In order to breathe underwater, fish have to extract dissolved oxygen from the water. They do this using their gills. Water enters the fish's mouth and then the fish forces water across its gills, past many tiny blood vessels, and out the gill slits. The gills take oxygen from the water and let water whisk away carbon dioxide and ammonia from the blood vessels in the gills. Gill filaments are the red, fleshy part of the gills; they take oxygen into the blood. Each filament has thousands of fine branches (lamellae) that are exposed to the water. The branches contain blood capillaries beneath a thin epithelium that separates the blood from the water, allowing oxygen and carbon dioxide to pass through easily.
Not all fish completely rely on their gills to breathe, however. Some fish species absorb a large part of their necessary oxygen through the skin, particularly when they're juveniles. Others have lungs or other accessory air chambers that have developed to breathe air from the surface of the water, and these species of fish may drown if they don't have access to the water's surface.
The gill filaments in fish have functions like lungs in people: it's the organ responsible for absorbing oxygen and expelling carbon dioxide. The gills also regulate levels of mineral ions and the pH of the blood, as well as being the primary site of nitrogenous waste excretion, in the form of ammonia.
The gill filaments of bony fishes are also called "primary lamellae." They are intricate structures that have a large surface area. Smaller "secondary lamellae" are offshoots of the primary filaments. The secondary lamellae contain small blood capillaries and the blood flows in the opposite direction of the water. As a result, the water flowing beside the secondary lamellae always has a higher oxygen concentration than that in the blood, so oxygen is absorbed along the full length of the secondary lamellae. In this way also, carbon dioxide is passively diffused from the blood into the water.
Actively swimming fish have gill filaments that are highly developed to maximize the absorption of oxygen. Sedentary fish that live on the bottom usually have gill filaments that absorb smaller volumes, since they are less active and don't use the oxygen as quickly.
Most fishes have three or more gill arches on each side of the body. These support the gill filaments and are cartilaginous or bony and shaped like a boomerang. Each gill arch consists of an upper and a lower limb that is joined in the back. Gill filaments and gill rakers are attached to the gill arches.
The gill arches offer support for the gills as well as the blood vessels. Arteries that enter the gills bring blood with low oxygen and a high concentration of wastes. Arteries that leave the gills contain blood with little waste that's rich with oxygen.
Gill rakers are bony projections that help the fish feed. They point forward and inward from the gill arches. Their number and shape vary based on the diet of the fish: widely spaced gill rakers are evident on fish that eat large prey, such as other fish, which prevent the prey item from getting free and escaping between the gills. A larger number of thinner, longer gill rakers are seen on fish that eat smaller prey. Species that consume plankton and tiny matter suspended in the water sport gill rakers that are extremely long and thin. Some fish have more than 150 just on the lower arch.These help to collect food particles in the throat that can be swallowed, while water is passed out through the gill slits.