All modern fishes have four respiratory gill arches and a 5th non-respiratory arch on each side of the buccal cavity. Each respiratory arch is composed of a cartilaginous supporting structure which bears gill rakers in the front and respiratory tissue in the rear. The gill rakers act like a strainer to keep food items from passing through the gills. Piscivorous fish have short, stubby rakers while planktivorous fish have gill rakers that are fine and feathery. The respiratory tissue is comprised of a paired series of filaments, similar to a feather. There are two series of filaments on each arch. One series of filaments is termed a hemibranch, while both together are termed a holobranch.
There are numerous plate-like lamellae on each filament. The lamellae are the site of blood/water exchange. In a healthy gill the blood is separated from the water by two layers of epithelial cells. If there is a source of gill irritation there is often hyperplasia of the epithelial cells reducing exchange efficiency. [An Example] Fishes have a countercurrent system where blood and water flow opposite directions at the lamellae. This greatly increases exchange efficiency.
Blood flows from the heart toward the head in the ventral aorta. From the ventral aorta, afferent branchial arteries branch off to each gill arch and run up the center of the cartilaginous arch, where another branch comes off at each filament and is called the afferent filamental artery. Blood then flows through the lamellar lacunae where the exchange of respiratory gases takes place. Pillar cells connect the sides of the lamellae preventing it from ballooning due to blood pressure. Pillar cells also direct most of the blood flow into the marginal channel [ha!], where the water flow is greatest and, therefore, gas exchange most efficient. Microscopic anatomy indicates that some blood may flow between the pillar cells and also into a central compartment (central venous sinus). This central sinus probably serves a nutritive function for the gill. From the lamellar lacunae, the oxygenated blood flows into the efferent filamental artery and into the efferent branchial artery and then to the dorsal aorta out to the body.
Lamellar anatomy is quite variable between species. For example, in tuna, the arrangement of the pillar cells form not one, but several channels. Moreover, the lamellae of one filament are connected to the lamellae of the other filament so the filaments cannot separate, thus the hemibranch is like a sieve. This allows the tuna to ram ventilate at high swimming speeds without blowing the filaments apart.
Much speculation and research has centered on whether a non-respiratory shunt exists in the gills of fish. Such an alternative pathway for blood flow would be beneficial, since during times of low respiratory need the fish could direct blood away form the water, minimizing osmoregulatory loss or gain [See Chapter VII]. The preponderance of evidence favors the existence of a non-respiratory pathway, at least in some species. One mechanism may involve contraction of the pillar cells to redirect blood to and away from maximum exposure to the water as it circulates through the lamellae.
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