
The salt gland is an
organ
Organ and organs may refer to:
Biology
* Organ (biology), a group of tissues organized to serve a common function
* Organ system, a collection of organs that function together to carry out specific functions within the body.
Musical instruments
...
for excreting excess
salt
In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...
s. It is found in the cartilaginous fishes subclass
elasmobranchii
Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including modern sharks ( division Selachii), and batomorphs (division Batomorphi, including rays, skates, and sawfish). Members of this subclass are characterised by h ...
(sharks, rays, and skates),
seabird
Seabirds (also known as marine birds) are birds that are adaptation, adapted to life within the marine ecosystem, marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent ...
s, and some
reptile
Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...
s. Salt glands can be found in the
rectum
The rectum (: rectums or recta) is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. Before expulsion through the anus or cloaca, the rectum stores the feces temporarily. The adult ...
of sharks. Birds and reptiles have salt glands located in or on the skull, usually in the eyes, nose, or mouth. These glands are lobed containing many secretory
tubules which radiate outward from the excretory canal at the center. Secretory tubules are lined with a single layer of
epithelial cells
Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...
. The diameter and length of these glands vary depending on the salt uptake of the species.
Salt glands maintain
salt balance and allow marine vertebrates to drink
seawater
Seawater, or sea water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has approximat ...
.
Active transport
In cellular biology, active transport is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellula ...
via
sodium–potassium pump
The sodium–potassium pump (sodium–potassium adenosine triphosphatase, also known as -ATPase, pump, or sodium–potassium ATPase) is an enzyme (an electrogenic transmembrane ATPase) found in the membrane of all animal cells. It performs s ...
, found on the
basolateral membrane
The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extra ...
, moves salt from the
blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells.
Blood is com ...
into the gland, where it is excreted as a concentrated
solution
Solution may refer to:
* Solution (chemistry), a mixture where one substance is dissolved in another
* Solution (equation), in mathematics
** Numerical solution, in numerical analysis, approximate solutions within specified error bounds
* Solu ...
.
In birds

The avian salt gland has two main ducts: a medial and a lateral. Salt gland activations occurs from increased osmolarity in the blood, stimulating the hypothalamic information processing, sending a signal through the
parasympathetic nerve activating
vasodilation
Vasodilation, also known as vasorelaxation, is the widening of blood vessels. It results from relaxation of smooth muscle cells within the vessel walls, in particular in the large veins, large arteries, and smaller arterioles. Blood vessel wa ...
, the release of hormones (acetylcholine and vasoactive intestinal peptide).
Acetylcholine
Acetylcholine (ACh) is an organic compound that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Par ...
binds to the receptor on the basolateral membrane of the gland. This in turn activates calcium release in the epithelial cells, opening potassium channels (flowing potassium out of the cells) on the basolateral membrane and chloride channels on the apical membrane to flow out of the cell. Ions are moved into the epithelial cells by a
Na-K-Cl cotransporter, also in the basolateral membrane. Increases in sodium opens the sodium-potassium
ATPase
ATPases (, Adenosine 5'-TriPhosphatase, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, ATP hydrolase, adenosine triphosphatase) are a class of enzymes that catalyze the decomposition of ATP into ADP and a free phosphate ion or ...
channels, removing the excess sodium back out across the basolateral membrane and allowing for potassium to come into the cell. An electrical gradient is formed from the chloride ions, allowing sodium to be passed through the
tight junctions
Tight junctions, also known as occluding junctions or ''zonulae occludentes'' (singular, ''zonula occludens''), are multiprotein junctional complexes between epithelial cells, sealing and preventing leakage of solutes and water. They also play a ...
of the epithelial cells into the salt gland along with minimal amounts of water. As well, mitochondria-rich cells are associated with changes in salt concentration, increasing with higher amounts and decreasing with lower exposure, assisting in the movement of salts. These glands excrete the
hypertonic sodium-chloride (with few other ions) by the stimulus of central and peripheral
osmoreceptors and volume receptors.
The supraorbital gland is a type of lateral nasal gland found in some species of marine birds, particularly
penguin
Penguins are a group of aquatic flightless birds from the family Spheniscidae () of the order Sphenisciformes (). They live almost exclusively in the Southern Hemisphere. Only one species, the Galápagos penguin, is equatorial, with a sm ...
s, which removes
sodium chloride
Sodium chloride , commonly known as Salt#Edible salt, edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is transparent or translucent, brittle, hygroscopic, and occurs a ...
from the bloodstream. The gland's function is similar to that of the kidneys, though it is much more efficient at removing salt, allowing penguins to survive without access to fresh water. The supraorbital gland is also possessed by the
European herring gull
The European herring gull (''Larus argentatus'') is a large gull, up to long. It breeds throughout the northern and western coasts of Europe. Some European herring gulls, especially those resident in colder areas, bird migration, migrate furthe ...
- allowing the seagull to drink seawater without becoming ill, although it prefers to drink fresh water when available.
Contrary to popular belief, the gland does not directly convert saltwater to freshwater. The term ''supraorbital'' refers to the area just above the eye socket (which is known as the
orbit
In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...
).
Living in saltwater environments would naturally pose a large problem for penguins because the ingestion of
saltwater would be detrimental to a penguin's health. Although penguins do not directly drink water, it is taken in when they consume
prey
Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not ki ...
. As a result, saltwater enters their system and must be effectively
excreted. The supraorbital gland has thus enabled the penguins' survival in such environments due to its water-filtering capability. The gland is located just above the eyes and surrounds a
capillary bed
A capillary is a small blood vessel, from 5 to 10 micrometres in diameter, and is part of the microcirculation system. Capillaries are microvessels and the smallest blood vessels in the body. They are composed of only the tunica intima (the inn ...
in the head. This capillary bed constantly strains out the salt in the saltwater that a penguin takes in. Since the byproduct of the gland has roughly five times as much salt as would normally be found in the animal's fluids, the supraorbital gland is highly efficient.
The penguin excretes the salt byproduct as a
brine
Brine (or briny water) is a high-concentration solution of salt (typically sodium chloride or calcium chloride) in water. In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawat ...
through its
bill. Often, the fluid drips out, and this gives the appearance of a runny nose. However, the fluid may also be
sneezed out. In the absence of saltwater, caused by captivity, the supraorbital gland will lie dormant as it has no other purpose. Having a dormant supraorbital gland does not negatively affect the health of a penguin.
In reptiles
The need for salt excretion in reptiles (such as
marine iguana
The marine iguana (''Amblyrhynchus cristatus''), also known as the sea iguana, saltwater iguana, or Galápagos marine iguana, is a species of Iguanidae, iguana found only on the Galápagos Islands (Ecuador). Unique among modern lizards, it is a m ...
s and
sea turtle
Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerh ...
s) and birds (such as
petrel
Petrels are tube-nosed seabirds in the phylogenetic order Procellariiformes.
Description
Petrels are a monophyletic group of marine seabirds, sharing a characteristic of a nostril arrangement that results in the name "tubenoses". Petrels enco ...
s and
albatross
Albatrosses, of the biological family Diomedeidae, are large seabirds related to the procellariids, storm petrels, and diving petrels in the order Procellariiformes (the tubenoses). They range widely in the Southern Ocean and the North Paci ...
es) reflects their having much less efficient
kidney
In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organ (anatomy), organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and rig ...
s than mammals.
Unlike the
skin
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other animal coverings, such as the arthropod exoskeleton, have different ...
of
amphibian
Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...
s, that of reptiles and birds is impermeable to salt, preventing its release.
The evolution of a salt gland in early reptiles and birds allowed them to eat aquatic plants and animals with high salt concentrations. This evolutionary development does not account for the gland in
elasmobranchs, suggesting
convergent evolution
Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...
.
Some theories suggest
mammal
A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
ian
tear duct
The nasolacrimal duct (also called the tear duct) carries tears from the lacrimal sac of the eye into the nasal cavity. The duct begins in the eye socket between the maxillary and lacrimal bones, from where it passes downwards and backwards. The o ...
s and
sweat gland
Sweat glands, also known as sudoriferous or sudoriparous glands, , are small tubular structures of the skin that produce sweat. Sweat glands are a type of exocrine gland, which are glands that produce and secrete substances onto an epithelial s ...
s may be evolutionarily related to salt glands. While human tears are high in
potassium
Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...
, most
phylogeneticists disagree with the association.
See also
*
Halotolerance
Halotolerance is the adaptation of living organisms to conditions of high salinity. Halotolerant species tend to live in areas such as hypersaline lakes, coastal dunes, saline deserts, salt marshes, and inland salt seas and springs. Halophiles ar ...
*
Lacrimal gland
The lacrimal glands are paired exocrine glands, one for each eye, found in most terrestrial vertebrates and some marine mammals, that secrete the aqueous layer of the tear film. In humans, they are situated in the upper lateral region of each o ...
*
Osmoregulation
Osmoregulation is the active regulation of the osmotic pressure of an organism's body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism's water content; that is, it maintains the fluid balance and the concentration ...
*
Preorbital gland
The preorbital gland is a paired exocrine gland found in many species of artiodactyls, which is homologous to the lacrimal gland found in humans. These glands are trenchlike slits of dark blue to black, nearly bare skin extending from the media ...
References
Further reading
*Evans, D. H. 1993. ''Osmotic and Ionic Regulation.'' pp. 315–336. In Evans, D. H. 1993. The Physiology of Fishes. CRC Press, Boca Raton, Florida.
*Goldstein, D. L. 2002. ''Water and Salt Balance in Seabirds.'' pp. 467–480. In Schreiber, E. A. and J. Burger. (eds.) 2002. Biology of Marine Birds. CRC Press, Boca Raton, Florida.
*Schmidt-Nielsen, K. 1959. Salt Glands. pp. 221–226. In Wessells, N. K. (comp.) 1974. Vertebrate Structures and Functions. W. H. Freeman and Company, San Francisco, CA.
*Wǖrsig, B. G., T. A. Jefferson and D. J. Schmidly. 2000. The Marine Mammals of the Gulf of Mexico. Texas A&M Press, College Station, TX.
External links
*
*
{{Animalosmo
Glands
Vertebrate anatomy