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Chapter 44

BIOL 1111 Chapter 44: Osmoregulation and Excretion

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BIOL 1111
Michael Balsai

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Chapter 44 Osmoregulation and Excretion Overview: A Balancing Act • The physiological systems of animals operate within a fluid environment.  The relative concentrations of water and solutes must be maintained within narrow limits, despite variations in the animal’s external environment. • Metabolism also poses the problem of disposal of wastes.  The breakdown of proteins and nucleic acids is problematic because ammonia, the primary metabolic waste from breakdown of these molecules, is very toxic. • Aosmoregulation, regulating solute balance and the gain and loss of water and excretion, the removal of nitrogen-containing waste products of metabolism. Concept 44.1 Osmoregulation balances the uptake and loss of water and solutes • All animals face the same central problem of osmoregulation.  Over time, the rates of water uptake and loss must balance.  Animal cells—which lack cell walls—swell and burst if there is a substantial net loss of water., or shrivel and die if there is a • Water enters and leaves cells by osmosis, the movement of water across a selectively permeable membrane.  Osmosis occurs whenever two solutions separated by a membrane solution).osmotic pressure, or osmolarity (moles of solute per liter of  Th(mosm/L). measurement of osmolarity is milliosmoles per liter ▪ 1 mosm/L is equivalent to a total solute concentration of 10−3 M. ▪ Tseawater has an osmolarity of about 1,000 mosm/L. while • If two solutions separated by a selectively permeable membrane have the same osmolarity, they are said to be isoosmotic. • There is no net movement of water by osmosis between isoosmotic solutions, although water molecules do 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. Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Educati44-1Inc.  Water flows by osmosis from a hypoosmotic solution to a hyperosmotic one. Osmoregulators expend energy to control their internal osmolarity; osmoconformers are isoosmotic with their surroundings. • Twater loss.o basic solutions to the problem of balancing water gain with  Onsurroundings as an osmoconformer.s—is to be isoosmotic to the ▪ Aosmolarity, osmoconformers often live in water that has a very stable composition and, hence, they have a very constant internal osmolarity. • Iosmolarity because its body fluids are not isoosmotic with the outsideal environment.  An osmoregulator must discharge excess water if it lives in a hypoosmotic environment or take in water to offset osmotic 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. • Whenever animals maintain an osmolarity difference between the body and the external environment, osmoregulation has an energy cost.  Because diffusion tends to equalize concentrations in a system, osmoregulators must expend energy to maintain the osmotic gradients via active transport.  The energy costs depend mainly on how different an animal’s osmolarity is from its surroundings, how easily water and solutes can move across the animal’s surface, and how much membrane-transport work is required to pump solutes.  Osmoregulation accounts for nearly 5% of the resting metabolic rate of many marine and freshwater bony fishes. • Most animals, whether osmoconformers or osmoregulators, cannot tolerate substantial changes in external osmolarity and are said to be stenohaline.  In contrast, euryhaline animals—which include both some osmoregulators and osmoconformers—can survive large fluctuations in external osmolarity.  For example, various species of salmon migrate back and forth between freshwater and marine environments.  The food fish, tilapia, is an extreme example, capable of adjusting to that of seawater.ation between freshwater and 2,000 mosm/L, twice • Most marine invertebrates are osmoconformers. Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Educatio44-2nc.  Their osmolarity is the same as seawater.  However, they differ considerably from seawater in their concentrations of most specific solutes.  Thus, even an animal that conforms to the osmolarity of its surroundings does regulate its internal composition. • Marine vertebrates and some marine invertebrates are osmoregulators.  For most of these animals, the ocean is a strongly dehydrating environment because it is much saltier than internal fluids, and water is lost from their bodies by osmosis.  Marine bony fishes, such as cod, are hypoosmotic to seawater and constantly lose water by osmosis and gain salt by diffusion and from the food they eat.  The fishes balance water loss by drinking seawater and actively transporting chloride ions out through their skin and gills. ▪ Sodium ions follow passively.  They produce very little urine. • Marine sharks and most other cartilaginous fishes (chondrichthyans) use a different osmoregulatory “strategy.”  Like bony fishes, salts diffuse into the body from seawater, and these salts are removed by the kidneys, a special organ called the rectal gland, or in feces.  Unlike bony fishes, marine sharks do not experience a continuous osmotic loss because high concentrations of urea and trimethylamine oxide (TMAO) in body fluids leads to an osmolarity slightly higher than seawater. ▪ TMAO protects proteins from damage by urea.  Consequently, water slowly enters the shark’s body by osmosis and in food, and is removed in urine. • In contrast to marine organisms, freshwater animals are constantly gaining water by osmosis and losing salts by diffusion.  This happens because the osmolarity of their internal fluids is much higher than that of their surroundings.  However, the body fluids of most freshwater animals have lower solute concentrations than those of marine animals, an adaptation to their low-salinity freshwater habitat.  Many freshwater animals, including fish such as perch, maintain water balance by excreting large amounts of very dilute urine, and regaining lost salts in food and by active uptake of salts from their surroundings. • Salmon and other euryhaline fishes that migrate between seawater and freshwater undergo dramatic and rapid changes in osmoregulatory status.  While in the ocean, salmon osmoregulate as other marine fishes do, by drinking seawater and excreting excess salt from the gills.  When they migrate to fresh water, salmon cease drinking, begin to produce lots of dilute urine, and their gills start taking up salt from the Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Education44-3c. dilute environment—the same as fishes that spend their entire lives in fresh water. • Dehydration dooms most animals, but some aquatic invertebrates living in temporary ponds and films of water around soil particles can lose almost all their body water and survive in a dormant state, called anhydrobiosis, when their habitats dry up.  For example, tardigrades, or water bears, contain about 85% of their weight in water when hydrated but can dehydrate to less than 2% water and survive in an inactive state for a decade until revived by water. • Anhydrobiotic animals must have adaptations that keep their cell membranes intact.  While the mechanism that tardigrades use is still under investigation, researchers do know that anhydrobiotic nematodes contain large amounts of sugars, especially the disaccharide trehalose.  Trehalose, a dimer of glucose, seems to protect cells by replacing water associated with membranes and proteins.  Many insects that survive freezing in the winter also use trehalose as a membrane protectant. • The threat of desiccation is perhaps the largest regulatory problem confronting terrestrial plants and animals.  Humans die if they lose about 12% of their body water.  Camels can withstand twice that level of dehydration. • Adaptations that reduce water loss are key to survival on land.  Most terrestrial animals have body coverings that help prevent dehydration.  These include waxy layers in insect exoskeletons, the shells of land snails, and the multiple layers of dead, keratinized skin cells of most terrestrial vertebrates.  Being nocturnal also reduces evaporative water loss. • Dwater from moist surfaces in their gas exchange organs, in urine and feces, and across the skin.  Land animals balance their water budgets by drinking and eating moist foods and by using metabolic water from aerobic respiration. • Ssurvive in deserts without drinking.minimizing water loss that they can  Fo90% of the loss from metabolic water and gain the remaining 10% inr their diet of seeds.  These and many other desert animals do not drink. Water balance and waste disposal depend on transport epithelia. • The ultimate function of osmoregulation is to maintain the composition of cellular cytoplasm, but most animals do this indirectly by managing the composition of an internal body fluid that bathes the cells. Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Education44-4c.  In animals with an open circulatory system, this fluid is hemolymph.  In vertebrates and other animals with a closed circulatory system, the cells are bathed in an interstitial fluid that is controlled through the composition of the blood.  The maintenance of fluid composition depends on specialized structures ranging from cells that regulate solute movement to complex organs such as the vertebrate 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.  Some transport epithelia directly face the outside environment, while others line channels connected to the outside by an opening on the body surface.  The cells of the epithelium are joined by impermeable tight junctions that form a barrier at the tissue-environment barrier. • In most animals, transport epithelia are arranged into complex tubular networks with extensive surface area.  For example, the salt-secreting glands of some marine birds, such as the albatross, secrete an excretory fluid that is much more salty than the ocean.  The counter-current system in these glands removes salt from the blood, allowing these organisms to drink seawater during their months at sea. • The molecular structure of plasma membranes determines the kinds and directions of solutes that move across the transport epithelium.  For example, the salt-excreting glands of the albatross remove excess sodium chloride from the blood.  By contrast, transport epithelia in the gills of freshwater fishes actively pump salts from the dilute water passing by the gill filaments into the blood.  Transport epithelia in excretory organs often have the dual functions of maintaining water balance and disposing of metabolic wastes. habitat 44.2 An animal’s nitrogenous wastes reflect its phylogeny and • 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. • Nitrogenous breakdown products of proteins and nucleic acids are among the most important wastes in terms of their effect on osmoregulation.  During their breakdown, enzymes remove nitrogen in the form of ammonia, a small and very toxic molecule.  Some animals excrete ammonia directly, but many species first convert the ammonia to other compounds that are less toxic but costly to produce. Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Education44-5c. • Animals that excrete nitrogenous wastes as ammonia need access to lots of water.  This is because ammonia is very soluble but can be tolerated only at very low concentrations.  Therefore, ammonia excretion is most common in aquatic species.  Many invertebrates release ammonia across the whole body surface. +  Inthe gill epithelium. ammonia is lost as ammonium ions (NH ) at 4 + + ▪ Freshwater fishes are able to exchange NH +or N4 from the fluids.ment, which helps maintain Na concentrations in body • Ammonia excretion is much less suitable for land animals.  Because ammonia is so toxic, it can be transported and excreted only in large volumes of very dilute solutions.  Most terrestrial animals and many marine organisms (which tend to lose water to their environment by osmosis) do not have access to sufficient water. • Instead, mammals, most adult amphibians, sharks, and some marine bony fishes and turtles excrete mainly urea.  Urea is synthesized in the liver by combining ammonia with carbon dioxide and is excreted by the kidneys. • The main advantage of urea is its low toxicity, about 100,000 times less than that of ammonia.  Urea can be transported and stored safely at high concentrations.  This reduces the amount of water needed for nitrogen excretion when of ammonia. concentrated solution of urea rather than a dilute solution • The main disadvantage of urea is that animals must expend energy to produce it from ammonia.  In weighing the relative advantages of urea versus ammonia as the form of nitrogenous waste, it makes sense that many amphibians excrete mainly ammonia when they are aquatic tadpoles. ▪ They switch largely to urea when they are land-dwelling adults. • Land snails, insects, birds, and many reptiles excrete uric acid as the main nitrogenous waste.  Like urea, uric acid is relatively nontoxic.  But unlike either ammonia or urea, uric acid is largely insoluble in water and can be excreted as a semisolid paste with very little water loss.  While saving even more water than urea, it is even more energetically expensive to produce. • Uric acid and urea represent different adaptations for excreting nitrogenous wastes with minimal water loss. • Mode of reproduction appears to have been important in choosing among these alternatives. Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Education,44-6.  Soluble wastes can diffuse out of a shell-less amphibian egg (ammonia) or be carried away by the mother’s blood in a mammalian embryo (urea).  However, the shelled eggs of birds and reptiles are not permeable to liquids, which means that soluble nitrogenous wastes trapped within the egg could accumulate to dangerous levels. ▪ Even urea is toxic at very high concentrations.  Uric acid precipitates out of solution and can be stored within the egg as a harmless solid left behind when the animal hatches. • The type of nitrogenous waste also depends on habitat.  For example, terrestrial turtles (which often live in dry areas) excrete mainly uric acid, while aquatic turtles excrete both urea and ammonia.  In some species, individuals can change their nitrogenous wastes when environmental conditions change. ▪ For example, certain tortoises that usually produce urea shift to uric acid when temperature increases and water becomes less available. • Excretion of nitrogenous wastes is a good illustration of how response to the environment occurs on two levels.  Over generations, evolution determines the limits of physiological responses for a species.  During their lives, individual organisms make adjustments within these evolutionary constraints. • The amount of nitrogenous waste produced is coupled to the energy budget and depends on how much and what kind of food an animal eats.  Because they use energy at high rates, endotherms eat more food— and thus produce more nitrogenous wastes—per unit volume than ectotherms.  Carnivores (which derive much of their energy from dietary proteins) excrete more nitrogen than animals that obtain most of their energy from lipids or carbohydrates. Concept 44.3 Diverse excretory systems are variations on a tubular theme • Awater are very different, the solutions all depend on the regulation of solute movements between internal fluids and the external environment.  Much of this is handled by excretory systems, which are central to homeostasis because they dispose of metabolic wastes and control body fluid composition by adjusting the rates of loss of particular solutes. body fluids.ory systems produce urine by refining a filtrate derived from Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Education44-7c. • While excretory systems are diverse, nearly all produce urine in a process that involves several steps.  First, body fluid (blood, coelomic fluid, or hemolymph) is collected. ▪ The initial fluid collection usually involves filtration through selectively permeable membranes consisting of a single layer of transport epithelium. ▪ Hydrostatic pressure forces water and small solutes into the excretory system.  This fluid is called the filtrate.  Filtration is largely nonselective. ▪ It is important to recover small molecules from the filtrate and return them to the body fluids. ▪ Excretory systems use active transport to reabsorb valuable solutes in a process of selective reabsorption. ▪ Nonessential solutes and wastes are left in the filtrate or added to it by selective secretion, which also uses active transport.  The pumping of various solutes also adjusts the osmotic movement of water into or out of the filtrate. ▪ The processed filtrate is excreted as urine. • Flatworms have an excretory system called protonephridia, consisting of a branching 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.  Excreted urine is very dilute in freshwater flatworms.  Apthe body. the tubules reabsorb most solutes before the urine exits  Insystem is osmoregulation, while most metabolic wastes diffuse across the body surface or are excreted into the gastrovascular cavity.  However, in some parasitic flatworms, protonephridia do dispose of nitrogenous wastes.  Protonephridia are also found in rotifers, some annelids, larval molluscs, and lancelets. • Mopenings that collect body fluids from the coelom through a ciliated funnel, the nephrostome, and release the fluid to the outside through the nephridiopore.  Each segment of an annelid worm has a pair of metanephridia. • An earthworm’s metanephridia have both excretory and osmoregulatory functions.  Asthe lumen reabsorbs most solutes and returns them to the blood in the capillaries. Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Education,44-8.  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 organs called 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 into the tubule 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 several key adaptations contributing to the tremendous success of insects on land. • The kidneys of vertebrates usually function in both osmoregulation and excretion.  Like the excretory organs of most animal phyla, kidneys are built of tubules.  The osmoconforming hagfishes, which are not vertebrates but are among the most primitive living chordates, have kidneys with segmentally arranged excretory tubules. ▪ This suggests that the excretory segments of vertebrate ancestors were segmented.  However, the kidneys of most vertebrates are compact, nonsegmented organs containing numerous tubules arranged in a highly organized manner.  The vertebrate excretory system includes a dense network of other structures that carry urine out of the tubules and kidney ands and eventually out of the body. Concept 44.4 Nephrons and associated blood vessels are the functional units of the mammalian kidney • Mammals have a pair of bean-shaped kidneys.  Each kidney is supplied with blood by a renal artery and drained by a renal vein.  In humans, the kidneys account for less than 1% of body weight, but they receive about 20% of resting cardiac output. • Urine exits each kidney through a duct called the ureter, and both ureters drain through a common urinary bladder. Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Educatio44-9nc.  During urination, urine is expelled from the urinary bladder through a tube called the urethra, which empties to the outside near the vagina in females or through the penis in males.  Sphincter muscles near the junction of the urethra and the
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