Fish gill
Most fish exchange gases like oxygen and carbon dioxide using gills on both sides of the pharynx (throat).
Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide.
Within the gill filaments, capillary blood flows in the opposite direction to the water, causing countercurrent exchange.
Most species employ a counter-current exchange system to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other.
The high surface area is crucial to the gas exchange of aquatic organisms as water contains only a small fraction of the dissolved oxygen that air does.
[7] Rather than using lungs "Gaseous exchange takes place across the surface of highly vascularised gills over which a one-way current of water is kept flowing by a specialised pumping mechanism.
[6] Fish gill slits may be the evolutionary ancestors of the tonsils, thymus gland, and Eustachian tubes, as well as many other structures derived from the embryonic branchial pouches.
Though all but the most primitive bony fish lack a spiracle, the pseudobranch associated with it often remains, being located at the base of the operculum.
[6] Fish transfer oxygen from the sea water to their blood using a highly efficient mechanism called countercurrent exchange.
The effect of this is that the blood flowing in the capillaries always encounters water with a higher oxygen concentration, allowing diffusion to occur all the way along the lamellae.
The gills' large surface area tends to create a problem for fish that seek to regulate the osmolarity of their internal fluids.
Therefore, freshwater fishes must utilize their gill ionocytes to attain ions from their environment to maintain optimal blood osmolarity.
The base of the arch may also support gill rakers, small projecting elements that help to filter food from the water.
[13] Most sharks rely on ram ventilation, forcing water into the mouth and over the gills by rapidly swimming forward.
In slow-moving or bottom dwelling species, especially among skates and rays, the spiracle may be enlarged, and the fish breathes by sucking water through this opening, instead of through the mouth.
Their kind of gill respiration is shared by the "fishes" because it was present in their common ancestor and lost in the other living vertebrates.
But based on this shared trait, we cannot infer that bony fish are more closely related to sharks and rays than they are to terrestrial vertebrates.
[18] Endoparasites (parasites living inside the gills) include encysted adult didymozoid trematodes,[19] a few trichosomoidid nematodes of the genus Huffmanela, including Huffmanela ossicola which lives within the gill bone,[20] and the encysted parasitic turbellarian Paravortex.
