For both invertebrates and vertebrates, one method of restricting gain of water (and loss of ions) would be to have the impermeable body surface. Though, even if this were case, water and ion movement across gills could still happen relatively unhindered. Water which is gained by invertebrates is excreted as urine - urine flow rate in freshwater is much higher than that of equivalent marine species. Though, excretion of urine also results in loss of ions and so exacerbates diffusive ion losses that happen in the animals. To compensate for loss of ions, active uptake mechanisms transport ions from freshwater back into animal. In several freshwater invertebrates, site of ion uptake is not known and it is believed to happen across general body surface area. Though, in few invertebrates site of uptake is known with some degree of certainty. In freshwater crustaceans, for instance, it is known that active transport of ions happens across gills; in aquatic insect larvae, active transport of ions has been illustrated to happen in anal gills.
Freshwater vertebrates face same osmotic and ionic problems as freshwater invertebrates. When thinking of freshwater vertebrates, it is only essential to consider osmotic and ionic relations of teleosts - there are very few elasmobranchs which are true freshwater species. Similar to invertebrates, main site of osmotic water gain in teleosts is gills. Excess water is removed by production of large quantities of very dilute urine. Though urine is dilute, it contains some dissolved solutes, and as large volumes of urine are produced, urine excretion may result in relatively large loss of ions. This in turn compromises ion loss that is already happening by diffusion from plasma to water. Few losses of ions can be compensated for by gain of ions from food. Though, main source of ion gain is by active transport of ions in gills. It is believed that transport of ions across general body surface is unimportant.
Factors Affecting Evaporative Water Loss in Animals:
The number of factors influences evaporative water loss from the animal.
Arthropods are largest proportion of terrestrial invertebrates. They comprise insects and spiders, of which former are the most several. Other members of phyla, crustaceans are predominantly aquatic' animals. One of defining characteristic features of insects is presence of exoskeleton. Exoskeleton is covered by wax that forms cuticle of insect. Presence of cuticle is one means by which evaporative water loss from general body surface area can be reduced. Though, it is significant to note that cuticle is not completely impermeable to water, as there is still some element of water loss across it. Even so, cuticle still represents formidable barrier to evaporation. Disruption to arrangement of waxes covering exoskeleton, by physical or thermal damage, for instance, results in increased water loss by evaporation.
The second site of evaporative water loss from insects is respiratory system, using spiracles. Though several tracheas that originate from spiracles are covered with chitin, water loss from here may still represent the important burden to animal; to limit such loss, several insects utilize cyclic respiration. Loss of water via feces and urine production in insects is negligible as water from urine and faeces are reabsorbed prior to release into environment as waste products. This condition is aided further by fact that insects excrete nitrogenous waste as uric acid that is really insoluble in water, so it can be excreted with loss of little water.
The clearest means of gaining water for insects is by drinking, for instance, from pools, rainfall, and so on. Though, this is source not available to every insect like those which live in hot arid environments and desert regions. Other potential sources of water comprise food and production of water during metabolism of food stuffs (metabolic water). In terms of water in food, maybe richest source of water is from plants. Lastly, few insects, like cockroaches, balance the water budget by absorbing water vapor from air in environment around them.
Terrestrial vertebrates include birds, reptiles, and mammals. Amphibians are frequently ignored as they are not really terrestrial animals by the nature. Reptiles, that comprise lizards, snakes, crocodilians and tortoises, contain dry, scaly skin that is well adapted to terrestrial life in that it signifies significant barrier to evaporative water loss. Additionally, they excrete the nitrogenous waste as uric acid, that needs loss of very little water. They are also able to produce extremely dry faeces that further limit water loss.
In terms of water gain, drinking of water may present the problem due to hot, arid environments where several animals are found. This means that water in food, together with water got during metabolism of food, signifies most significant gain of water. Few lizards and tortoises make dilute urine that is stored in bladder. This may be reabsorbed when animals are dehydrated.
Birds exhibit similar adaptations to maintaining the appropriate water balance as observed in reptiles. Though, water balance of birds may be further compromised by fact that they should maintain the more or less constant body temperature. One method in which heat-stressed bird may lose heat is to lose water by evaporation that causes cooling. This may be got by phenomenon of gular fluttering. It is rapid oscillatory movement of mouth and throat that promotes water loss. It is analogous to panting in mammals. Gular fluttering represents the potential disruption to animal's water balance, but, due to birds is able to satisfy water needs by drinking for simple reason that they are able to fly to find sources of water, this is not a important problem. Though, one potential problem with drinking water, mainly for marine birds, is that water has very high salt content. Together with ingestion of large amounts of salt in their food, this means that the birds require excreting large amounts of salt. To get this, marine birds have paired nasal salt glands. When birds are facing the salt overload they start to secrete solution which is fundamentally concentrated solution of NaCl. Glands are rendered inactive until bird becomes salt stressed. Like glands are also found in reptiles, which, though mainly terrestrial animals, may spend part of the life in marine environment.
Water loss by birds is more reduced by fact that they excrete very dry urine (uric acid) like reptiles. This technique of limiting water loss is so well-organized that water content of feces may be as low as 25%. This has certainly contributed to success of birds and reptiles living in hot, arid environments.
Terrestrial mammals like reptiles and birds have same potential routes for loss and gain of water. Evaporative loss of water from general body surface area is minimized by existence of comparatively impermeable skin and of fur and hair. Of greater significance, in terms of water loss, is evaporative loss from respiratory tract - this may account for the large proportion of water lost from animal.
Though, mechanisms have evolved that serve to limit the loss. One such mechanism is breathing out air which is at lower temperature than normal body temperature. This phenomenon is observed in all mammals. In inspiration, walls of nasal passages transfer heat to air entering respiratory system. When animal breathes out, warm air from respiratory system passes over this cooled surface and condensation of water occurs. Water and salt loss also happen in the animals capable of sweating. In this condition, though, these losses are means of regulating body temperature and not true osmotic response.
Water gain for several mammals is just attained by drinking. Though, this is not possible for desert dwelling mammals. Kangaroo rat (Dipodomys specfabilis), for instance, doesn't drink, but survives on metabolic water - oxidation of glucose, for example, produces carbon dioxide, ATP, and water. Few mammals, for instance, whales and dolphins, are completely marine-living mammals. It might be believed that such animals would face harsh osmotic problems because of gain of large amounts of salt from food. This may well be the case, but animals have very proficient kidneys that can create highly concentrated urine; therefore, ensuring that excess salt they consume is excreted. Though, it is not possible to create urine of infinite concentration. Usually, it is only possible to create urine that is three to four times more concentrated than plasma from which it has been formed.
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