1. Lecture #11 – 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. Water and Metabolic Waste
Osmoregulation ≠ Excretion
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

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

5. Critical Thinking
The cellular metabolism of ___________ will produce nitrogenous waste.

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

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

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. Specialized chloride cells in the gills actively accumulate chloride, resulting in removal of both Cl- and Na+
Figure showing how chloride cells in fish gills regulate salts
From Gorka, by 13 November 2008

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???

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
The waterbear song by MalWebb

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. 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
Figure showing a comparison between osmoregulation strategies of marine and freshwater fish

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???

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

30. 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
Figure showing the Na/K pump and membrane ion channels. This figure is used in the next 9 slides.

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???

34. Critical Thinking
What kind of electrochemical gradient???

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. 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
Figure showing different forms of nitrogenous waste in different groups of animals

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

42. Critical Thinking
Why does ammonia raise pH???

43. Critical Thinking Why does ammonia raise pH???
Base – a substance that accepts protons

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
Urea is produced in the liver, carried in blood to the kidneys

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 1o maintains water and solute balance
In other flatworms, the protonephridia excrete nitrogenous waste

51. 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
Figure showing annelid metanephridia

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???

55. 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
Figure showing arthropod malphigian tubules. Same or similar figure is used in the next 3 slides.

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???

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

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

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

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

66. 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)
Diagram of nephron structure

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
Diagram showing the interior epithelia of the glomerulus surrounding the capillaries in the corpuscle

70. Filtration in the Corpuscle
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 maintains pressure

71. Filtration in the Corpuscle
Diagram of renal corpuscle

72. 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

73. Epithelial cells lining Bowman’s Capsule have extensions that make filtration slits – podocytes!
Diagram of podocytes and porous capillary
The bulk is the cell body

74. 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

75. 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)

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

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

78. The Proximal Tubule – Secretion
Body pH is partly maintained by secretion of excess H+
Proximal tubule epithelia cells also make and secrete ammonia (NH3) which neutralizes the filtrate pH by bonding to the secreted protons
Drugs and other toxins processed by the liver are secreted into the filtrate

79. 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…

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

81. Critical Thinking
What’s driving water transport???

82. Critical Thinking
What’s driving water transport???

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

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

85. 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

86. 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

87. 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

88. 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
Diagram of the distal tubule and collecting duct. This diagram is used in the next 2 slides.

89. 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

90. 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

91. 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

92. 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

93. Hands On Back to the rat Dissect out the excretory system
Snip away the membranes to reveal the kidneys, ureters, bladder and urethra
If you have demolished your rat, work with a team member