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Osmoregulation and Excretion
Transcript of Osmoregulation and Excretion
Organization of the Nephron
Hormonal circuits link kidney function, water balance, and blood pressure
Function of ADH
The Renin-Angiotensin-Aldosterone System
Solute Gradients and Water Concenration
Adaptations of the Vertebrate Kidney to Diverse Environments
by Caitlyn Brashears
Osmoregulation: A Balancing Act
Osmoregulation: the general process by which animals control solute concentrations and balance water gain and loss.
Osmoregulation is a process of homeostasis
For any animal's physiological system to function properly, the relative concentrations of water and solutes must be kept within narrow limits.
Ions, such as sodium, must be maintained at concentrations which allow for the normal activity of body cells.
Energetics of Osmoregulation
Transport Epithelia in Osmoregulation
Excretion: the process that rids the body of nitrogenous metabolites and other waste products
In maintaining the internal fluid environment it is also important to dispose of the hazardous metabolite produced by the dismantling of protiens and nucleic acids: ammonia.
All animals face the same central problem of osmoregulation: over time, the rates of water uptake and loss must balance.
Animal cells lyse if there is a continuous net uptake of water or shrivel if there is a substantial net loss of water.
Water enters and leaves cells by osmosis - the movement of water across a selectively permeable membrane.
osmosis occurs when 2 solutions separated by a membrane differ in osmotic pressure, or osmolarity (moles of solute per liter of solution)
the unit of measurement of osmolarity is milliosmoles per liter (mosm/L)
1 mosm/L is equivalent to a total solute concentration of 10^-3 M.
If two solutions separated by a selectively permeable membrane have the same osmolarity they are isoosmotic, and there is not net movement of water by osmosis for water molecules cross at equal rates in both directions.
When two solutions differ in osmolarity, the one with the greater concentration of solutes is referred to as hyperosmotic, and the more dilute solution is hypoosmotic.
There are two solutions to the problem of balancing water gain and water loss.
One is to be an
which is isoosmotic with its surroundings, and because of this they have no tendency to gain or lose water.
The other is to be an osmoregulator, which expends energy to control its internal osmolarity. The osmolarity of an osmoregulator is not isoosmotic to its surroundings.
Most animals, whether osmoconformers or osmoregulators, cannot tolerate substantial changes in external osmolarity are said to be stenohaline. On the contrary euryhaline animals, which include certain osmoconformers and osmoregulators, can survive large fluctuations in external osmolarity. Examples:
many barnacles and mussels covered and uncovered by ocean tides are euryhaline osmoconformers
various species of salmon which migrate through both saltwater and freshwater are euryhaline osmoregulators.
All osmoconformers are marine animals.
They often live in water that has a very stable composition - hence they have a stable internal osmolarity.
An osmoregulator must discharge excess water if it lives in a hypoosmotic environment or take in water to offset water loss if it inhabits a hyperosmotic environment.
Osmoregulation enables animals to live in environments that are uninhabitable to osmoconformers, such as freshwater and terrestrial habitats.
It also enables many marine animals to maintain internal osmolarities different from that of seawater.
Marine vertebrates and some marine invertebrates are osmoregulators.
For most of these animals the ocean is an extremely dehydrating environment because it is much saltier than their internal fluids, and water is lost from their bodies by osmosis.
Most Marine invertebrates are osmoconformers.
Their osmolarity is the same as the seawater, thus they face no substantial challenges in water balance.
However, because they differ considerably from the seawater in the concentrations of specific solutes, they must actively transport these solutes to maintain homeostasis.
Thus, even an animal that conforms to the osmolarity of its surroundings does regulate its internal composition to some degree.
Marine bony fishes are hypoosmotic to seawater and constantly lose water by osmosis. They balance this water loss by drinking large amounts of seawater and then make use of both their gills and kidneys to rid themselves of salts.
In the gills they actively transport chloride ions out of special chloride cells and sodium ions passively follow
In the kidneys excess ions are excreted with the loss of only small amounts of water.
Marine sharks and most other cartilaginous fishes use a different osmoregulatory strategy.
Sharks have an internal salt concentration much less than that of seawater, so salt diffuses into their bodies (especially across the gills). Salts are removed by the kidneys or the rectal gland.
However, unlike bony fishes, sharks are not hypoosmotic to seawater because their tissue contains high concentrations of urea and trimethylamine oxide (TMAO) - used to protect proteins from damage by urea.
As a result of the high concentrations of urea and TMAO the body fluids of sharks have an osmolarity very close to that of seawater, only slightly greater causing water to enter their bodies. This excess water is disposed of in the urine.
The body fluids of freshwater animals must be hyperosmotic to their environment because animal cells cannot tolerate salt concentrations as low as those of lake or river water. Thus, freshwater animals face the problem of gaining water by osmosis and losing salts by diffusion.
Many fresh water animals solve this problem by drinking almost no water and excreting large amounts of very dilute urine.
Salts lost by diffusion are replaced by eating or by up taking salts across gills - chloride cells in the gills of fish actively transport chloride ions into the body, and sodium ions follow.
Euryhaline fishes that migrate between seawater and freshwater undergo dramatic changes in osmoregulatory status.
while living in the ocean they carry out osmoregulation like other marine fishes by drinking seawater and excreting excess salt from the gills
while in freshwater they cease drinking and begin to produce large amounts of dilute urine.
Animals That Live in Temporary Waters
Extreme dehydration, desiccation, is fatal for most animals, but a few aquatic invertebrates that live in temporary ponds and films of water can lose almost all of their body water and survive.
These animals enter a dormant state when their habitats dry up, and adaptation called anhydrobiosis.
Anhydrobiosis requires adaptations that keep cell membranes intact. Studies on anhydrobiotic roundworms show that desiccated individuals contain large amounts of sugars, particularly trehalose.
Trehalose seems to protect the cells by replacing the water that is normally associated with proteins and membrane lipids.
Above is an example of a anhydrobiotic creature, a tardigade (or water bear).
The threat of dehydration is a major regulatory problem for terrestrial plants and animals. Humans, for example, die if they lose as little as 12% of their body water. Thus, adaptations that reduce water loss are key to survival on land.
The body coverings of most terrestrial animals help prevent dehydration. Examples:
waxy layers of insect exoskeletons
shells of land snails
layers of dead, keratinized skin cells covering most terrestrial vertebrates
Many terrestrial animals, especially desert animals are nocturnal, which reduces evaporative water loss, due to the lower temperature and higher relative humidity of night air.
Despite these and other adaptations, most terrestrial animals lose water through many routes:
through urine and feces
across the skin
from moist surfaces in gas exchange organs
Land animals offset this water loss by drinking and eating moist foods and by producing water metabolically through cellular respiration.
Over 40% of the Marine Iguana's water intake is seawater from the algea that it dives to feed on, which can greatly dehydrate a terrestrial creature. In order to deal with this problem the Marine Iguana has developed a unique adaptation - it 'sneezes salt' out of its nostrils from a salt gland located in its nasal cavity.
When an animal maintains an osmolarity difference between its body and the external environment, there is an energy cost. Because diffusion tends to equalize concentrations in a system, osmoregulators must expend energy to maintain the osmotic gradients that cause water to move in or out.
They do so by using active transport to manipulate solute concentrations in their body fluids.
The energy costs of osmoregulation depends on:
how different an animal's osmolarity is from its surroundings
how easily water and solutes can move across the animal's surface
how much work is required to pump solutes across the membrane
The energy cost to an animal of maintaining water and salt balance is minimized by a body fluid composition adapted to the salinity of the animal's habitat.
The body fluids of most freshwater animals have lower solute concentrations than the body fluids of their marine relatives.
The ultimate function of osmoregulation is to maintain the composition of the cellular contents, but most animals do this indirectly by managing the composition of the internal body fluid that bathes the cells.
in insects and other animals with an open circulatory system this fluid is the hemolymph
in vertebrates and other animals with a closed circulatory system the cells are bathed in an interstitial fluid that contains a mixture of solutes controlled indirectly by the blood
Maintaining the composition of such fluids depends on structures ranging from cells that regulate solute movement to complex organs, such as the human kidney.
In most animals osmotic regulation and metabolic waste disposal depend on the ability of a layer or layers of transport epithelium to move specific solutes in controlled amounts in specific directions.
Transport epithelia are typically arranged into complex tubular networks with extensive surface area.
Some transport epithelia directly face the outside environment, while other line channels connected to the outside by an opening on the body surface.
Transport epithelia that function in maintaining water balance also often funciton in disposal of metabolic wastes.
The Kangaroo Rat is so well adapted to its desert environment that it does not need to drink. It loses so little water that 90% of the loss is replaced by water generated metabolically. The remaining 10% comes form the small amount of water in its diet of seeds.
An animal's nitrogenous wastes relfect its phylogeny and habitat.
Forms of Nitrogenous Wastes
Because most metabolic wastes must be dissolved in water when they are removed from the body, the type and quantity of waste products may have a large impact on water balance.
The nitrogenous breakdown products of proteins and nucleic acids have the most potential to affect water balance.
During their breakdown, enzymes remove nitrogen in the form of ammonia, a very toxic molecule because it interferes with oxidative phosphorylation.
Some animals excrete ammonia directly, but many species first convert the ammonia to other compounds that are less toxic, but more costly to produce.
Ammonia is only tolerable in very low concentrations, thus animals that excrete ammonia need access to a lot of water. As a result, ammonia excretion is most common in aquatic (freshwater) species.
Ammonia is highly soluble and molecules easily pass through the membranes and thus are easily lost through diffusion.
In many invertebrates, ammonia release occurs across the whole body surface.
In fish most of the ammonia is lost across the epithelium of the gills.
Ammonia is so toxic that it can only be transported and excreted in large volumes of very dilute solute. As a result, most terrestrial animals and marine species produce urea from ammonia by combining ammonia with carbon dioxide.
The main advantage of urea is its very low toxicity. It can be transported and stored in high concentrations, so there is less water lost in excretion.
The main disadvantage of urea is its energy cost; animals must expend energy to produce urea from ammonia.
From a bioenergetic standpoint, we would predict that animals that spend that part of their lives in water and part on land would switch between excreting ammonia (saving energy) and excreting urea (saving water). Many amphibians do just that, using ammonia excretion while in their aquatic state and using mostly urea excretion as terrestrial adults.
Insects, land snails, reptiles, and many birds excrete uric acid.
The main advantage is that it is even less toxic than urea and it is largely insoluble so it can be excreted in a semisolid paste with very little water loss.
The main disadvantage is that it is more energetically expensive to produce than urea.
Influence of Evolution and Environment on Nitrogenous Wastes
The type of nitrogenous waste an animal produces is mainly affected by the availability of water. For example:
tortoises which often live in dry areas excrete mainly uric acid
aquatic turtules excrete both urea and ammonia
The mode of reproduction seems to have played an important role in determining the major type of nitrogenous wastes produced during the evolution of a particular group of animals. For example:
Soluble wastes can diffuse out of a shell-less amphibian egg, or be carried away from the embryo by the mother's blood, thus ammonia or urea could be a potential waste product.
However, shelled eggs produced by birds and reptiles are permeable to gases but not to liquids, thus soluble nitrogenous wastes released by an embryo would be trapped within the egg and could accumulate to dangerous levels. As a result , the evolution of uric acid as a waste product conveys a selective advantage because it precipitates out of solution and can be stored within the eggs as a harmless solid left behind after hatching.
Energy Budget and Diet
Regardless of the type of nitrogenous waste, the amount produced by an animal is coupled to the energy budget.
Endotherms which used energy at high rates, eat more fod and produce more nitrogenous wste than ectotherms.
The amount of nitrogenous waste produced is also linked to diet.
Predators, which derive much of their energy from protein, excrete more nitrogen than animals that rely mainly on lipids or carbohydrates as energy sources.
Water balance in all animals depends on the regulation of solute movement between internal fluids and the external environment. The majority of this movement is handled by excretory systems. These systems are central to homeostasis because they disposed of metabolic wastes and control fluid composition.
While excretory systems are diverse, nearly all produce urine in a process that involves similar steps.
Body fluid is brought into contact with the selectively permeable membrane of transport epithelium.
the membrane is not permeable to cells and other large molecules, but water and small solutes such as sugars, salts, amino acids, and nitrogenous wastes cross the membrane forming a solution called filtrate
in most cases hydrostatic pressure (blood pressure in many animals) drives the process of filtration
The process of selective reabsorption recovers useful molecules and water from the filtrate and returns them to the body fluids.
Valuable solutes - certain salts, vitamins, hormones, and amino acids - are reabsorbed by active transport.
Nonessential solutes are left in the filtrate or are added to it by selective secretion.
Selective secretion is carried out through active transport
The pumping of various solutes adjusts the osmotic movement of water into or out of the filtrate.
In the last step, excretion, the processed filtrate is released from the body as urine.
Flatworms have an excretory system called protonephrida consisting of a network of dead-end tubules.
these are capped by a flame bulb with a tuft of cilia that draws water and solutes from the interstitial fluid, through the flame bulb, and into the tubule system.
The urine in the tubules exits through openings called nephridiopores.
the tubules reabsorb most solutes before the urine exits the body.
Protonephridia are also found in other organisms and can serve a slightly different purpose depending on the organism's environment. For example:
In freshwater flatworms the major function of the flame-bulb system is osmoregulation, for most metabolic wastes diffuse across the body surface.
However, in some parasitic flatworms, which are isoosmotic to their environment, protonephridia dispose of nitrogenous wastes.
Most annelids have metanephridia, excretory organs that collect body fluids from the coelom through a ciliated funnel, the nephrostome. As the cilia in the nephrostome beat fluid is drawn into a collecting tubule, which includes a storage bladder which opens to the outside through the nephridopore.
Each segment of an annelid worm has a pair of metanephridia immersed in coelomic fluids and enveloped by a capillary network.
An earthworm's metanephridia have both excretory and osmoregulatory functions.
As urine moves along the tubule, the transport epithelium bordering the lumen reabsorbs most solutes and returns them to the blood in the capillaries.
Nitrogenous wastes remain in the tubule and are dumped outside.
Because earthworms experience a net uptake of water from damp soil, their metanephridia balance water influx by producing dilute urine.
Insects and other terrestrial arthropods have Malpighian tubules that remove nitrogenous wastes and also function in osmoregulation.
These open into the digestive system and dead-end at tips that are immersed in the hemolymph.
The transport epithelium lining the tubules secretes certain solutes, including nitrogenous wastes, from the hemolymph into the lumen of the tubule.
Water follows the solutes by osmosis, and the fluid then passes back to the rectum, where most of the solutes are pumped back into the hemolymph
Water again follows the solutes, and the nitrogenous wastes, primarily insoluble uric acid, are eliminated along with the feces.
This system is highly effective in conserving water and is one of the several key adaptations contributing to the success of insects on land.
In vertebrates and some other chordates, a specialized organ called the kidney functions in both osmoregulation and excretion.
Like the excretory organs of most animals the kidneys consist of tubules.
The numerous tubules are arranged in a highly organized manner and are all closely associated with a network of capillaries.
The vertebrate excretory system also includes ducts and other structures that carry urine from the tubules out of the kidney and out of the body.
The Structure of the Mammalian Excretory System
Blood Vessels Associated with Nephrons
Filtration of the Blood
Pathway of the Filtrate
The excretory system of mammals centers on a pair of kidneys.
In humans each kidney is supplied with blood by the renal artery and drained by the renal vein. Blood flow through the kidneys is voluminous, the kidneys receive roughly 25% of the blood exiting the heart.
Urine exits each kidney through a duct called the ureter.
Both ureters drain into the urinary bladder.
During urination urine is expelled from the bladder through a tube called the urethra.
The mammalian kidney has an outer renal cortex and an inner renal medulla. Microscopic excretory tubules and their associated blood vessels pack both regions.
Weaving back and forth across the cortex and medulla is the nephron, the functional unit of the kidney.
A nephron consists of a single long tubule as well as a ball of capillaries called a glomerulus. One end of the tubule forms a cup-shaped swelling called Bowman's capsule, which surrounds the glomerulus.
Each human kidney contains about a million nephrons.
Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of the Bowman's capsule.
The porous capillaries and cells of the are permeable to water and small solutes, but not to blood cells or large molecules.
Thus the filtrate in the Bowman's capsule contains salts, glucose, amino acids, vitamins, nitrogenous wastes, and other small molecules.
1. The filtrate passes into the proximal tubule.
2. Next it goes through the loop of Henle
a hairpin turn with a descending limb and an ascending limb
3. It then enters the distal tube, the last region of the nephron, and empties into a collecting duct which receives processed filtrate from many nephrons.
4. The filtrate then flows from all of the collecting ducts of the kidney into the renal pelvis which is drained by the ureter.
The nephron and collecting duct are lined by transport epithelium that process the filtrate, forming urine.
One of the epithelium's most important tasks is the reabsorption of solutes and water.
The kidneys normally process about 1,600L of blood a day (that is 300 times the amount contained in the body) and only about 1.5L of the 1,600L is expelled as urine.
Among vertebrates only mammals and birds have loops of Henle.
In the human kidney 85% of the nephrons are cortical nephrons and the other 15% are juxtamedullary nephrons.
have short loops of Henle
are almost entirely confined to the renal cortex
have loops that extend deeply into the renal medulla
enable mammals to produce urine that is hyperosmotic to body fluids, an adaptation which is extremely important in the conservation of water
Each nephron is supplied with blood by an afferent arteriole, and offshoot of the renal artery that branches from the capillaries of the glomerulus.
The capillaries converge as they leave the glomerulus, forming an efferent arteriole.
Branches of the efferent arteriole form the peritubular capillaries, which surround the proximal and distal tubules.
Another set of capillaries extend downward and from the vasa recta, hairpin-shaped capillaries that serve the long loop of Henle of juxtamedullary nephrons.
Each ascending portion of the vasa recta lies next to the descending portion of the loop of Henle, and vice versa.
Both the tubules and capillaries are immersed in interstitial fluid, through which various substances diffuse between the plasma within the capillaries and the filtrate within the nephron tube.
Though they do not exchange materials directly, the vasa recta and the loop of Henle function together as a part of a countercurrent system that enhances nephron efficiency.
I know the picture is blurry, sorry. If you have a book look on page 963.
The picture is a bit blurry so if you have a book turn to page 961.
Ascending limb of the loop of Henle
A Closer Look at the Nephron
Descending limb of the loop of Henle
Reabsorption of water continues as the filtrate moves into the descending limb of the loop of Henle.
Numerous water channels formed by aquaporin proteins make the transport epithelium freely permeable to water.
There are little to no channels for salt and other small solutes, thus there is a very low permeability for these solutes.
For water to move out of the tubule by osmosis, the interstitial fluid bathing the tubule must be hyperosmotic to the filtrate.
This condition is met along the entire arm of the descending loop because the osmolarity of the interstitial fluid increases progressively from the outer cortex to the inner medulla of the kidney.
As a result, the filtrate undergoes a loss of water, and thus an increase of solute concentration as it descends the limb.
Reabsorption in the proximal tubule is critical for the recapture of ions, water, and valuable nutrients from the huge initial filtrate volume.
Salts, glucose, potassium ions, and other essential substances are actively or passively transported from the filtrate to the interstitial fluids and then into the peritubular capillaries.
Water often follows these solutes through osmosis.
As the filtrate passes through the proximal tubule the materials to be excreted become more concentrated.
Some toxic materials are actively secreted into the filtrate form surrounding tissues.
The process of filtration in the proximal tubule helps maintain a constant pH in the body fluids.
The transport epithelium secrete H+ but also synthesize and secrete ammonia, which acts like a buffer to trap H+ in the form of ammonium ions.
The proximal tubules also reabsorb about 90% of the buffer bicarbonate from the filtrate, thus further maintaining pH balance.
When the filtrate reaches the tip of the loop it then travels up the ascending limb back into the cortex.
The ascending limb contains ion channels, but not water channels - it is impermeable to water.
The ascending limb has two regions a thin segment near the loop tip and a thick segment near the distal tube.
Thin segment: in this section the NaCl in the filtrate diffuses out of the permeable tubule into the interstitial fluid, thus maintaining the osmolarity of the interstitial fluid in the medulla.
Thick segment: the movement of NaCl into the interstitial fluid through active transport as opposed to diffusion.
As a result of losing salt, not water, through the ascending limb the filtrate becomes even more concentrated.
The distal tubule plays a key role in regulating the K+ and NaCl concentration of body fluids.
This regulation involves variation in the amount of K+ that is secreted into the filtrate and the amount of NaCl that is reabsorbed.
The distal tubule also contributes to pH regulation by the controlled secretion of H+ and reabsorption of bicarbonate.
The collecting duct carries the filtrate through the medulla to the renal pelvis.
As the filtrate passes along the transport epithelium of the collecting duct, hormonal control of permeability and transport determines the concentration of the urine.
When the kidneys are conserving water:
The epithelium remains impermeable to salt and, in the renal cortex, to urea.
As the filtrate passes from the cortex to the medulla it becomes increasingly concentrated as it loses water by osmosis the hyperosmotic interstitial fluid.
In the inner medulla the duct becomes permeable to urea, to maintain the hyperosmotic nature of the interstitial fluid in the medulla.
The net result is urine that is hyperosmotic to the general body fluids.
When the kidneys are producing dilute urine:
The epithelium lacks water channels and NaCl is actively transported out of the filtrate, thus it reabsorbs salts without allowing water to follow by osmosis, thus making the urine less concentrated.
look on page 965
It is imperative for water conservation that the kidney produces urine hyperosmotic to general body fluids. The production of hyperosmotic urine is only possible because considerable energy is expended for the active transport of solutes against concentration gradients.
Thus the nephrons - primarily the loop of Henle - can be viewed as energy consuming machines which establish an osmolarity gradient suitable for extracting water from filtrate.
The two primary solutes affecting osmolarity are NaCl and urea.
The ability of the kidney to concentrate hyperosmotic urine depends on a countercurrent multiplier system between the ascending and descending limbs of the loop of Henle
countercurrent multiplier system: expend energy to create concentration gradients
As the filtrate flows from the cortex to the medulla in the descending limb of the loop of Henle, water leaves the tubule by osmosis.
The osmolarity of the filtrate increases as solutes, including NaCl, become more concentrated.
The highest osmolarity occurs at the elbow of the loop of Henle.
This maximizes the diffusion of salt out of the tubule as the filtrate rounds the curve and enters the ascending limb.
The descending limb produces progressively saltier filtrate, and the ascending limb exploits this concentration of NaCl to help maintain a high osmolarity in the interstitial fluid of the renal medulla.
The loop of Henle
The loop of Henle has several qualities of a countercurrent system.
Though the 2 limbs of the loop are not in direct contact, they are close enough to exchange substances through interstitial fluid
The nephron can concentrate slat in the inner medulla largely because exchange between opposing flows in the descending and ascending limbs overcomes the tendency for diffusion to even out salt concentrations throughout the kidney's interstitial fluid
The Vasa Recta + loop of Henle
The vasa recta is also a countercurrent system, with descending and ascending vessels carrying blood in opposite directions through the kidney's osmolarity gradient.
As the descending vessel conveys blood toward the inner medulla, water is lost from the blood and NaCl diffuses into it.
The fluxes are reversed as blood flows back toward the cortex in the ascending vessel.
Thus, the vasa recta can supply the kidney with nutrients and other important substances without interfering with the osmolarity gradient necessary to excrete hyperosmotic urine.
Variations in nephron structure and function equip the kidneys of different vertebrates for osmoregulation in their various habitats.
The juxtamedullary nephron is a key adaptation to terrestrial life, enabling mammals to get rid of salts and nitrogenous wastes without squandering water.
Mammals that excrete the most hyperosmotic urine have loops of Henle that extend deep into the medulla. Long loops maintain steep osmotic gradients in the kidney, resulting in urine becoming very concentrated.
Desert animals such as this Australian Hopping Mouse tend to have loops of Henle that extend very deeply into the medulla. The Australian Hopping mouse, as a result, can produce urine that is 25 times more concentrated than its general body fluids
Terrestrial mammals living in moist conditions have loops of Henle of intermediate length and the capacity to produce urine intermediate in concentration to that produced by freshwater and desert mammals.
Animals with access to plenty of water, such as this beaver, have loops of Henle which do not penetrate deeply into the medulla and thus have comparitively less concentrated urine.
Birds and Reptiles
Most birds live in an environment that is dehydrating, and like mammals they have kidneys with juxtamedullary loops of Henle. However, their loops of Henle extend less far into the medulla than those of mammals. Thus their main water conservation adaptation is having uric acid as the nitrogen waste molecule.
The kidneys of reptiles have only cortical nephrons and produce urine that is isoosmotic or hypoosmotic to body fluids. However, the epithelium of the chamber called the cloaca helps conserve fluid by reabsorbing some of the water present in urine and in feces. Reptiles also excrete uric acid to conserve water.
Freshwater Fishes and Amphibians
Freshwater fishes are hyperosmotic to their surroundings, so they must excrete excess water continuously, thus they produce very dilute urine. Their kidneys produce filtrate at a high rate. They conserve salts by reabsorbing ions from the filtrate in their distal tubules, leaving water behind.
When in fresh water, the kidneys of frogs excrete dilute urine while the skin accumulates certain salts from the water. On land, frogs conserve body fluid by reabsorbing water across the epithelium of the urinary bladder.
Marine Bony Fish
The tissues of marine bony fishes gain excess salts from their surroundings and lose water. To cope with these conditions, marine fishes have fewer and smaller nephrons, and their nephrons lack a distal tubule. Also, their kidneys have small glomeruli or lack glomeruli entirely. In keeping with these features, filtration rates are low and very little urine is excreted.
The main function of the kidneys in marine bony fishes is to get rid of divalent ions, which they take in by incessantly drinking sea water. They rid themselves of these ions by secreting them into the proximal tubules of the nephrons and excreting them in urine.
In mammals, both the volume and osmolarity of urine are adjusted according to an animal's water and salt balance and its rate of urea production.
A combination of nervous and hormonal controls manages the osmoregulatory function of the kidney. Antidiuret hormone is a key hormone in this regulatory circuitry.
ADH is produced in the hypothalamus of the brain and stored in the posterior pituitary gland.
Osmoreceptor cells in the hypothalamus monitor the osmolarity of blood and regulate release of ADH from the posterior pituitary.
1. In response to an increase in osmolarity above 300mOsm/L, more ADH is released into the bloodstream.
2. When ADH reaches the kidney its main targets are the distal tubules and collecting ducts
ADH brings about changes that make the epithelium more permeable to water.
3. The resulting increase in water reabsorption:
reduces urine volume
lowers blood osmolarity back toward the set point of 300mOsm/L
4. As the osmolarity of the blood subsides, a negative feedback mechanism reduces the activity of osmoreceptor cells in the hypothalamus, and ADH secretion is reduced.
An intake of a large volume of water leads to a decrease in ADH secretion to a very low level.
The resulting decrease in permeability of the distal tubules and collecting ducts reduces water reabsorption resulting in discharge of large volumes of urine.
ADH influences water uptake in the kidney by regulating the water-selective channels formed by aquaporins.
The binding of ADH to receptor molecules leads to a temporary increase in the number of aquaporin molecules in the membranes of collecting duct cells.
Additional channels recapture more water, reducing urine volume.
ADH Receptor Mutation
Mutations that prevent ADH production or that inactivate the ADH receptor gene block the increase in channel number and thus the ADH response.
This disorder, called diabetes insipidus, can cause severe dehydration and solute imbalance due to the production of urine that is abnormally large in volume and very dilute.
Alcohol and ADH
Many factors can inhibit ADH secretion. For example, alcohol can disturb water balance by inhibiting ADH release, leading to excessive urinary water loss and dehydration.
This may cause some of the symptoms of a hangover.
The second regulatory mechanism that helps to maintain homeostasis is the renin-angiotensin-aldosterone system (RAAS). The RAAS involves a tissue called the juxtaglomerular apparatus (JGA), located near the afferent arteriole.
When blood pressure or blood volume in the afferent arteriole drops, the JGA releases the enzyme renin.
Renin initiates chemical reactions that cleave a plasma protein called angiotensinogen, creating angiotensin II.
Functioning as a hormone, angiotensin II raises blood pressure by constricting arterioles, which decreases blood flow to many capillaries, including those to the kidney.
Angiotensin stimulates the adrenal glands to release the hormone aldosterone, which acts on the nephrons' distal tubules, making them reabsorb more sodium and water and increasing blood volume and pressure.
Homeostatic regulation of the Kidney
The RAAS operates as a part of a complex feedback circuit that results in homeostasis:
A drop in blood pressure and blood volume triggers renin release from the JGA.
The rise in blood pressure and volume resulting from the various actions of angiotensin II and aldosterone reduces the release of renin.
RAAS vs. ADH
release in response to an increase in blood osmolarity - when body is dehydrated
lowers blood Na+ concentration
stimulates water reabsorption in kidney
responds to excessive loss of both salt and body fluids (which changes blood volume without increasing osmolarity)- a major wound or severe diarrhea
stimulates Na+ reabsorption
increases blood pressure
Another hormone, atrial natriuretic peptide (ANP) opposes the RAAS. The walls of the atria of the heart release ANP in response to an increase in blood volume and pressure.
ANP inhibits the release of renin form the JGA
inhibits NaCl reabsorption
reduces aldosterone release from the adrenal glands
these actions lower blood volume and pressure.
Thank you for your attention and for more information read chapter 44.