Freshwater animals void excess water gained by osmosis by producing large amounts of dilute urine (Hyposmotic to blood; U/P < 1) like a freshwater animal like a goldfish or a frog, the daily osmotic water influx is equal to one-third of its body weight and this will help remove a lot of that excess water that has been taken passively via osmosis.
• The urine of freshwater animals, in addition to being produced in abundance, is typically markedly Hyposmotic to their blood plasma and contains much lower concentrations of Na+ and Cl- than the plasma. That is, the U/P ratios (urine: plasma ratios) for osmotic pressure, Na+, and Cl- are far less than 1 in these animals and when the osmotic U/P ratio is less than 1, urine production tends to raise the plasma osmotic pressure. Similarly, when the U/P ratio for an ion is less than 1, urine production tends to raise the plasma concentration of that ion. Typically, therefore, the kidneys of a freshwater animal not only solve the animal's volume-regulation problem by voiding the animal's excess volume of water, but also aid osmotic and ionic regulation by helping to maintain a high osmotic pressure and high ion concentrations in the blood.
• Kidneys are regulatory organs and they characteristically adjust their function in ways that help to maintain stability of volume and composition in the body fluids. If an animal experiences an increase in the rate at which it takes in water by osmosis, its kidneys ordinarily increase their rate of urine production.
• Kidneys limit [ion] in urine, but still a loss of ions
• Any factor that increases an animal's rate of osmotic water influx tends to increase the animal's rate of ion loss. We see, therefore, that volume regulation and ionic regulation are basically at conflict with each other in freshwater animals.
• Freshwater animals actively take up major ions into their blood directly from the water, while simultaneously removing metabolic wastes.
• The site of active ion uptake is usually the gills or the general integument. In teleost (bony) fish and decapod crustaceans (e.g., crayfish), the site of uptake is the gill epithelium. In frogs, active ion uptake occurs across the gills when the animals are tadpoles but across the skin when they are adults.5 Active ion uptake also occurs across the general integument in leeches and aquatic oligochaete worms.
• The active uptake of ions from the ambient water requires ATP. Thus active ion uptake places demands on an animal's energy resources.
• The mechanisms that pump Na+ and Cl- from the ambient water into the blood are typically different and independent from each other.
• The Cl- pump typically exchanges bicarbonate ions (HCO -) for Cl- ions, in this way remaining electro-neutral.
• The Na+ pump typically exchanges protons (H+) for Na+ ions (or possibly exchanges ammonium ions, NH4+, for Na+ in some groups of animals), thereby remaining electro-neutral.
• The HCO3- and H+ pumped from the blood into the ambient water by the Cl- and Na+ pumps are produced by aerobic catabolism, being formed by the reaction of metabolically produced CO2 with H2O. Thus the Na+ and Cl- pumps participate in removal of metabolic wastes.
• Because HCO3- and H+ are principal players in acid-base regulation, the Na+ and Cl- pumps sometimes play critical roles in the acid-base physiology of freshwater animals.
Replacement of water losses to replace water lost by osmosis, teleost fish actually drink seawater.
1. Ingestion of hyperosmotic seawater causes diffusion of body water into seawater in the gut. Water in the gut increases in volume and it comes more dilute.
2. In later parts of the gut: Na and Cl are actively transported out of gut and into blood and this requires ATP. It favors osmotic uptake of water into the blood. By the time ingested seawater is completely processed, about 50-85% of the H2O in the seawater is absorbed into the blood.
Marine teleost fish will excrete excess divalent ions in the urine whereas excess monovalent ions are excreted by the gills.
• Passive process, drinking seawater: ions are increased in the blood.
• Divalent ions -> excreted in urine
• Monovalent ions -> excreted by gills
• Excreted urine is isosmotic to plasma (osmotic U/P = 1)
• Ionic U/P ratios of monovalent ions < 1
• Ionic U/P ratios of divalent ions > 1like Mg and SO4
• The kidneys thereby void the major divalent ions preferentially in relation to water and keep plasma concentrations of those ions from increasing.
• Ionic regulation via urine production conflicts with osmoregulation: teleost fish limit urine to what is necessary to excrete of solutes that are not excreted by other routes.
• Nitrogenous wastes and the principal ions, Na+ and Cl-, are voided across the gills. Thus the role of the kidneys in marine teleosts is largely limited to excretion of divalent ions, and the rate of urine production can be low. The excretion of Chloride happens by secondary active transport and its carried out by special cells in the gills called mitochondria-rich cells so they have lots of mitochondria to generate lots of ATP to carried out these active processes.
External NaCl excretion by gills: Na+, Cl-, nitrogenous wastes voided across gills. There is a Na-K pump creates electrochemical gradient favoring Na+ diffusion out of the cell. Na+ is diffused back into cell at NKCC (sodium, potassium, 2 chloride cotransporter) with K+, 2Cl-. So as sodium comes back into the cell, it takes with it one potassium and 2 chlorides with it. Cl- entry creates gradient favoring outward diffusion via Cl- channels in the apical membranes. Excretion of Na+ may be active or passive depending on the species.
• The elimination of Cl- and Na+ by the gills of marine teleost fish provides our first example of extrarenal salt excretion: excretion of inorganic ions by structures other than the kidneys.
• Fish that is able to excrete NaCl without also excreting water: gills are the sites in marine teleost fish where the primary site of osmotic regulation is accomplished.
o Many of the adaptations for terrestrial life in these groups also aid osmotic regulation or water loss that they have inherited in a marine habitat. For instance, all of these animals are air breathers and they don't have the problem of passive movements of water and ions across their respiratory exchange membranes which is not the case for fish. Another advantage of their terrestrial heritage is that they have inherited integuments that are adapted to limiting water losses in the dehydrating terrestrial environment, so they tend to exhibit low integumentary permeabilities. Despite these adaptations, they still have problems with water loss and salt loading from the environment.
o They lose water, for example, by pulmonary evaporation during breathing; they also lose water to some extent across their skin, not only when they are immersed in seawater, but also when they are exposed to the air. These animals often gain excess salts from the foods they eat; for example, when they prey on marine plants or invertebrates that are isosmotic to seawater, they ingest body fluids that have far higher salt concentrations than their own. In addition, they probably often take in quantities of seawater with the foods they eat, although, for the most part, they are thought not to drink seawater.
Marine elasmobranch fish are hyperosmotic, but hypoionic to seawater. These are animals like sharks, skates and rays. They are unique in terms of the separation of the osmotic regulation and the ionic regulation. They maintain low blood concentrations of Na and Cl, lower than seawater, but they are still slightly hyperosmotic to water. They have high concentrations of urea and TMAO as a result of this decoupled of ionic concentrations and osmotic regulation. Urea is a nitrogenous waste product and is the major way that mammals release their waste, but these will produce ammonia instead to to the job of urea so urea will accumulate in the body fluids of these fish and this can be very damaging because it can alter protein structures. Urea in high concentrations can alter the structures of proteins, and the concentration of urea is kept low in most vertebrates (about 2-7 mM in human plasma). Plasma concentrations of urea in marine elasmobranchs—usually 300-400 mM—are "out of sight" by comparison. These higher concentrations of urea don't damage their body because they are counterbalanced by TMAO. TMAO serves as a counteracting solute. In the amounts present, TMAO offsets the effects of urea, evidently by opposing effects of urea on deleterious interactions of proteins with solvent water, interactions that if unopposed cause protein unfolding. In this fish osmotic regulation and ionic regulation are decoupled from each other. Excess salts are removed from the body fluids of elasmobranchs by the kidneys and, extrarenally, by rectal salt glands. Unlike teleost, therefore, elasmobranchs need not drink and need not incur an extra NaCl load to gain H2O from ingested seawater. o Water challenges: permeable integuments and urea is nitrogen waste product and they produce isosmotic urine. Moreover, although amphibians are notably adept at simply shutting off urine outflow when faced with dehydration, they are unable, when they do excrete urine, to produce a urine any more concentrated in total solutes than their blood plasma.
o Hormonal control of anti-dehydration by ADH: its an endocrine system and its produced by the posterior pituitary gland released to the blood and then acts on the kidney and the bladder. Their functions are that they reduce rate of urine production and elevates the urine concentration and it does this by altering the number of aquaporins. It also stimulates bladder cells for the reuptake of water, so they take up water from the bladder and use it like a canteen. Also they do water uptake through ventral skin like frogs that sits on their pelvic patch (dark) against moist substrates to take up water across the skin.
Desert amphibians exhibit behavioral adaptations to a xeric environment for example the spadefoot toads. They go at night when its cooler and then in the day they go underground and they also employ seasonal dormancy to simply "retire from the scene" and protect their water status during dry season. The Spadefoot toads, spend many months of each year in dormancy. Overall, these desert amphibians are reclusive animals, holed up in secluded places during much of their lives. For some, dormancy dominates their lives more than activity. Nonetheless, they are able to survive in deserts despite the high permeability of their skin and other vulnerabilities.
• Humidic animals have highly permeable integuments to water and with this they can reduce water loss by choosing humid habitats which will decrease water pressure gradient between bodies and air. These animals often lose water through their integuments at rates that are 50-100% as great as rates of evaporation from open dishes of water of equivalent surface area! With such a high integumentary permeability, a humidic animal can restrict its integumentary rate of evaporation only by limiting the difference in water vapor pressure that exists across its integument. This explains why humidic animals are tied to humid habitats, where the air has a high water vapor pressure and this can reduce that gradient between the external environment and the tissues of the animal's body.
• The xeric animals have integuments with a low permeability to water. Indeed, the evolution of a low integumentary permeability to water is one of the most important steps toward a xeric existence. In all the major xeric groups—vertebrate and invertebrate—microscopically thin layers of lipids are responsible for low integumentary water permeability. In mammals, birds, and nonavian reptiles, the lipid layers are structurally heterogeneous, lamellar complexes of lipids and keratin, less than 10 m thick, located in the stratum corneum, the outermost layer of the epidermis of the skin. The principal lipids present are ceramides, cholesterol, and free fatty acids. Mammals, birds, and nonavian reptiles differ in histological details of the lipid layers, and they evidently evolved their lipid layers independently. In insects and arachnids, the lipids responsible for low integumentary permeability—such as long-chain hydrocarbons and wax esters—are contained in the outermost layer of the exoskeleton (cuticle).