing the surface area exposed to gas exchange. The fish are still subject to gravity, but this flow of water (the gas exchange medium itself) keeps the filaments open (supporting the gas exchange surface), so no extensive support structure is needed. The primary lamellae (Ella filaments) are covered with numerous small folds known as secondary lamellae. Blood is responsible for transporting respiratory gases (oxygen/carbon dioxide) between the gills and the respiratory cells in the body. In the lamella, blood travels through tiny capillaries in the opposite direction of water flow. This is known as a countercurrent system. This transport system ensures that a concentration gradient is maintained across the gas exchange surface by displacing oxygenated blood with deoxygenated blood. This is an adaptation to the oxygen-poor aquatic environment in which fish live. The underwater lifestyle keeps the gas exchange surfaces (gills) moist. The lamellae (primary and secondary) give the gills a large surface area, increasing diffusion rates and thus gas exchange.