Gills

Gills

Gills and lungs are the two structures commonly used by animals for respiration. Both are characterized by large amounts of surface area that function in gas exchange. The difference between them is that gills involve external extensions from the body surface, whereas lungs possess internal foldings. Gills have evolved independently several times in a variety of animal groups.

Among the annelids, certain species of terrestrial worms have long slender, branching gills which extend from the body. Horseshoe crabs possess structures known as book gills, which are actually modified appendages that function in gas exchange. Crustaceans also have gills that have been modified from thoracic or abdominal appendages.

Gill structures are highly diverse among crustaceans. Generally, the more aquatic crustaceans have more elaborate gills, while the more terrestrial species are characterized by simplified gills. That is a consequence of the greater availability of oxygen on land, from the air, than in water.

In echinoderms, the group that includes starfish, the large surface area provided by the many appendages and by the tube feet are used in gas exchange. However, some species supplement these with gills around the oral cavity. Mollusks possess gills within their mantles. These are oriented to face the water current. Vertebrates such as salamanders are also characterized by external gills, which in their case are filamentous structures that extend from the head region. In some species, only the aquatic larvae have gills. However, in many species that remain aquatic their entire lives, the gills may be retained into adulthood.

Fish also use gills in gas exchange. The gills of fish are supported by a series of bony gill arches. The gill arches lie between the gill clefts, through which oxygenated water flows. The gill arches support tissue that includes the tiny blood vessels which carry in deoxygenated blood and carry away oxygenated blood, as well as the gill filaments, where gas exchange actually occurs. The gill filaments each have numerous secondary gill lamellae that further increase the surface area available for gas exchange.

Fish that have high energy demands, such as those which swim quickly, have more surface area associated with their gills. The operculum of bony fishes is a covering that protects the entire gill area. It also covers the operculum chamber, which is essential to the process of pumping water over the gills.

Gas exchange across the gills occurs in what is described as a counter current fashion. This is a very efficient mode of gas exchange because water flows in one direction and blood flows in the other. The consequence of countercurrent flow is that well-oxygenated blood encounters well-oxygenated water, and is able to pull more oxygen from the water, while less-oxygenated blood encounters less-oxygenated water. Oxygenation thus occurs along the entire pathway where the water and blood are juxtaposed.

Fish push oxygen-rich water across their gills by one of two methods. In ram ventilation, fast-swimming species such as sharks swim with their mouths open. Water is forced into the mouth and out over the gills.

Most bony fish species, however, employ a second method, buccal and opercular pumping. In this method, serial expansions and contractions of the mouth cavity and the opercular cavity occur, resulting in the continuous flow of water over the gills. First, the mouth, or buccal, cavity expands, drawing water in. Then, the mouth closes and the buccal cavity contracts. This forces water to flow over the gills into the opercular cavity. At the same time, the opercular cavity expands, which draws more water in from the buccal cavity. Water exits through the operculum.

Some fish will use buccal and opercular pumping while swimming slowly and ram ventilation when swimming faster. Ram ventilation is essential in some species, which suffocate if they are not able to swim fast enough.

The gill arches have played a crucial role in vertebrate evolution. Over evolutionary time, they have been modified to form other essential skeletal structures. The vertebrate jaw, which characterizes all vertebrates aside from lamprey and hagfish, was modified from a single pair of gill arches.

In mammals there has been a further modification of this pair of gill arches. Mammals have evolved a "new jaw," and the original bones of the jaw joint now function as middle-ear bones in mammals. The incus and malleus are both homologous to the jaw bones in other vertebrates, which are homologous to gill arches in primitive fish groups. The middle-ear bones are involved in conducting sound between the eardrum and the inner ear, where neural processing occurs. The third mammalian middle-ear bone, the stapes, occurs in all terrestrial vertebrates, and is also derived from the gill arches.

see also Blood; Respiratory System.

Jennifer Yeh

Bibliography

Gould, James L., and William T. Keeton. Biological Science, 6th ed. New York: W. W. Norton, 1996.

Hildebrand, Milton, and Viola Hildebrand. Analysis of Vertebrate Structure. New York: John Wiley, 1994.

Withers, Philip C. Comparative Animal Physiology. Fort Worth, TX: Saunders College Pub., 1992.