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A gill is a vital organ used by fish and other aquatic animals to breathe by extracting oxygen from the surrounding water. On most aquatic animals that utilize gills for respiration, they are found on or near the head, and are protected by a structure called a gill flap. These specialized respiratory organs work by filtering the oxygen out of the water as it passes across numerous filaments. Similar to lungs, the oxygen is absorbed into the blood through the thin walls of tiny blood vessels. In order to obtain enough oxygen to support life, a great deal of water must pass through the gills, and scientists believe it has been used by various creatures for millions of years.
Most aquatic animals who remain submerged underwater use gills for respiration. Examples include fish, eels, and crabs, and the gill performs the job that the lung does in air-breathing animals. The primary function of gills is the extraction of oxygen from the water, and the release of carbon dioxide. Usually gills are located on or near the animal's head in pairs with one on each side, and the number of sets varies depending on the kind of aquatic animal. Each one is protected by a structure called a gill flap, which has the appearance of a long, deep, slightly curved slit in the skin. The flap covers and protects the gill's structure, and also helps to direct water across its surface.
A gill functions by filtering enormous the quantities of water that flows through the structure. As water enters through the flap, large particles are removed by structures called gill-rakers, which are like a screen made of soft tissue. Then the water reaches the filaments, which are composed of reddish-colored soft tissue and look similar to a fine-toothed comb. The filaments have a lot of surface area exposed to the water with multitudes of tiny thin-walled blood vessels very close to the outer membrane.
As the water passes through the filaments, oxygen is able to pass through the walls of the tiny blood vessels and enter the bloodstream. Carbon dioxide is also able to exit the blood through the vessel walls so that it can be released back into the water by the gills. Since there is a much lower concentration of oxygen in water than in air, tremendous amounts must pass through the gills in order to supply enough of the gas to sustain life. In addition to the water that is naturally drawn in as the aquatic organism swims, many animals are able suck more in by "pumping" their gills when extra quantities are needed. Gills are extremely efficient respiratory organs that many scientists believe have been employed for millions of years since the first creatures began life in the sea.
An eel is a fish.