1 Lecture #12 – Animal Osmoregulation and Excretion

2 Key Concepts Water and metabolic waste The osmotic challenges of different environments The sodium/potassium pump and ion channels Nitrogenous waste Osmoregulation and excretion in invertebrates Osmoregulation and excretion in vertebrates

3 All organismal systems exist within a water based environment  The cell solution is water based  Interstitial fluid is water based  Blood and hemolymph are water based All metabolic processes produce waste  Metabolic processes that produce nitrogen typically produce very toxic ammonia Water and Metabolic Waste

4 Critical Thinking The cellular metabolism of _____________ will produce nitrogenous waste.

5 Critical Thinking The cellular metabolism of proteins, nucleic acids, and ATP will produce nitrogenous waste.

6 Water and Metabolic Waste All animals have some mechanism to regulate water balance and solute concentration All animals have some mechanism to excrete nitrogenous waste products Osmoregulation and excretion systems vary by habitat and evolutionary history

7 Animals live in different environments Marine….Freshwater….Terrestrial All animals must balance water uptake vs. water loss and regulate solute concentration within cells and tissues

8 The osmotic challenges of different environments – water balance Water regulation strategies vary by environment  Body fluids range from 2-3 orders of magnitude more concentrated than freshwater  Body fluids are about one order of magnitude less concentrated than seawater for osmoregulators  Body fluids are isotonic to seawater for osmoconformers  Terrestrial animals face the challenge of extreme dehydration

9 The osmotic challenges of different environments – solute balance All animals regulate solute content, regardless of their water regulation strategy Osmoregulation always requires metabolic energy expenditure

10 The osmotic challenges of different environments – solute balance In most environments, ~5% of basal metabolic rate is used for osmoregulation  More in extreme environments  Less for osmoconformers Strategies involve active transport of solutes and adaptations that adjust tissue solute concentrations

11 Water Balance in a Marine Environment Marine animals that regulate water balance are hypotonic relative to salt water (less salty) Where does water go???

12 Critical Thinking Marine animals that regulate water balance are hypotonic relative to salt water – where does water go???

13 Critical Thinking Marine animals that regulate water balance are hypotonic relative to salt water – where does water go??? Remember water potential! Ψ = P - s

14 Critical Thinking Marine animals that regulate water balance are hypotonic relative to salt water – where does water go??? Water will always move from high ψ to low ψ Pressure is not important in this instance (no cell wall) Solute concentration is much higher in the saltwater environment than in the cytoplasm Water is constantly moving out of the animal by osmosis

15 Water Balance in a Marine Environment Marine animals that regulate water balance are hypotonic relative to salt water  They dehydrate and must drink lots of water  Marine bony fish excrete very little urine Most marine invertebrates are osmoconformers that are isotonic to seawater  Water balance is in dynamic equilibrium with surrounding seawater

16 Solute Balance in a Marine Environment Marine osmoregulators  Gain solutes because of diffusion gradient  Excess sodium and chloride transported back to seawater using metabolic energy, a set of linked transport proteins, and a leaky epithelium  Kidneys filter out excess calcium, magnesium and sulfates Marine osmoconformers  Actively regulate solute concentrations to maintain homeostasis

17 Figure showing how chloride cells in fish gills regulate salts Specialized chloride cells in the gills actively accumulate chloride, resulting in removal of both Cl - and Na +

18 Solute Balance in a Marine Environment Marine osmoregulators  Gain solutes because of diffusion gradient  Excess sodium and chloride transported back to seawater using metabolic energy, a set of linked transport proteins, and a leaky epithelium  Kidneys filter out excess calcium, magnesium and sulfates Marine osmoconformers  Actively regulate solute concentrations to maintain homeostasis

19 Water Balance in a Freshwater Environment All freshwater animals are regulators and hypertonic relative to their environment (more salty) Where does water go???

20 Critical Thinking All freshwater animals are regulators and hypertonic relative to freshwater – where does water go???

21 Critical Thinking All freshwater animals are regulators and hypertonic relative to freshwater – where does water go??? Solute concentration is much lower in the freshwater environment than in the cytoplasm Water is constantly moving by osmosis into the animal

22 Water Balance in a Freshwater Environment All freshwater animals are regulators They are constantly taking in water and must excrete large volumes of urine  Most maintain lower cytoplasm solute concentrations than marine regulators – helps reduce the solute gradient and thus limits water uptake Some animals can switch environments and strategies (salmon)

23 Some animals have the ability to go dormant by extreme dehydration

24 Solute Balance in a Freshwater Environment Large volume of urine depletes solutes  Urine is dilute, but there are still losses Active transport at gills replenishes some solutes Additional solutes acquired in food

25 Figure showing a comparison between osmoregulation strategies of marine and freshwater fish Marine osmoregulators dehydrate and drink to maintain water balance; regulate solutes by active transport Freshwater animals gain water, pee alot to maintain water balance; regulate solutes by active transport

26 Water Balance in a Terrestrial Environment Dehydration is a serious threat  Most animals die if they lose more than 10-12% of their body water Animals that live on land have adaptations to reduce water loss

27 Critical Thinking Animals that live on land have adaptations to reduce water loss – such as???

28 Critical Thinking Animals that live on land have adaptations to reduce water loss – such as??? Waxy cuticle on arthropod exoskeletons Mollusk and reptile shells and scales Layers of dead skin cells Fur that develops an insulating boundary layer Eating wet food Retaining metabolic water Small openings from respiratory surfaces to outside environment

29 Solute Balance in a Terrestrial Environment Solutes are regulated primarily by the excretory system  More later

30 Figure showing the Na/K pump and membrane ion channels. This figure is used in the next 9 slides. The sodium/potassium pump and ion channels in transport epithelia ATP powered Na + /Cl - pumps regulate solute concentration in most animals  First modeled in sharks, later found in other animals Position of membrane proteins and the direction of transport determines regulatory function  Varies between different groups of animals

31 The Pump Metabolic energy is used to transport K + into the cell and Na + out  This produces an electrochemical gradient

32 Critical Thinking What kind of electrochemical gradient???

33 Critical Thinking What kind of electrochemical gradient??? Two K + in vs. 3 Na + out…..

34 Critical Thinking What kind of electrochemical gradient??? Two K + in vs. 3 Na + out….. Cell interior becomes more negative in charge and lower in Na + concentration

35 The Na + /Cl - /K + Cotransporter A cotransporter protein uses this gradient to move sodium, chloride and potassium into the cell

36 The Na + /Cl - /K + Cotransporter Sodium is cycled back out Potassium and chloride accumulate inside the cell

37 Selective Ion Channels Ion channels allow passive diffusion of chloride and potassium out of the cell Placement of these channels determines direction of transport – varies by animal

38 Additional Ion Channels In some cases sodium also diffuses between the epithelial cells  Shark rectal glands  Marine bony fish gills

39 Additional Ion Channels In other animals, chloride pumps, additional cotransporters and aquaporins are important  Membrane structure reflects function

40 Figure showing different forms of nitrogenous waste in different groups of animals Nitrogenous Waste Metabolism of proteins and nucleic acids releases nitrogen in the form of ammonia Ammonia is toxic because it raises pH Different groups of animals have evolved different strategies for dealing with ammonia, based on environment

41 Critical Thinking Why does ammonia raise pH??? Remember chemistry……

42 Critical Thinking Why does ammonia raise pH??? Remember chemistry..…ammonia is NH 3 …..a base…..protons are abundant…..

43 Critical Thinking Why does ammonia raise pH??? Remember chemistry..…ammonia is NH 3 …..a base…..protons are abundant….. Ammonia readily acquires a proton to become ammonium – NH 4 + This reduces proton concentration = raises pH Higher pH disrupts enzyme function

44 Nitrogenous Waste Metabolism of proteins and nucleic acids releases nitrogen in the form of ammonia Ammonia is toxic because it raises pH Different groups of animals have evolved different strategies for dealing with ammonia, based on environment

45 Nitrogenous Waste Most aquatic animals excrete ammonia or ammonium directly across the skin or gills Plenty of water available to dilute the toxic effects Freshwater fish also lose ammonia in their very dilute urine

46 Nitrogenous Waste Most terrestrial animals cannot tolerate the water loss inherent in ammonia excretion They use metabolic energy to convert ammonia to urea Urea is 100,000 times less toxic than ammonia and can be safely excreted in urine

47 Nitrogenous Waste Insects, birds, many reptiles and some other land animals use even more metabolic energy to convert ammonia to uric acid Uric acid is excreted as a paste with little water loss Energy expensive

48 Osmoregulation and excretion in invertebrates Earliest inverts still rely on diffusion  Sponges, jellies Most inverts have some variation on a tubular filtration system Three basic processes occur in a tubular system that penetrates into the tissues and opens to the outside environment  Filtration  Selective reabsorption and secretion  Excretion

49 Protonephridia in flatworms, rotifers, and a few other inverts System of tubules is diffusely spread throughout the body Beating cilia at the closed end of the tube draw interstitial fluid into the tubule Solutes are reabsorbed before dilute urine is excreted Figure showing flatworm protonephridia

50 Protonephridia in flatworms, rotifers, and a few other inverts In freshwater flatworms most N waste diffuses across the skin or into the gastrovascular cavity  Excretion 1 o maintains water and solute balance In other flatworms, the protonephridia excrete nitrogenous waste

51 Figure showing annelid metanephridia Metanephridia in the earthworms Tubules collect body fluid through a ciliated opening from one segment and excrete urine from the adjacent segment Hydrostatic pressure facilitates collection

52 Metanephridia in the earthworms Vascularized tubules reabsorb solutes and maintain water balance N waste is excreted in dilute urine

53 Critical Thinking Earthworms are terrestrial – why would they have to get rid of excess water by producing dilute urine???

54 Critical Thinking Earthworms are terrestrial – why would they have to get rid of excess water by producing dilute urine??? They are hypertonic to their moist environments and absorb excess water across their skin

55 Figure showing arthropod malphigian tubules. Same or similar figure is used in the next 3 slides. Malphigian tubules in insects and other terrestrial arthropods System of closed tubules uses ATP- powered pumps to transport solutes from the hemolymph Water follows ψ gradient into the tubules

56 Malphigian tubules in insects and other terrestrial arthropods Nitrogenous wastes and other solutes diffuse into the tubules on their gradients Dilute filtrate passes into the digestive tract

57 Malphigian tubules in insects and other terrestrial arthropods Solutes and water are reabsorbed in the rectum  Again, using ATP- powered pumps

58 Malphigian tubules in insects and other terrestrial arthropods Uric acid is excreted from same opening as digestive wastes Mixed wastes are very dry Effective water conservation has helped this group become so successful on land

59 Osmoregulation and excretion in vertebrates Almost all vertebrates have a system of tubules (nephrons) in a pair of compact organs – the kidneys Each nephron is vascularized Each nephron drains into a series of coalescing ducts that drain urine to the external environment Many adaptations to different environments  Most adaptations alter the concentration and volume of excreted urine

60 Critical Thinking Which of the world’s environments has produced the most concentrated urine???

61 Critical Thinking Which of the world’s environments has produced the most concentrated urine??? Deserts – some desert animals almost never drink water  They recycle metabolic water, absorb water from their food, and produce extremely concentrated urine

62 Diagram of the human excretory system The Human Excretory System Kidneys filter blood and concentrate the urine Ureter drains to bladder Bladder stores Urethra drains urine to the external environment

63 Diagram of the human excretory system showing closeup of nephron The Human Excretory System Each kidney is composed of nephrons  These are the functional sub-units of the kidney Each nephron is vascularized

64 Critical Thinking Each nephron is vascularized….. What exactly does that mean???

65 Critical Thinking Each nephron is vascularized….. What exactly does that mean??? Each nephron is surrounded by a capillary bed where water and solutes are reabsorbed after filtration

66 Diagram of nephron structure Nephron Structure Each nephron starts at a cup-shaped closed end  Corpuscle  Site of filtration Next is the proximal convoluted tubule in the outer region of the kidney (cortex)

67 Nephron Structure The Loop of Henle descends into the inner region of the kidney (medulla) The distal tubule drains into the collecting duct  All these tubules are involved with secretion, reabsorption and the concentration of urine

68 Remember the 2 major steps to urine formation: Filtration and reabsorption/secretion Enormous quantities of blood are filtered daily  1,100 – 2,000 liters of blood filtered daily  ~180 liters of filtrate produced daily Most water and many solutes are reabsorbed; some solutes are secreted  ~1.5 liters of urine produced daily Water conservation!!!

69 Filtration in the Corpuscle Occurs as arterial blood enters the glomerulus  A capillary bed with unusually porous epithelia Blood enters AND LEAVES the glomerulus under pressure Glomerulus is surrounded by Bowman’s Capsule  The invaginated but closed end of the nephron  The enclosed space creates pressure

70 Diagram of renal corpuscle Filtration in the Corpuscle

71 Filtration in the Corpuscle The interior epithelium of Bowman’s Capsule has special cells with finger-like processes that produce slits The slits allow the passage of water, nitrogenous wastes, many solutes Large proteins and red blood cells are too large to be filtered out and remain in the arteriole

72 Diagram of podocytes and porous capillary Epithelial cells lining Bowman’s Capsule have extensions that make filtration slits – podocytes!

73 Materials are filtered through pores in the capillary epithelium, across the basement membrane and through filtration slits into the lumen of Bowman’s Capsule, passing then into the tubule

74 Filtration in the Corpuscle Anything small enough to pass makes up the initial filtrate  Water  Urea  Solutes  Glucose  Amino acids  Vitamins… Filtration forced by blood pressure Large volume of filtrate produced (180l/day)

75 Diagram showing overview of urine production Stepwise – From Filtrate to Urine

76 The Proximal Tubule Secretion – substances are transported from the blood into the tubule Reabsorption – substances are transported from the filtrate back into the blood

77 The Proximal Tubule – Secretion Body pH is partly maintained by secretion of excess H +  Proximal tubule epithelia cells also make and secrete ammonia (NH 3 ) which neutralizes the filtrate pH by bonding to the secreted protons Drugs and other toxins processed by the liver are secreted into the filtrate

78 The Proximal Tubule – Reabsorption Tubule epithelium is very selective Waste products remain in the filtrate Valuable resources are transported back to the blood  Water (99%)  NaCl, K +  Glucose, amino acids  Bicarbonate  Vitamins…

79 Diagram of tubule membrane proteins including Na/K pump The Proximal Tubule – Reabsorption ATP powered Na + /Cl - pump builds gradient Transport molecules speed passage  Note increased surface area facing tubule lumen

80 Critical Thinking What’s driving water transport???

81 Critical Thinking What’s driving water transport??? The solute gradient produces lower ψ inside epithelial cells – water follows the solutes

82 The Loop of Henle Differences in membrane permeability set up osmotic gradients that recover water and salts and concentrate the urine

83 Diagram of Loop of Henle. This diagram is used in the next 3 slides Three Regions

84 The Descending Limb Permeable to water Impermeable to solutes Water is recovered because of the increase in solutes in the surrounding interstitial fluids from the cortex to the inner medulla

85 The Thin Ascending Limb Not permeable to water Very permeable to Na + and Cl - These solutes are recovered through passive transport Solutes help maintain the interstitial fluid gradient

86 The Thick Ascending Limb Na + and Cl - continued to be recovered by active transport High metabolic cost, but helps to maintain the gradient that concentrates urea in the urine

87 Diagram of the distal tubule and collecting duct. This diagram is used in the next 2 slides. The Distal Tubule Filtrate entering the distal tubule contains mostly urea and other wastes Na +, Cl - and water continue to be reabsorbed  The amount depends on body condition  Hormone activity maintains Na + homeostasis Some secretion also occurs

88 The Collecting Duct The final concentration of urine occurs as the filtrate passes down the collecting duct and back through the concentration gradient in the interstitial fluid of the kidney  Water reabsorption is regulated by hormones to maintain homeostatis  Dehydrated individuals produce more concentrated urine

89 The Collecting Duct Some salt is actively transported The far end of the collecting duct is permeable to urea Urea trickles out into the inner medulla  Helps establish and maintain the concentration gradient

90 The Big Picture Blood is effectively filtered to remove nitrogenous waste Filtrate is effectively treated to isolate urea and return the good stuff to the blood Water is conserved – an important adaptation to terrestrial conditions

91 REVIEW – Key Concepts Water and metabolic waste The osmotic challenges of different environments The sodium/potassium pump and ion channels Nitrogenous waste Osmoregulation and excretion in invertebrates Osmoregulation and excretion in vertebrates