The salt gland , or supraorbital gland , is a special organ in birds , which makes it possible to efficiently remove excess salt (sodium chloride) from the body [1] [2] and is part of the osmoregulatory system [3] . It allows seabirds to satisfy their fluid needs through seawater [1] and is the main organ that removes most of the salts from the body of these birds.
The salt gland was first discovered in seagulls by Schmidt-Nielsen in a study of how seabirds are able to maintain their osmotic balance without fresh water from the outside [4] , and later it was shown by its staff that it plays an important role in salt metabolism in many seabirds [4] [5] . Salt glands are modified lateral nasal glands located on the frontal bones above the orbit [6] . Saline solution flows through the nostrils and flows to the tip of the beak [2] .
Most developed in seabirds [7] ( tube-nosed gulls , scallops , pelicans , etc.), but also function in some desert forms ( African ostrich , desert chicken , etc.) [2] . In birds that are not associated with the consumption of excess salt, the nasal glands are poorly developed and practically do not function [2] . Most of the salts are secreted by these glands as part of a clear, colorless liquid that drips from the tip of the beak. The liquid is a 5% sodium chloride solution, that is, the concentration of salt in it is several times higher than in tear fluid, and almost twice as high as in sea water [1] .
The glands secrete a hypertonic solution of sodium chloride. In birds, the concentration of sodium chloride in the secret produced by the salt glands is about 5 times higher than in the blood [1] .
Building
Microscopic studies of the salt gland indicate that it is formed by many parallel cylindrical lobules. Each lobule, in turn, contains several thousand tubules that radially branch from the central duct. In these tubules, the diameter of which is several microns, the processes of saline secretion occur. Active salt release occurs through the epithelial layer of branching tubules formed from special secretory cells. There are deep folds on the basal side of the membrane of these cells. In addition, there are a lot of mitochondria in these cells [4] . As in other epithelium with transport function, neighboring cells are in close contact with each other, forming tight contacts. Such a structure prevents the abundant seepage of water or dissolved substances from one side of the epithelial layer to the other, bypassing the cells themselves. Secretion by the salt glands does not include the filtration process, as in the case of urine formation in the renal glomeruli. The salt glands in birds are built as a countercurrent system, which contributes to the concentration of salts in the fluid they release. Parallel to the tubules is the capillary network, the movement of blood in which is opposite to the flow of saline in the tubules [4] . This nature of the flow helps maintain along the entire length of the tubules a minimum concentration gradient of sodium chloride between the blood and the secreted fluid. Due to this, the concentration gradient necessary for the upward transfer of salts from blood plasma to the secretion of the gland is minimized [4] . In the mechanism of ion transfer in the bird’s salt gland, the main concentration gradient between the blood and the fluid secreted by the gland is established at the boundary between the gland of the gland itself and the secretory cell, in the effective apical membrane of the cell [5] .
Gland secretion regulation
The secretory activity of the gland is directly regulated by the nervous system ( parasympathetic ). Neuroendocrine regulation also occurs. Studies of the endocrine regulation of the salt gland of birds suggest that a number of hormones, such as corticosterone and arginine vasotocin , have a regulatory effect on salt secretion [4] .
The secretory activity of the salt gland, apparently, is regulated by the principle of positive feedback and is dependent on the concentration of sodium chloride in the blood. The initial trigger signal is a change in the osmotic concentration of blood, since the salt glands in birds begin to activate only after ingestion of sea water or after the introduction of hypertonic solutions. Receptors that respond to excess salt in the body are found in birds in the heart and are excited by increased blood tonicity. However, the immediate stimulus that causes and supports secretion is the release of acetylcholine by the ends of the cholinergic nerve endings of the salt gland. Irritation of the facial nerve also has a stimulating effect on the secretion of the salt gland [5] .
Notes
- ↑ 1 2 3 4 Systematics of birds: a training manual / T. A. Atemasova. - Kh.: KhNU named after V.N. Karazin, 2015. - 200 p. ISBN 978-966-285-152-6
- ↑ 1 2 3 4 Ilyichev V.D., Kartashev N.N., Shilov I.A. General ornithology: Textbook for students. biol. specialist. un-com. - M.: Higher School, 1982. - 464 s
- ↑ Ginetsinsky A.G. Physiological mechanisms of water-salt balance. A. G. Ginetsinsky. Moscow-Leningrad, Science. 1964
- ↑ 1 2 3 4 5 6 Animal physiology: mechanisms and adaptations. The textbook in two volumes. Volume 2. Eckert R., Randall D., Augustine D. M .: World, 1992. 344c. ISBN 5-03-001458-6
- ↑ 1 2 3 Khochachka P., Somero J. Strategy for biochemical adaptation. Moscow. World 1977.398 s
- ↑ Kemp P., Arms K. Introduction to Biology M .: 1988. - 672 p.
- ↑ Goldenstein, DL 2002. Water and Salt Balance in Seabirds. p. 467-480. In Schreiber, EA and J. Burger. (eds.) 2002. Biology of Marine Birds. CRC Press, Boca Raton, FL.