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, the release of hormones (acetylcholine and vasoactive intestinal peptide).
Acetylcholine 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 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 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
penguins, which removes
sodium chloride 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 - allowing the seagull to drink seawater without becoming ill, although it prefers to drink fresh water when available. Contrary to popular misconceptions, 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). 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. 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 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 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 ==