Osmoregulation and Excretion

K. J. O’Driscoll, L. K. Staniels, and D. E. Facey

(presented by Saint Michael's College, Colchester, VT, USA)

Osmoregulation is the regulation of water and ion concentrations in the body. Keeping this regulation precise is critical in maintaining life in a cell. Balance of water and ions is partly linked to excretion, the removal of metabolic wastes from the body.

The following information should answer some of your questions relating to this topic area. We have discussed the major components in water/ion regulation, as well as specific examples of animals and their struggle to achieve osmotic balance.


How do animals regulate their water intake in different environments?

There are three main types of osmoregulatory environments in which animals live: freshwater, marine, and terrestrial. Aquatic animals are either euryhaline or stenohaline, depending on their ability to tolerate different salinities. Animals whose internal osmotic concentration is the same as the surrounding environment are considered osmoconformers, whereas those that maintain an osmotic difference between their body fluid and the surrounding environment are osmoregulators.  Freshwater animals (all osmoregulators) include invertebrates, fishes, amphibians, reptiles, and mammals. The freshwater animals are generally hyperosmotic to their environment. The problems that they face because of this are that they are subject to swelling by movement of water into their bodies owing to the osmotic gradient, and they are subject to the continual loss of body salts to the surrounding environment (which has a low salt content). They way these animals deal with these problems is to produce a large volume of dilute urine. The kidney absorbs the salts that are needed, and the rest of the water is excreted. Another way these animals deal with lack of salt is by obtaining it from the food they ingest. A key salt replacement mechanism for freshwater animals is active transport of salt from the external dilute medium across the epithelium into the interstitial fluid and blood. Amphibian’s skin and fish gills are active in this process. Freshwater animals tend to take in water passively and to remove it actively through osmotic work of kidneys (in vertebrates) or kidney-like organs (in invertebrates).

Among marine animals, most invertebrates are osmoconformers whereas most vertebrates are somoregulators.  Marine animals do not need to expend much energy in regulating the osmolarity of their body fluids. There is a tendency for mairne fishes to lose water to the environment through the gill epithelium. The net result of combined osmotic work of the gills and kidneys in the marine teleosts is a net retention of water. Marine reptiles (iguanas, sea turtles, crocodiles, and sea snakes) drink seawater to obtain a supply of water but are unable to produce a concentrated urine that is significantly hyperosmotic to their body fluids. They compensate for this by the use of specialized glands for the secretion of salts in a strong hyperosmotic fluid. Salt glands are generally located above the orbit of the eye and nose in lizards. The salt glands of marine reptiles secrete a sufficiently concentrated salt solution to enable them to drink saltwater even though their kidneys are unable to produce urine more concentrated than seawater. Marine animals with these salt glands compensate for the inability of their kidney to produce urine that is strongly hypertonic relative to body fluids. Marine animals lacking salt glands avoid drinking seawater, and obtain water entirely from their food intake and metabolism. These animals depend on their kidneys for maintaining osmotic balance. Sea lions, seals, and a couple of marine mammals that live in saltwater do not have external salt-secreting organs like that of the birds and reptiles, yet they still survive in the ocean. Mammals cannot drink seawater, and would become quickly dehydrate if they did. These mammals face the same problems as the desert animals. Because mammals cannot consume seawater, a different method of hydration needs to be found. They have highly efficient kidneys capable of producing very hypertonic urine. These animals also rely on metabolic water (water produced as an endproduct of cellular metabolism) and water from feeding on fishes and invertebrates.

Air breathing animals are subject to dehydration through their respiratory epithelia. Humans and most other air-breathing animals require a constant source of fresh drinking water to excrete accumulated salts and metabolic waste products.

Water regulation and temperature regulation are closely related. Animals living in harsh heat environments such as deserts have to compensate for the lack of water. The kangaroo rat avoids the daytime heat, and emerges late at night. These rats like other desert mammals have efficient kidneys, and excrete highly concentrated urine. 90% of the water that they use is called metabolic water and is the major source for all desert animals. Metabolic water is derived from cellular oxidation. Camels have a different way of dealing with the unforgiving heat and lack of water in the desert. They are too large to hide in a hole, so when deprived of drinking water they allow their body temperatures to rise. In doing this they limit the amount of water lost by evaporation/perspiration. At night the animals’ body temperature can stay at 35 degrees Celsius and during the day rise to over 41 degrees Celsius. It too produces concentrated urine and dry feces. When water sources are too limited the camel will not produce urine but will store the urea in the tissues. This is particularly unusual because along with tolerating dehydration it can deal with high urea levels in its body. An interesting fact is that when water becomes available they will consume up to 80 liters in 10 minutes.


How do fish and insects regulate osmotic balance?

Cartilaginous fishes such as sharks, rays, and skates, have plasma that is approximately isosmotic to seawater. This unusually high osmotic concentration (compared to that of other vertebrates) is maintained by high levels of urea and trimethylamine oxide (TMAO) in the blood. In most vertebrates, levels of urea this high would damage proteins, but the presence of the TMAO helps to stabilize these protein molecules against the adverse effects of urea. Excess inorganic electrolytes, such as Na+ and Cl- which diffuse into the blood at the gills, are excreted by way of the kidneys and also by means of a special excretory organ called the rectal gland that is located at the end of the alimentary canal.

The body fluids of marine teleosts, like those of higher vertebrates, are hypotonic to seawater, so there is a tendency for these fishes to lose water to the environment, especially across the gill epithelium. To replace the water, they drink salt water and actively secrete the excess salt ingested with the seawater back into the environment. By absorption, 70% to 80% of the ingested water enters the bloodstream, along with most of the NaCl and KCl. Active transport is responsible for the elimination of Na+, Cl-, and some K+ across the gill epithelium into the seawater, and by secretion of divalent salts by the kidney. The net result of the combined osmotic work of gills and kidneys in the marine teleost is a net retention of water. The kidney nephrons in certain marine teleosts have neither glomeruli nor Bowman’s capsules. The urine is formed entirely by secretion because there is no specialized mechanism for the production of a filtrate.

Freshwater fishes tend to take in water passively and remove it actively through the osmotic work of kidneys. They lose salts to the dilute environment and replace them by actively absorbing ions from the surrounding fluids into their bodies through the gills. Freshwater teleosts’ bodies are hypertonic to the environment and water diffuses into them, so they maintain water balance by producing large volumes of dilute urine.

Ticks, mites, and other terrestrial arthropods have the ability to extract water vapor directly from the air. The way they accomplish this is by producing very concentrated solutions that absorb water from air. This solution is usually found in the rectum, which also removes water from the feces. Ticks, along with this solution, have tissues in the mouth useful in the uptake of water. The salivary gland excretes highly concentrated KCl solution, which absorbs water from air.


What are the sources of water gain and loss?

Animals acquire most of their water in food, drink and a smaller amount by oxidative metabolism. Animals lose water by urinating, defecating, and by evaporative loss due to sweating and breathing. For aquatic animals, evaporation is unimportant, but these animals experience the uptake and loss of water across the body surface by osmosis. Animals that are protected by a covering that stops water loss and gain have specialized epithelia which are not waterproof, that must be exposed to the environment in order to exchange gases. Examples of these epithelia occur in gills, lungs, and tracheae.

The nasal passages of mammals play an important role in reducing water loss through this pathway. Respiratory surfaces are a major avenue for water loss in air-breathing animals. The internalization of the respiratory surfaces in a body cavity such as the lungs reduces evaporative loss in terrestrial vertebrates. Because the body temperature of birds and mammals is generally higher than external temperatures, evaporative loss of water is greater. Warm expired air contains more water than the cooler inspired air, as the water holding capacity of air increases with temperature.

A mechanism, termed a temporal countercurrent system retains most of the respiratory water vapor by condensing it on cooled nasal passages during expiration. The nasal passages of mammals plays an important role in reducing the loss of water and heat from the body. The importance of the nasal passages in cooling expired air can be detected easily by placing your hand in front of your nose when breathing, and comparing this to putting your hand in front of your mouth when breathing.


What are the major problems that animals face with regard to osmoregulation?

In most animals, the majority of cells are not in direct contact with the external environment but are bathed by an internal body fluid. Homeostatic mechanisms hamper changes in an animal’s body fluid, which both gives protection from harmful external environments and impedes quick exchange between intracellular compartments. The cells of the animal cannot survive much additional water gain or loss. Water continuously enters and leaves an animal cell across the plasma membrane, however, uptake and loss must balance. Animal cells swell and burst if there is a net uptake of water or shrivel and die it there is a net loss of water.

Other problems associated with osmoregulation are body and environment temperatures. The enzyme activity in the body function between temperatures of 0o – 40o Celsius. The way animals deal with temperature and regulating it is by way of water loss. So animals in hot environments need to limit the amount of water loss due to evaporation and respiration. The importance of water in temperature regulating leads to conflicts and compromises between physiological adaptations to environmental temperatures and osmotic stresses in terrestrial animals.


What are the major structures involved in osmoregulation and excretion?

Organisms in different environments utilize different structures in osmoregulation and excretion. The major structures involved are the integument, the respiratory surface, the kidney, and the salt gland. All animals use at least one of these structures in their osmoregulatory processes. The common characteristic in structures such as gills, skin, kidneys and the integument are cells called transport epithelia, anatomically and functionally polarized cells which determine the osmoregulatory capabilities of the structure, through properties such as permeability to various solutes.

The integument functions in osmoregulation by acting as a barrier between the extracellular compartment and the environment to regulate water gain and loss, as well as solute flux. The permeablity of the integument to water and solutes varies from animal to animal.

Respiratory surfaces such as the alveoli of the lung, and gills in aquatic animals also serve in osmoregulation and excretion. Respiratory surfaces are the chief avenues for the excretion of carbon dioxide and metabolic water, as well as other gaseous wastes, in animals.

The kidney is the main organ involved in maintaining water balance and excreting harmful substances in mammals. For further explanation, see "How does the mammalian kidney produce urine?"

Elasmobranchs, marine birds, and some reptiles have a structure called a salt gland to secrete NaCl from their bodies. These animals require a lower internal NaCl concentration than the surrounding seawater, which causes a concentration gradient favoring the influx of salt. Therefore, they need a way to secrete it. The solution is provided by glands in the rectum of sharks and the skulls of marine birds and reptiles which produces a concentrated salt solution for secretion. The sodium ions are removed from the blood by these glands not by filtration, but by the sodium-transport mechanism (the sodium-potassium pump). This Na+/ K+ / ATPase activity allows for the movement of NaCl from the blood across the epithelium into the lumen of the salt gland for secretion. Interestingly, the shark rectal gland, bird nasal gland, fish gill, and the thick ascending Loop of Henle in the kidney all contain salt-secreting cells that transport NaCl by the same basic mechanism.  Active transport produces an increase in the chloride concentration in the cytoplasm of epithelial cells.  This results in the diffusion of chloride ions out of the cell across the apical surface.  The build-up of chloride ions at the apical surface attracts sodium ions to diffuse between the cells (the paracellular route).

Insects have a network of Malpighian tubules extending throughout much of the body cavity and attached to the alimentary canal between the midgut and the hindgut. The secretory cells which line the walls of these long, thin tubules secrete KCl, NaCl, and phosphate from the hemolymph (blood) into the lumen of the tubule. Smaller molecules, such as water, amino acids, and sugars diffuse down their concentration gradient and into the lumen. The fluid then flows along the tubule and into the gut. As the fluid passes through the hindgut, water and valuable ions are transported back into the hemolymph, leaving behind a concentrated waste for excretion from the body.


Why and how do organisms excrete metabolic wastes (particularly nitrogenous wastes)?

Waste products generated in metabolic processes are often toxic, and therefore must be eliminated before they can harm the organism. The major metabolic wastes produced by animals include carbon dioxide, metabolic water, and nitrogenous wastes. Small aquatic organisms are able to get rid of wastes by simple diffusion across membranes. More complex animals with circulatory systems rely on kidneys to filter wastes out of the blood and eliminate them from the body.

Carbon dioxide and metabolic water produced in respiration easily diffuse into the environment from respiratory surfaces. Nitrogenous waste excretion is more difficult, yet necessary. Elevated ammonia levels in the body can lead to convulsions, coma, and even death. This is because ammonium ions can substitute for potassium ions in ion-exchange mechanisms. Ammonia can also adversely affect metabolism and amino acid transport. Excessive amounts of ammonia in the system elevates bodily pH, which causes changes in the tertiary structure of proteins, and thus cellular functions can be altered.

There are three main types of nitrogenous wastes: ammonia, urea, and uric acid. The type of waste an animal excretes depends on its living environment, because nitrogenous waste excretion is accompanied by a certain amount of water loss. Ammonotelic (ammonia-excreting) animals generally live only in aquatic habitats, because ammonia is extremely toxic, and a large volume of water is required to maintain the excreted ammonia level lower than the body level. This is needed because ammonia excretion relies on passive diffusion, so a gradient is required between the organism and the environment in order for the ammonia to flow from high concentration to low concentration.

Whereas most excretion of ammonia occurs across the gills of aquatic animals, mammals do excrete some ammonia in the urine. Amino groups are enzymatically transformed into glutamate, and then changed to glutamine in the liver. Glutamine can cross the kidney membranes (whereas amino acids can not). In the kidney tubules, the glutamine is deaminated to ammonia and then excreted in the urine.

Although ammonia excretion is present in some forms in mammals, the major nitrogenous waste excreted is urea. Urea is less toxic than ammonia, and requires less water for elimination. Therefore, ureotelic (urea-excreting) animals are most often (but not exclusively) terrestrial. A downside to urea excretion is that urea synthesis requires energy, in the form of ATP. Vertebrates synthesize urea in the liver using the ornithine-urea cycle. Teleosts and invertebrates produce urea from uric acid via the uricolytic pathway.

Birds, reptiles, and most terrestrial arthropods often are subject to very limited water availability, so even urea excretion is not possible. Therefore, these uricotelic (uric acid-excreting) animals synthesize uric acid, which requires even less water than urea for elimination.  The ability to produce uric acid, which is relatively insoluble, is quite important to birds and reptiles prior to hatching.  Nitrogenous wastes can be safely stored within the egg in the form of uric acid, whereas a build-up of either ammonia or urea would be deadly. 


How does the mammalian kidney produce urine?

The kidney contains numerous functional units, called nephrons, which produce urine through a series of steps: glomerular filtration of the blood, tubular reabsorption of the glomerular filtrate, and tubular secretion of harmful substances .


Glomerular Filtration

Blood flows from the afferent arteriole into the glomerulus, a tuft of fenestrated capillaries enclosed in the Bowman’s capsule. Here 15 to 25 percent of the plasma’s water and solutes are filtered through a single-cell layer of the capillary walls, through a basement membrane, and into the lumen of the Bowman’s capsule. The filtrate then flows into the renal tubule, to undergo tubular reabsorption.

The rate of glomerular filtration depends on three factors: the hydrostatic pressure difference between the capillaries and the Bowman’s capsule (due to blood pressure), the colloid osmotic pressure, which opposes filtration, and the hydraulic permeability of the three-layered tissue separating the capillaries and the lumen of the Bowman’s capsule. Overall, blood pressure in the body has a major effect on the glomerular filtration rate, because the amount of blood passing through the glomerulus determines how much and how fast the fluid can be filtered.  Among the dangers of very low blood pressure, therefore, is the loss of kidney function.  This is a primary reason for the use of inflatable "shock suits" on the lower body in cases of extreme blood loss from trauma.  By reducing blood flow to the legs, blood pressure in the trunk is kept higher, in an effort to maintain kidney function.

The glomerular filtration rate can be regulated by the body through endocrine responses. In the case of autoregulation, increased blood pressure stretches walls of the afferent arteriole, which responds by contracting - thereby reducing fluctuation of blood pressure in the glomerulus.  A drop in blood pressure brings about a decrease in the glomerular filtration rate, which, in turn, results in a decrease in sodium ions in the filtrate.  (The filtrate moves through the nephron more slowly, allowing more sodium to be reabsorbed along the way.).  This lower sodium level in the filtrate is detected by the macula densa, modified cells of the wall of the distal convoluted tubule that lie adjacent to the afferent and efferent arterioles of the glomerulus.  In response to the low sodium, cells of the juxtaglomerular apparatus (JGA) release renin.  This triggers a series of biochemical reactions which bring about an increase of blood pressure, and thereby an increase in GFR.  This series of reactions includes an increase in angiotensin II, which helps bring blood pressure back up by (1) causing vasoconstriction in arterioles throughout much of the body, and (2) promoting increased synthesis of antidiiuretic hormone (ADH), which increases resorption of water in the collecting ducts of the kidney, thereby increasing blood volume.  Angiotensin II also promotes the release of aldosterone from adrenal cortex, which promotes retention of both sodium and water, thereby helping to bring blood pressure back up. 

The glomerular filtration rate can also be regulated by sympathetic nerve responses, which can result in the constriction or dilation of the afferent arterioles in times of great bodily stress.  Vasoconstriction increases blood pressure, and therefore GFR, whereas vasodilation decreases blood pressure and GFR.


Tubular Reabsorption 

As the glomerular filtrate moves through the nephron, it changes composition dramatically. About 99% of the water in the original filtrate is reabsorbed, and less than 1% of the original NaCl content appears in the final urine. How does this happen? First, the filtrate moves into the proximal tubule, where about 70% of the sodium ions are reabsorbed through active transport. Water and chloride ions follow passively. Glucose and amino acids are reabsorbed here. Approximately 75% of the glomerular filtrate is reabsorbed in the proximal tubule. This is aided by the so-called "brush border" of microvilli cells which function in increasing the surface area for reabsorption.

After moving through the proximal tubule, the filtrate moves on to the descending limb of the Loop of Henle. The cells in this area have no brush border, and there is no active salt transport here. The cells have a low permeability to urea and salt, but are very permeable to water.  As the filtrate descends the Loop of Henle, water diffuses out because of the high salt concentration in the surrounding tissue.  This is part of the urine-concentrating system of the nephron. The Loop of Henle acts as a countercurrent multiplier.  For this reason, mammals living in marine or desert environments have longer loops of Henle and can conserve more water by producing a more concentrated urine.

The filtrate moves along the Loop of Henle to the thin segment of the ascending limb, which is highly permeable to Na+ and  Cl-, and impermeable to water and urea. As the filtrate moves up the thin segment, Na+ and  Cl- diffuse out because there is a higher concentration in the filtrate than in the surrounding tissues.  The filtrate then moves to the medullary thick ascending limb, which is involved in the active transport of Na+ and  Cl- outward from the lumen into the interstitial space. This causes the fluid reaching the distal tubule to be hypoosmotic to the interstitial fluid, and allows for the passive transport of water out of the tubule.

The distal tubule functions in transporting K+, H+, and NH3 into the lumen, and Na+, Cl-, and HCO3- out. Transport of salts in this area is under endocrine control and adjusted according to osmotic conditions.

The filtrate then moves into the collecting duct, which carries the fluid to the renal pelvis, to the ureters, and out of the body through the urethra. The epithelium of the collecting duct is permeable to water, but not salt or urea. The permeability o the collecting duct to water is controlled by the hormone ADH from the posterior pituitary gland. This response causes the filtrate, which is hypoosmotic to the interstitial fluid at this point, to lose water by osmosis and therefore increase the concentration of salts and urea in the urine. At the bottom of the collecting duct, the epithelium is permeable to urea. The diffusion of some urea out of the filtrate and into the surrounding tissue helps produce the interstitial concentration gradient necessary for the diffusion of water out of the descending limb of the Loop of Henle.   The urea also helps draw water out of the filtrate passing down the collecting duct, thereby enabling the kidney to excrete urine that is hypertonic to the general body fluids, a property that is important in water conservation.


Tubular Secretion

In several places along the nephron, substances that are not part of the initial filtrate (because the molecules are too big to be filtered through the glomerulus) are actively transported from the blood into the filtrate for elimination from the body in the urine.  Some substances, such as toxins and drugs, are processed in the liver and conjugated with glucouronic acid.  This marks them for removal from blood capillaries in the kidney and transport into the lumen of the nephron to become part of the filtrate.


How do the kidneys regulate pH?

Regulation of pH is governed by the carbon dioxide/bicarbonate buffering system in the body, which consists of three steps:

CO2 + H2O <==> H2CO3 <==> HCO3- + H+

CO2 + OH- + H+ <==> HCO3- + H+

HOH <==> OH- + H+

The excretion of acid by the kidney is one of the two major factors which influence this system (the other being the excretion of carbon dioxide by the lungs). The excretion of hydrogen ions (acid) in the urine is primarily responsible for maintaining the plasma HCO3- concentration. Mammalian urine is mildly acidic, with a pH of about 6, and contains no bicarbonate. However, the initial glomerular filtrate has a high bicarbonate concentration and a low hydrogen ion concentration. Therefore, in the process of urine formation, acid must be added to the filtrate, and bicarbonate must be removed.   Therefore, the excretion of Hand the recovery of HCO3- are both important mechanisms by which the kidneys help the body regulate pH.

This process is accomplished by special cells in the distal tubule and collecting duct, called A-type cells and B-type cells. The A-type cells are acid-secreting cells that have a proton ATPase in the apical membrane and a Cl-/ HCO3- exchange system in the basolateral membrane. The cells also contain carbonic anhydrase, which hydrates carbon dioxide passing through the membrane to form protons and bicarbonate ions. The protons formed are pumped back into the lumen and can react with the bicarbonate in the filtrate to form carbon dioxide and water, which can diffuse back into the cell, and create an uptake of bicarbonate back into the blood.

B-type cells, on the other hand, are base-secreting cells. They have a different form of chloride/bicarbonate exchanger in the apical membrane than the A-type cells, and secrete bicarbonate into the lumen of the tubule in exchange for chloride ions.

Regulation of pH is accomplished then, by altering the activity of A and B-type cells, which determines whether bicarbonate is reabsorbed or secreted.

Another mechanism used in pH regulation is the uptake of H+ by HPO4- and NH3 in the lumen to trap excess H+ in the filtrate. This occurs in order to bind H+ with something so that these protons will not move back into the epithelial cells and the blood, which would lower pH.