Osmoregulation and Excretion


I.         There are two basic solutions to the problem of balancing water gain with water loss.

            A.        The first, available only to marine animals, is to be isoosmotic to the surroundings as an osmoconformer.

            B.        The second is to be an osmoregulator - an animal that must control its internal osmolarity because its body fluids are not isoosmotic with the outside environment.

                        1.         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.

            C.        Whenever animals maintain an osmolarity difference between the body and the external environment, osmoregulation has an energy cost.

                        1.         Because diffusion tends to equalize concentrations in a system, osmoregulators must expend energy to maintain the osmotic gradients via active transport.

                        2.         Osmoregulation accounts for nearly 5% of the resting metabolic rate of many marine and freshwater bony fishes.

            D.        Most animals, whether osmoconformers or osmoregulators, cannot tolerate substantial changes in external osmolarity and are said to be stenohaline.

            E.        In contrast, euryhaline animals—which include both some osmoregulators and osmoconformers—can survive large fluctuations in external osmolarity.

            F.        Most marine invertebrates are osmoconformers. Their osmolarity is the same as seawater.

            G.        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.

                        1.         Marine bony fishes are hypoosmotic to seawater and constantly lose water by osmosis and gain salt by diffusion and from the food they eat.

                        2.         The fishes balance water loss by drinking seawater and actively transporting chloride ions out through their skin and gills. Sodium ions follow passively.

                        3.         They produce very little urine.

            H.        Marine sharks and most other cartilaginous fishes (chondrichthyans) use a different osmoregulatory process.

                        1.         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.

                        2.         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.

                        3.         Consequently, water slowly enters the shark’s body by osmosis and in food, and is removed in urine.

            I.         In contrast to marine organisms, freshwater animals are constantly gaining water by osmosis and losing salts by diffusion.

                        1.         This happens because the osmolarity of their internal fluids is much higher than that of their surroundings.

                        2.         Many freshwater animals 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.

                                    a.         Salmon and other euryhaline fishes that migrate between seawater and freshwater undergo dramatic and rapid changes in osmoregulatory status.

                                    b.         While in the ocean, salmon osmoregulate as other marine fishes do, by drinking seawater and excreting excess salt from the gills.

                                    c.         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 dilute environment—the same as fishes that spend their entire lives in fresh water.

            J.         Dehydration is fatal for 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.

                        1.         Anhydrobiotic animals must have adaptations that keep their cell membranes intact.

                                    a.         Some anhydrobiotic organisms contain large amounts of sugars, especially the disaccharide trehalose.

                                    b.         Trehalose, a dimer of glucose, seems to protect cells by replacing water associated with membranes and proteins.

                                    c.         Many insects that survive freezing in the winter also use trehalose as a membrane protectant.

            K.        The threat of desiccation is perhaps the largest regulatory problem confronting terrestrial plants and animals.

                        1.         Adaptations that reduce water loss are key to survival on land.

                                    a.         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.

                                    b.         Being nocturnal also reduces evaporative water loss.

                        2.         Despite these adaptations, most terrestrial animals lose considerable water from moist surfaces in their respiratory surface, in urine and feces, and across the skin.

                                    a.         Land animals balance their water budgets by drinking and eating moist foods and by using metabolic water from aerobic respiration.

                        3.         Some animals are so well adapted for minimizing water loss that they can survive in deserts without drinking by using the water produced during cellular respiration.

II.        Water balance and waste disposal.

            A.        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.

            B.        Nitrogenous breakdown products of proteins and nucleic acids are among the most important wastes in terms of their effect on osmoregulation.

                        1.         During their breakdown, enzymes remove nitrogen in the form of ammonia, a small and very toxic molecule.

                        2.         Some animals excrete ammonia directly, but many species first convert the ammonia to other compounds that are less toxic but costly to produce.

                        3.         Animals that excrete nitrogenous wastes as ammonia need access to lots of water.

                                    a.         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.

                                    b.         Many invertebrates release ammonia across the whole body surface.

                                    c.         In fishes, most of the ammonia is lost as ammonium ions (NH4+) at the gill epithelium.

                        4.         Ammonia excretion is much less suitable for land animals.

                                    a.         Because ammonia is so toxic, it can be transported and excreted only in large volumes of very dilute solutions.

                                    b.         Most terrestrial animals and many marine organisms (which tend to lose water to their environment by osmosis) do not have access to sufficient water.

                        5.         Instead, mammals, most adult amphibians, sharks, and some marine bony fishes and turtles excrete mainly urea.

                                    a.         Urea is synthesized in the liver by combining ammonia with carbon dioxide and is excreted by the kidneys.

                                    b.         The main advantage of urea is its low toxicity, about 100,000 times less than that of ammonia. This means urea can be transported and stored safely at high concentrations, reducing the amount of water needed for nitrogen excretion.

                                    c.         The main disadvantage of urea is that animals must expend energy to produce it from ammonia.

                        6.         Land snails, insects, birds, and many reptiles excrete uric acid as the main nitrogenous waste.

                                    a.         Like urea, uric acid is relatively nontoxic.

                                    b.         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.

                                    c.         While saving even more water than urea, it is even more energetically expensive to produce.

            C.        Mode of reproduction appears to have been important in choosing among these alternatives.

                        1.         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).

                        2.         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.

                        3.         Uric acid precipitates out of solution and can be stored within the egg as a harmless solid left behind when the animal hatches.

III.       Most excretory systems produce urine by refining a filtrate derived from body fluids. While excretory systems are diverse, nearly all produce urine in a process that involves several steps.

            A.        First, body fluid (blood, coelomic fluid, or hemolymph) is collected.

                        1.         The initial fluid collection usually involves filtration through selectively permeable membranes consisting of a single layer of transport epithelium.

                        2.         Hydrostatic pressure forces water and small solutes into the excretory system. This fluid is called the filtrate. Filtration is largely nonselective.

            B.        It is important to recover small molecules from the filtrate and return them to the body fluids.

                        1.         Excretory systems use active transport to reabsorb valuable solutes in a process of selective reabsorption.

            C.        Nonessential solutes and wastes are left in the filtrate or added to it by selective secretion, which also uses active transport.

            D.        The pumping of various solutes also adjusts the osmotic movement of water into or out of the filtrate.

            E.        The processed filtrate is excreted as urine.

IV.      Diverse excretory systems are variations on a tubular theme.

            A.        Flatworms have an excretory system consisting of a branching network of dead-end tubules. These contain cilia that draw water and solutes from the interstitial fluid into the tubule system. The urine in the tubules exits through openings.

            B.        Each segment of an annelid worm has a pair of tubes collect body fluids from the coelom and release the fluid to the outside.

            C.        Insects and other terrestrial arthropods have organs 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.

            D.        The kidneys of vertebrates usually function in both osmoregulation and excretion and are also built of tubules.

V.        Nephrons and associated blood vessels are the functional units of the mammalian kidney.

            A.        Mammals have a pair of bean-shaped kidneys, each one being supplied with blood by a renal artery and drained by a renal vein.

            B.        Urine exits each kidney through a duct called the ureter, and both ureters drain through a common urinary bladder. 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.

            C.        The mammalian kidney has two distinct regions, an outer renal cortex and an inner renal medulla.

                        1.         Both regions are packed with microscopic excretory tubules, nephrons, and their associated blood vessels.

                        2.         Each nephron consists of a single long tubule and a ball of capillaries, called the glomerulus.

                        3.         The blind end of the tubule forms a cup-shaped swelling, called Bowman’s capsule, that surrounds the glomerulus.

            D.        Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule.

                        1.         The porous capillaries are permeable to water and small solutes but not to blood cells or large molecules such as plasma proteins.

                        2.         The filtrate in Bowman’s capsule contains salt, glucose, amino acids, vitamins, nitrogenous wastes such as urea, and other small molecules.

                        3.         From Bowman’s capsule, the filtrate passes through three regions of the nephron: the proximal tubule; the loop of Henle, a hairpin turn with a descending limb and an ascending limb; and the distal tubule.

                                    a.         The distal tubule empties into a collecting duct, which receives processed filtrate from many nephrons.

                                    b.         The many collecting ducts empty into the renal pelvis, which is drained by the ureter.

            E.        The nephron and the collecting duct are lined by a transport epithelium that processes the filtrate to form the urine.

                        1.         Their most important task is to reabsorb solutes and water.

                        2.         The nephrons and collecting ducts reabsorb nearly all of the sugar, vitamins, and other organic nutrients from the initial filtrate and about 99% of the water.

                        3.         This reduces 180 L of initial filtrate to about 1.5 L of urine to be voided.

            F.        Each nephron is supplied with blood by an afferent arteriole, a branch of the renal artery that subdivides into the capillaries of the glomerulus.

                        1.         The capillaries converge as they leave the glomerulus, forming an efferent arteriole.

                        2.         This vessel subdivides again into the peritubular capillaries, which surround the proximal and distal tubules.

                        3.         Additional capillaries extend downward to form the vasa recta, a loop of capillaries that serves the loop of Henle.

                        4.         The tubules and capillaries are immersed in interstitial fluid, through which substances diffuse.

            G.        Filtrate from Bowman’s capsule flows through the nephron and collecting ducts as it becomes urine.

            H.        Secretion and reabsorption in the proximal tubule substantially alter the volume and composition of filtrate.

                        1.         For example, the cells of the transport epithelium help maintain a constant pH in body fluids by controlled secretions of hydrogen ions or ammonia.

                        2.         The proximal tubules reabsorb about 90% of the important buffer bicarbonate (HCO3-).

                        3.         Drugs and other poisons that have been processed in the liver pass from the peritubular capillaries into the interstitial fluid and then across the epithelium to the nephron’s lumen.

                        4.         Valuable nutrients, including glucose, amino acids, and K+, are actively or passively absorbed from filtrate.

            I.         One of the most important functions of the proximal tubule is reabsorption of most of the NaCl and water from the initial filtrate volume.

                        1.         Salt in the filtrate diffuses into the cells of the transport epithelium.

                        2.         The epithelial cells actively transport Na+ into the interstitial fluid.

                        3.         This transfer of positive charge is balanced by the passive transport of Cl- out of the tubule.

                        4.         As salt moves from the filtrate to the interstitial fluid, water follows by osmosis.

            J.         Reabsorption of water continues as the filtrate moves into the descending limb of the loop of Henle.

                        1.         This transport epithelium is freely permeable to water but not very permeable to salt and other small solutes.

                        2.         For water to move out of the tubule by osmosis, the interstitial fluid bathing the tubule must be hyperosmotic to the filtrate.

                        3.         Because the osmolarity of the interstitial fluid becomes progressively greater from the outer cortex to the inner medulla, the filtrate moving within the descending loop of Henle continues to lose water.

            K.        In contrast to the descending limb, the transport epithelium of the ascending limb of the loop of Henle is permeable to salt, not water.

                        1.         As filtrate ascends the thin segment of the ascending limb, NaCl diffuses out of the permeable tubule into the interstitial fluid, increasing the osmolarity of the medulla.

                        2.         The active transport of salt from the filtrate into the interstitial fluid continues in the thick segment of the ascending limb.

                        3.         By losing salt without giving up water, the filtrate becomes progressively more dilute as it moves up to the cortex in the ascending limb of the loop.

            L.        The distal tubule plays a key role in regulating the K+ and NaCl concentrations in body fluids by varying the amount of K+ that is secreted into the filtrate and the amount of NaCl reabsorbed from the filtrate.

                        1.         Like the proximal tubule, the distal tubule also contributes to pH regulation by controlled secretion of H+ and the reabsorption of bicarbonate (HCO3-).

            M.       By actively reabsorbing NaCl, the transport epithelium of the collecting duct plays a large role in determining how much salt is actually excreted in the urine.

                        1.         Though the degree of its permeability is under hormonal control, the epithelium is permeable to water but not to salt or (in the renal cortex) to urea.

                        2.         As the collecting duct traverses the gradient of osmolarity in the kidney, the filtrate becomes increasingly concentrated as it loses more and more water by osmosis to the hyperosmotic interstitial fluid.

                        3.         In the inner medulla, the duct becomes permeable to urea.

                        4.         Because of the high urea concentration in the filtrate at this point, some urea diffuses out of the duct and into the interstitial fluid.

                        5.         Along with NaCl, this urea contributes to the high osmolarity of the interstitial fluid in the medulla.

                        6.         This high osmolarity enables the mammalian kidney to conserve water by excreting urine that is hyperosmotic to general body fluids.

VI.      The mammalian kidney’s ability to conserve water is a key terrestrial adaptation.

            A.        The osmolarity of human blood is about 300 mosm/L, but the kidney can excrete urine up to four times as concentrated—about 1,200 mosm/L.

                        1.         At an extreme of water conservation, Australian hopping mice, which live in desert regions, can produce urine concentrated to 9,300 mosm/L—25 times as concentrated as their body fluid.

            B.        In a mammalian kidney, the cooperative action and precise arrangement of the loops of Henle and the collecting ducts are largely responsible for the osmotic gradients that concentrate the urine.

            C.        In addition, the maintenance of osmotic differences and the production of hyperosmotic urine are only possible because considerable energy is expended by the active transport of solutes against concentration gradients. The two primary solutes in this osmolarity gradient are NaCl and urea.

            D.        The juxtamedullary nephrons, which maintain an osmotic gradient in the kidney and use that gradient to excrete a hyperosmotic urine, are the key to understanding the physiology of the mammalian kidney as a water-conserving organ.

                        1.         Filtrate passing from Bowman’s capsule to the proximal tubule has an osmolarity of about 300 mosm/L.

                        2.         As the filtrate flows through the proximal tubule in the renal cortex, large amounts of water and salt are reabsorbed.

                        3.         The volume of the filtrate decreases substantially, but its osmolarity remains about the same.

            E.        The ability of the mammalian kidney to convert interstitial fluid at 300 mosm/L to 1,200 mosm/L as urine depends on a countercurrent multiplier between the ascending and descending limbs of the loop of Henle.

                        1.         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.

                        2.         The osmolarity of the filtrate increases as solutes, including NaCl, become more concentrated.

                        3.         The highest molarity occurs at the elbow of the loop of Henle.

                        4.         This maximizes the diffusion of salt out of the tubule as the filtrate rounds the curve and enters the ascending limb, which is permeable to salt but not to water.

                        5.         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.

            F.        The loop of Henle has several qualities of a countercurrent system.

                        1.         Although the two limbs of the loop are not in direct contact, they are close enough to exchange substances through the interstitial fluid.

                        2.         The nephron can concentrate salt 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.

            G.        The vasa recta is also a countercurrent system, with descending and ascending vessels carrying blood in opposite directions through the kidney’s osmolarity gradient.

                        1.         As the descending vessel conveys blood toward the inner medulla, water is lost from the blood and NaCl diffuses into it.

                        2.         These fluxes are reversed as blood flows back toward the cortex in the ascending vessel.

                        3.         Thus, the vasa recta can supply the kidney with nutrients and other important substances without interfering with the osmolarity gradient necessary to excrete a hyperosmotic urine.

            H.        The countercurrent-like characteristics of the loop of Henle and the vasa recta maintain the steep osmotic gradient between the medulla and the cortex.

                        1.         This gradient is initially created by active transport of NaCl out of the thick segment of the ascending limb of the loop of Henle into the interstitial fluid.

                        2.         This active transport and other active transport systems in the kidney consume considerable ATP, requiring the kidney to have one of the highest relative metabolic rates of any organ.

            I.         By the time the filtrate reaches the distal tubule, it is actually hypoosmotic to body fluids because of active transport of NaCl out of the thick segment of the ascending limb.

                        1.         As the filtrate descends again toward the medulla in the collecting duct, water is extracted by osmosis into the hyperosmotic interstitial fluids, but salts cannot diffuse in because the epithelium is impermeable to salt.

                        2.         This concentrates salt, urea, and other solutes in the filtrate.

                        3.         Some urea leaks out of the lower portion of the collecting duct, contributing to the high interstitial osmolarity of the inner medulla.

            J.         Before leaving the kidney, the urine may obtain the osmolarity of the interstitial fluid in the inner medulla, which can be as high as 1,200 mosm/L.

            K.        Although isoosmotic to the inner medulla’s interstitial fluid, the urine is hyperosmotic to blood and interstitial fluid elsewhere in the body.

            L.        This high osmolarity allows the solutes remaining in the urine to be secreted from the body with minimal water loss.

VII.     Regulation of blood osmolarity is maintained by hormonal control of the kidney by negative feedback circuits.

            A.        One hormone important in regulating water balance is antidiuretic hormone (ADH).

                        1.         ADH is produced in the hypothalamus of the brain and stored in and released from the pituitary gland, which lies just below the hypothalamus.

                        2.         Osmoreceptor cells in the hypothalamus monitor the osmolarity of the blood.

                        3.         When blood osmolarity rises above a set point of 300 mosm/L, more ADH is released into the bloodstream and reaches the kidney.

                        4.         ADH induces the epithelium of the distal tubules and collecting ducts to become more permeable to water.

                        5.         This amplifies water reabsorption, reducing urine volume, and helps prevent further increase of blood osmolarity above the set point.

            B.        By negative feedback, the subsiding osmolarity of the blood reduces the activity of osmoreceptor cells in the hypothalamus, and less ADH is secreted.

                        1.         Only a gain of additional water in food and drink can bring osmolarity all the way back down to 300 mosm/L.

                        2.         ADH alone only prevents further movements away from the set point.

            C.        Conversely, if a large intake of water has reduced blood osmolarity below the set point, very little ADH is released.

                        1.         This decreases the permeability of the distal tubules and collecting ducts, so water reabsorption is reduced, resulting in an increased discharge of dilute urine.

                        2.         Alcohol can disturb water balance by inhibiting the release of ADH, causing excessive urinary water loss and dehydration (causing some symptoms of a hangover).

                        3.         Normally, blood osmolarity, ADH release, and water reabsorption in the kidney are all linked in a feedback loop that contributes to homeostasis.

            D.        A second regulatory mechanism involves a special tissue called the juxtaglomerular apparatus (JGA), located near the afferent arteriole that supplies blood to the glomerulus.

                        1.         When blood pressure or blood volume in the afferent arteriole drops, the enzyme renin initiates chemical reactions that convert a plasma protein angiotensinogen to a peptide called angiotensin II.

                        2.         Acting as a hormone, angiotensin II increases blood pressure and blood volume in several ways.

                        3.         It raises blood pressure by constricting arterioles, decreasing blood flow to many capillaries, including those of the kidney.

                        4.         It also stimulates the proximal tubules to reabsorb more NaCl and water.

                        5.         This reduces the amount of salt and water excreted and, consequently, raises blood pressure and volume.

                        6.         It also stimulates the adrenal glands, located atop the kidneys, to release a hormone called aldosterone. This acts on the distal tubules, which reabsorb Na+ and water, increasing blood volume and pressure.

            E.        In summary, the renin-angiotensin-aldosterone system (RAAS) is part of a complex feedback circuit that functions in homeostasis.

                        1.         A drop in blood pressure triggers a release of renin from the JGA.

                        2.         In turn, the rise in blood pressure and volume resulting from the various actions of angiotensin II and aldosterone reduce the release of renin.

                        3.         While both ADH and RAAS increase water reabsorption, they counter different osmoregulatory problems.

                                    a.         The release of ADH is a response to an increase in the osmolarity of the blood, as when the body is dehydrated from excessive loss or inadequate intake of water.

                                    b.         However, a situation that causes excessive loss of salt and body fluids—an injury or severe diarrhea, for example—will reduce blood volume without increasing osmolarity.

                                    c.         The RAAS will detect the fall in blood volume and pressure and respond by increasing water and Na+ reabsorption.

                        4.         Normally, ADH and the RAAS are partners in homeostasis.

                                    a.         ADH alone would lower blood Na+ concentration by stimulating water reabsorption in the kidney.

                                    b.         But the RAAS helps maintain balance by stimulating Na+ reabsorption.

            F.        Still another hormone, atrial natriuretic factor (ANF), opposes the RAAS.

                        1.         The walls of the atria release ANF in response to an increase in blood volume and pressure.

                        2.         ANF inhibits the release of renin from the JGA, inhibits NaCl reabsorption by the collecting ducts, and reduces aldosterone release from the adrenal glands.

                        3.         These actions lower blood pressure and volume.