Lecture

1. Lecture Osmoregulation and Excretion

2. Osmoregulation • balancing the uptake and loss of water and solutes • based on the controlled movement of solutes between internal fluids and the external environment

3. Lower soluteconcentration Higher soluteconcentration • • Lower free H2Oconcentration Higher free H2Oconcentration • • Osmosis and Osmolarity Selectively permeablemembrane water moves from low solute to high solute concentration • cells require a balance between uptake and loss of water • water enters and leaves a cell through osmosis • movement of water is determined by osmolarity or osmotic pressure • Osmolarity = total solute concentration (in moles/liter) • solute concentration of a solution determines the movement of water across a selectively permeable membrane Solutes Water Hypoosmotic side: Hyperosmotic side: - If two solutions are isoosmotic, the movement of water is equal in both directions - If two solutions differ in osmolarity, the net flow of water is from the hypoosmotic (low solute) to the hyperosmotic (high solute) solution Net water flow

4. Osmotic pressure -Osmosis is controlled by tonicity= degree to which a the concentration of a specific solute surrounding a cell causes water to enter or leave the cell -SO it is the concentration of solutes that causes the water to move -experiment – U shaped tube divided by a membrane permeable to water only -increase the solute concentration in the right half of the tube -this causes water to flow toward the solutes -pressure needed to counteract this rise in water= osmotic pressure -therefore increasing solute concentration increases osmotic pressure -water will move in to decrease this OP -Osmotic pressure is important in determining how much fluid remains in your blood plasma and how much leaves to surround the cells in your tissues

5. Osmotic Challenges to Aquatic Animals • Osmoconformers– marine animals • are isoosmoticwith their surroundings and do not regulate their osmolarity • Osmoregulators– aquatic animals that expend energy to control water uptake and loss • existin a hyperosmotic or hypoosmotic environment • can be freshwater, marine or animals that live in temporary waters • can also be terrestrial animals too!

6. aquatic animals can be classified on how well they tolerate changes in external osmolarity • stenohaline- cannot tolerate substantial changes in external osmolarity • euryhaline- can survive large fluctuations in external osmolarity

7. Marine Animals • all osmoconformers are marine • internal osmolarity of its tissue fluids is the same as its environment • but they can differ significantly in the concentrations of specific solutes • must actively transport these solutes to maintain homeostasis • but not all marine animals are osmoconformers!!! • most marine animals are osmoregulators

8. (a) Osmoregulation in a marine fish Marine Animals Gain of waterand salt ionsfrom food Excretionof salt ionsfrom gills Osmotic waterloss through gillsand other partsof body surface • most marine vertebrates and invertebrates are osmoregulators • marinebony fishes are hypoosmotic to seawater • they constantly lose water by osmosis • they balance water loss by drinking seawater • this seawater brings in salt ions • excess salt is eliminated through their gills and kidneys • gills – chloride cells actively transport chloride ions out and allow sodium ions to follow passively • kidneys – excess Ca, Mg and sulfate ions are excreted with minimal loss of water Key Excretion of salt ions andsmall amounts of water inscanty urine from kidneys Gain of waterand salt ionsfrom drinkingseawater Water Salt

9. Marine sharks • considered osmoconformers • sharks and other chondroichthyes (cartilaginous fishes) use a unique method of osmoregulation • sharks do NOT drink seawater – WHY?? • unlike bony fish – sharks are not hypo-osmotic to seawater • shark tissues contain a high level of urea to maintain osmotic pressure close to that of seawater – they are almost isoosmotic • so no loss of water and no need to drink seawater • to protect their proteins from urea damage - body fluids contain the organic molecule = trimethylamine oxide (TMAO) • like marine bony fish – sharks have an internal salt concentration that is lower than seawater • salt tends to move in - especially across the gills • rectal glands remove excess NaCl via the cloaca (done by chloride cells in marine bony fishes) • but they are not exactly isoosmotic – water will slowly move into the shark over time

10. (b) Osmoregulation in a freshwater fish Freshwater Fish Uptake ofsalt ionsby gills Gain of waterand some ionsin food Osmotic watergain throughgills and otherparts of bodysurface • body fluids of freshwater fish are hyper-osmotic • constantly gain water by osmosis and lose salts by diffusion • these fish do NOT drink water • excrete large amounts of very dilute urine • salts are replenished by eating • some fish – e.g. salmon – travel between salt and freshwater environments • these fish are euryhaline • when in rivers – osmoregulate like other freshwater fish • when in the sea – osmoregulate like marine fishes • increase their production of cortisol which increases production of chloride cells in the gills – increased excretion of salt Key Excretion of salt ions andlarge amounts of water indilute urine from kidneys Water Salt

11. (b) Osmoregulation in a freshwater fish (a) Osmoregulation in a marine fish Gain of waterand salt ionsfrom food Excretionof salt ionsfrom gills Uptake ofsalt ionsby gills Osmotic waterloss through gillsand other partsof body surface Gain of waterand some ionsin food Osmotic watergain throughgills and otherparts of bodysurface Key Key Excretion of salt ions andsmall amounts of water inscanty urine from kidneys Gain of waterand salt ionsfrom drinkingseawater Excretion of salt ions andlarge amounts of water indilute urine from kidneys Water Water Salt Salt

12. (b) Dehydratedtardigrade “Temporary” aquatic fish 50 m • some aquatic animals can survive periods of dehydration or dessication • e.g. live in ponds or films of water • enter a dormant state when their environments dry up = anhydrobiosis • e.g. tardigrades or water bears • requires adaptations that keep their cell membranes intact • e.g. some nematodes – dessicated worms contain large amounts of sugars • trehalose– sugar replaces the water normally associated with proteins and lipids 50 m (a) Hydrated tardigrade

13. Terrestrial animals • threat of dehydration is constant • routes for water loss • urine & feces • across respiratory surfaces • across skin • adaptationsto reduce water loss are key to survival on land • 1.) body coverings of most terrestrial animals help prevent dehydration • 2.) behavioral modifications - desert animals & nocturnal lifestyle • 3.) acquisition of water through eating moist food and producing water metabolically through cellular respiration

14. Water balance ina kangaroo rat(2 mL/day) Water balance ina human(2,500 mL/day) • kangaroo rats lose so little water that they can recover 90% of the loss from metabolic water and gain the remaining 10% in their diet of seeds. • these and many other desert animals do not drink. Ingestedin food (750) Ingestedin food (0.2) Ingestedin liquid(1,500) Watergain(mL) Derived frommetabolism (1.8) Derived frommetabolism (250) Feces (100) Feces (0.09) Waterloss(mL) Urine(1,500) Urine(0.45) Evaporation (900) Evaporation (1.46)

15. Osmoregulation and Energy • Osmoregulators must expend energy to maintain osmotic gradients • the amount of energy differs based on: • 1. how different the animal’s osmolarity is from its surroundings • 2. how easily water and solutes move across the animal’s surface • 3. the work required to pump solutes across the membrane • to minimize this energy cost – most animals have body fluids that are close to the salinity of their environment

16. Transport Epithelia in Osmoregulation • most animals osmoregulate by regulating the solute content of body fluid that bathes their cells • in those animals with an open circulatory system – tissues are bathed in hemolymph • in those animals with a closed system – cells are bathed in interstitial fluid – derived from blood plasma • the composition of these fluids are maintained by osmoregulatory structures • range from single cells to complex tissues/organs

17. Transport Epithelia in Osmoregulation • most animals rely upon the presence of a transport epithelia • found in specific organs • one or more layers of epithelial cells that are specialized for moving solutes in specific directions • they are typically arranged in complex tubular networks

18. 1 2 4 3 Excretory Systems Filtration Capillary • variations on a tubular theme • filtration – collects a filtrate from the blood into the tubule • reabsorption– transport epithelium lining the tubule reclaim needed ions and water • secretion– toxins and excess ions are extracted from body fluids and added to the filtrate • excretion – the altered filtrate (urine) leaves the system • absorption = entrance of new materials into the blood (e.g. via digestive absorption) Filtrate Excretorytubule Reabsorption Secretion Urine Excretion

19. Excretory Systems • animal systems to dispose of metabolic wastes and control body fluid composition • 1. protonephridia • 2. metanephridia • 3. Malphigian tubules • 4. Nasal glands of marine birds • 5. Kidneys

20. Nephridia • protonephridia: • freshwater flatworms • network of dead-end tubules connected to external openings in the worm • branch throughout the worm • tubules end as flame bulbs – contains a tuft of cilia • cilia beating draws in interstitial fluid and processes it • filtrate is released into the connected tubule and is expelled outside as a dilute urine • osmoregulatory function – gets rid of excess water only Nucleusof cap cell Flamebulb Cilia Interstitialfluid flow Tubule Opening inbody wall Tubules ofprotonephridia Tubulecell

21. Nephridia Coelom Capillarynetwork • metanephridia: • annelids • collect fluid directly from the coelom • pair found in each worm segment • fluid drawn in via a ciliated funnel – opening is called the nephrostome • coiled tubule lined with a transport epithelium – reabsorbs most solutes and moves them into the capillary network • remaining filtrate (urine) moves outside via the nephridiopore • osmoregulatory and excretory functions • removal of excess water and nitrogenous wastes Components of a metanephridium: Collecting tubule Internal opening Bladder External opening

22. Digestive tract Malphigian tubules Rectum Hindgut • insects and terrestrial arthropods • osmoregulation and excretion function • dead-end tips immersed in hemolymph • open into the midgut • no filtration of fluids by this system • transport epithelium secretes wastes and certain solutes into the tubules • water follows by osmosis • as fluid flows into the rectum – water & solutes are reclaimed and waste are expelled out the anus • main waste = uric acid Intestine Midgut(stomach) Malpighiantubules Salt, water, andnitrogenouswastes To anus Feces and urine Malpighiantubule Rectum Reabsorption HEMOLYMPH

23. (b) Secretorytubules (a) Location of nasal glandsin a marine bird (c) Countercurrentexchange Nasal glands Secretory cellof transportepithelium Lumen ofsecretorytubule Artery Vein Nasal saltgland • marine birds, iguanas and some marine turtles • allows them to drink seawater – yet their urine has little NaCl in it • nasal glands within the beak produced a concentrated NaCl solution • remove excess sodium chloride from the blood – by counter-current exchange • birds drink seawater – removal of NaCl from the water via these glands Ducts Nasal gland Saltions Capillary Secretory tubule Transportepithelium Nostril with saltsecretions Salt secretion Blood flow Key Salt movement Blood flow Central duct

24. Kidney • Kidneys: formation of urine • contains the functional unit for filtration = Nephron • production of urine, absorption of water and salts • Ureters: transfer of urine from kidneys to bladder • Urethra: transfer of urine from bladder to outside

25. Nephron Nephron Types Juxtamedullary nephron -about one million nephrons -kidneys filter 180 L fluid per day!!!! -each nephron is a renal corpuscle + renal tubules -renal corpuscle: filtering unit consisting of a tangled cluster of capillaries -> glomerulus + glomerular capsule (Bowman’s capsule) -renal tubules: for reabsorption of water and ions leading to final urine volume and composition Cortical nephron • path of filtrate through the nephron: • Bowman’s capsule  Proximal convoluted tubule  Loop of Henle  Distal convoluted tubule • several nephrons dump into a common Collecting Duct Renalcortex Renalmedulla

26. Nephron Organization Reabsorption through the Nephron Afferent arteriolefrom renal artery Glomerulus Bowman’scapsule Proximaltubule • water and salts are reclaimed from the filtrate flowing through the nephron • water and salts are returned to the blood supply • nephron is associated with capillary networks • 1. Peritubular capillary network – “covers” the PCT and DCT • 2. Vasa recta – “covers” the loop of Henle Peritubularcapillaries Distaltubule Efferentarteriolefrom glomerulus Branch ofrenal vein Descendinglimb LoopofHenle Vasarecta Collectingduct Ascendinglimb

27. Nephron Nephron Types Juxtamedullary nephron • renal corpuscle, PCT and DCT are located in the kidney’s cortex • the loop of Henle and Collecting duct are found in the medulla • two types of nephrons depending on where the renal corpuscle is located in the cortex • 1. Cortical – closer to the periphery of the cortex • 2. Juxtamedullary – deeper in the cortex; closer to the medulla Cortical nephron Renalcortex Renalmedulla

28. Nephron Path of filtrate/urine: • 80-85% of all nephrons • shorter loop of Henle • loop is covered with an extensive capillary network (vasa recta) • cortical nephrons make “normal” urine

29. Juxtamedullary Nephron • 15-20% of nephrons are juxtamedullary nephrons • Renal corpuscles close to medulla and long loops of Henle extend into deepest medulla enabling excretion of dilute or concentrated urine • Loop of Henle’s ascending limb divided into thick and thin regions – different functions in making urine

30. Filtration – The Glomerulus • glomerulus:capillary tangle derived from afferent arterioles (into) and lead into efferent arterioles (out) • surrounded by a glomerular capsule (Bowman’s capsule) • glomerular capsule: site of initial filtration and the first step in the formation of urine • consists of visceral and parietal layers • space between the visceral and parietal layers = glomerular/Bowman’s capsule • visceral layer consists of a layer of epithelial cells (podocytes) that cover the capillaries • endothelial cells of the capillary have gaps between them • spaces between the podocytes + spaces between the endothelial cells forms the filtration membrane

31. Transport Epithelium of the Renal Tubule • following filtration at the Bowman’s capsule – the filtrate moves through the rest of the nephron for the reabsorption of salts and water • the transport epithelium of the PCT is a single layer of cuboidal epithelial cells with microvilli • the cells change their shapes and structures in the loop of Henle (ascending limb= squamous; descending limb= cuboidal) and DCT (cuboidal) • each cell type has a unique function in reabsorbing salts and water

32. Two Reabsorption Routes • one of two routes before re-entering the blood • Paracellularreabsorption • 50% of reabsorbed materialmoves between cells bydiffusion • cells are linked by “leaky” tight junctions • Transcellular reabsorption • material moves throughthe cell by active transport

33. Tubular Reabsorption = The Tubules Distal tubule Proximal tubule NaCl Nutrients H2O H2O HCO3 K HCO3 NaCl • the transport epithelium lining the nephron reabsorbs about 99% of the filtered water and many of the solutes and returns them to the blood plasma • principal materials reabsorbed – glucose, amino acids, urea, Na+, K+, Ca+, Cl-, HCO3- and HPO4- • return to the blood through reabsorption into the peritubular capillary network or vasa recta H NH3 H K Filtrate CORTEX Loop ofHenle NaCl H2O OUTERMEDULLA NaCl Collectingduct Key Urea Active transport NaCl H2O Passive transport INNERMEDULLA

34. The Proximal Convoluted Tubule • PCTis the site of water reabsorption (PASSIVE) - associated with the ACTIVE reabsorption of sodium ions • active Na+ uptake from the PCT is by sodium pumps • sodium pumped out of the transport epithelium into the interstitial fluid between the nephron and the capillary • creates a Na+ gradient of higher Na+ in the filtrate and lower Na+ in the PCT cell • causes Na+ to diffuse out of the urine to replace it (carries a glucose with it) • chloride, bicarbonate and phosphate ions follow it - salt reabsorption • the active transport of ions into the blood plasma increases osmotic pressure within the blood • therefore water moves out of the PCT into the capillaries PASSIVELY!

35. Loop of Henle • active transport of Na+ continues through the loop of Henle • descending loop of Henleis quite permeable to water but impermeable to solute movement • salt reabsorption in the ascending limb determines how much water is reabsorbed from the descending limb • known as a counter current multiplier system • ascending loop is the opposite – permeable to salt • Na+ is pumped out of the cell into the blood • causes more Na+ to diffuse into the cell – carries with it Cl- and K+ ions (K+ ions return to the filtrate) • positively charged cations diffuse into the blood (more of a –ve charge in the interstitial fluid)

36. Reabsorption within Loop of Henle • counter current multiplier system • creates a Na gradient within the interstitial fluid around the nephron • determines the movement of water out of the filtrate and eventually into the blood • 1. Na+ pumps in the ascending limb pump Na+ into the interstitial fluid between the two limbs of the loop of Henle • - causes a temporary increase in osmotic pressure within the interstitial fluid • 2. increase in OP causes water to be “sucked out” of filtrate in descending limb into the interstitial fluid • 3. water then moves into the capillaries • 4. NaCl is returned to the ascending limb to start over again

37. DCT and Collecting Duct • DCT and collecting duct are normally impermeable to water !!!! • the DCT and CD become permeable upon action of hormones • two types of cells found in the DCT and CD • principal cells – contain receptors for the hormones aldosterone and ADH • increases the synthesis of: • 1. Na/K pumps – for more NaClreabsorption • 2. aquaporins- form water “pores” in the principal cell – more water reabsorption

38. DCT and Collecting Duct • intercalated cells – play a role in maintaining blood pH • pump H+ ions into the urine • the H+ ions combine with ammonia to form ammonium • urine is slightly acidic and smells like ammonia

39. Reabsorption & Secretion in the Collecting Duct • By end of DCT, 95% of solutes & water have been reabsorbed and returned to the bloodstream • Cells in the collecting duct make the final adjustments • principal cells reabsorb more Na+ and secrete K+ • intercalated cells reabsorb more K+ & bicarbonate ions and secrete H+

40. Nitrogenous Wastes • function of the kidney is two-fold • osmoregulation • excretion of metabolic wastes • metabolic wastes must be dissolved in water • major metabolic wastes are due to breakdown of proteins and nucleic acids • 1. Ammonia – very toxic, requires a lot of water to eliminate • 2. Urea • 3. Uric acid – less toxic, requires little water to eliminate

41. Ammonia • product of amino acid breakdown • can only be tolerated in very low concentrations • to decrease toxicity requires large amounts of water • no problem for aquatic animals • ammonia diffuses easily across cell membranes • in aquatic invertebrates – diffuses across the body wall • fish – ammonia is lost as ammonium across the surface of the gills • problem for terrestrial animals • also a problem for marine animals • since they lose large amounts of water via osmosis

42. Urea • mammals, adult amphibians, sharks and some marine fishes and turtles excrete urea • produced in the liver through the urea cycle • combination of NH3 with CO2 and H2O • requires energy • can be transported safely and stored in tissues without toxicity • urea is often retained in body fluids – contributes to osmotic pressure

43. Uric acid • in humans - formed from the breakdown of nucleic acids • breakdown of purines – A and G • also the main excretory product of insects, terrestrial snails, many reptiles and birds • does not dissolve well in water • excreted as a semi-solid with little water loss • even more energetically expensive to synthesis then urea • guano – droppings of birds • mixture of uric acid and brown feces • excellent source of nitrogen for soils

44. Osmoreceptors inhypothalamus triggerrelease of ADH. Thirst Hypothalamus Drinking reducesblood osmolarityto set point. ADH Pituitarygland Increasedpermeability Distaltubule STIMULUS:Increase in bloodosmolarity (forinstance, aftersweating profusely) H2O reab-sorption helpsprevent furtherosmolarityincrease. Antidiuretic Hormone Collecting duct Homeostasis:Blood osmolarity(300 mOsm/L) • also known as vasopressin • release is controlled by the hypothalamus control over the posterior-pituitary’s release of ADH • Antidiuretic hormone (ADH) makes the collecting duct epithelium more permeable to water • an increase in osmolarity of blood triggers the release of ADH, which helps to conserve water • binding of ADH to receptor molecules leads to a temporary increase in the number of aquaporin proteins in the membrane of collecting duct cells

45. ADH ADHreceptor LUMEN COLLECTINGDUCT CELL • ADH–regulates water reabsorption by increasing the permeability of the principal cells in the collecting duct to water • in the absence of ADH the principal cells of the CT have low permeability to water • ADH stimulates the insertion of aquaporinsinto the apical membrane • water permeability increases • when the OP of the blood plasma increases due to dehydration – osmoreceptors in the hypothalamus detect this drop and stimulate the release of ADH • increased OP of blood plasma also causes increased reabsorption of water from the urine through the aquaporin channels ADH cAMP Second-messengersignaling molecule Storagevesicle Exocytosis Aquaporinwater channel H2O H2O

46. Renin-Angiotensin-Aldosterone • when blood volume and BP drop – kidney secretes renininto the blood • in the blood renin cleaves angiotensinogen (made by liver hepatocytes) to form the active enzyme angiotensin I • the enzyme ACE (in the lung) – cleaves angiotensin I to form angiotensin II • angiotensin II • 1. enhances reabsorption of Na+, Cl+ and water in the PCT • 2. stimulates the release of aldosterone by the adrenal cortex • aldosterone: stimulates the principal cells of the DCT & collecting ducts to reabsorb more Na and Cl- and secrete more K into the blood • osmotic consequence of this causes an increased reabsorption of water • increase in blood volume and pressure results

47. Adaptations by the Vertebrate Kidney: Mammals • The juxtamedullary nephron is key to water conservation in terrestrial animals • juxtamedullary neurons with their long loops of Henle are responsible for the production of concentrated urine by allowing greater retention of water • also allows mammals to get rid of nitrogenous wastes and salts without losing too much water • Na+ gets pumped into the interstitial fluid between the loop and DCT and Collecting Duct • causes water to move out of DCT and Collecting Duct (but ADH must be produced for this to happen) so mammals that inhabit dry environments have long loops of Henle, while those in fresh water have relatively short loops

48. Birds and Other Reptiles • Birds have the same kinds of nephrons as mammals BUT • conserve water by excreting uric acid instead of urea • some reptiles have only cortical nephrons but also excrete nitrogenous waste as uric acid • cortical nephrons produce iso-osmotic or hypo-osmotic urine • the cloaca is used to reabsorb water

49. Freshwater Fishes and Amphibians • Freshwater fishes conserve salt in their DCTs and excrete large volumes of dilute urine • freshwater fish are hyper-osmotic to their environment • must excrete water continuously • kidneys have many nephrons producing filtrate at a high rate • Kidney function in amphibians is similar to freshwater fishes • amphibians conserve water on land by reabsorbing water from the urinary bladder

50. Marine Bony Fishes • Marine bony fishes are hypo-osmotic compared with their environment • lose water • so they gain water and salts from the environment • their kidneys have fewer and smaller nephrons • their nephron have small glomeruli and some lack glomeruli entirely • filtration rates are low, and very little urine is excreted • main function is to get rid of divalent ions like calcium, magnesium and sulfate ions • secrete these into the PCT and excrete them in their urine