Gill
In aquatic organisms,
gills are
respiration organs for the extraction of
oxygen from
water and the excretion of
carbon dioxide.
Unlike many small aquatic animals, which can absorb oxygen through the entire surface of their bodies, more complex aquatic organisms have localized respiratory organs called gills specially formed to present an adequate
surface area to the external environment. Gills are usually thin plates of tissue or slender tufted
processes and, with the exception of some aquatic
insects, they contain
blood or
coelomic fluid, which exchanges gases through their thin walls. In the aquatic insects, a unique type of respiratory organ is used, the
tracheal gill, which contains air tubes. The oxygen in these tubes is renewed through the gills. Other aquatic insects use a type of
physical gill, often
plastrons.
Respiration in
Echinoidea,
sea stars and
sea urchins, is done through a very primitive version of gills called papulae, thin protuberances on the surface of the body containing diverticula of the water
vascular system. In
crustaceans,
mollusks and some insects, they are tufted or plate-like structures at the surface of the body in which blood circulates. The gills of other insects are of the tracheal kind and also include both thin plates and tufted structures, and, in the
larval
dragon fly, the wall of the caudal end of the
alimentary tract (
rectum) is richly supplied with tracheae as a rectal gill. Water pumped into and out of the rectum provide oxygen to the closed tracheae.
Gills of
vertebrates are developed in the walls of the
pharynx along a series of gill slits opening to the exterior. Water taken into the mouth passes out of the slits, bathing the gills as it passes. Some
fish utilize the gills for the excretion of
electrolytes. In some
amphibians, the gills occupy the same position on the body but protrude as external tufts. In most species, a
countercurrent exchange system is employed to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other. The gill slits of fish are believed to be the evolutionary ancestors of the
Eustachian tubes.
Gills' large surface area tends to create a problem for fish seeking to regulate the
osmolarity of their internal fluids. Saltwater is less dilute than these internal fluids; as a consequence, saltwater fish lose large quantities of water osmotically through their gills. To regain the water, they drink large amounts of
seawater and excrete the
salt. Freshwater is more dilute than the internal fluids of fish, however, so freshwater fish gain water osmotically through their gills. They eliminate the water through their urine.
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Lung*
Book lungs
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Branchia*
Physical gill
