Lecture

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Lecture 11 Animal Osmoregulation and Excretion
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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

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Water and Metabolic Waste

• 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

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Critical Thinking

• The cellular metabolism of _____________ will
  produce nitrogenous waste.

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Critical Thinking

• The cellular metabolism of ___________ will
  produce nitrogenous waste.

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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

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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
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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

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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

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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

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Water Balance in a Marine Environment

• Marine animals that regulate water balance are
  hypotonic relative to salt water (less salty)
• Where does water go???

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Critical Thinking

• Marine animals that regulate water balance are
  hypotonic relative to salt water where does
  water go???

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Critical Thinking

• Marine animals that regulate water balance are
  hypotonic relative to salt water where does
  water go???

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Critical Thinking

• Marine animals that regulate water balance are
  hypotonic relative to salt water where does
  water go???

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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

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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

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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
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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

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Water Balance in a Freshwater Environment

• All freshwater animals are regulators and
  hypertonic relative to their environment (more
  salty)
• Where does water go???

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Critical Thinking

• All freshwater animals are regulators and
  hypertonic relative to freshwater where does
  water go???

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Critical Thinking

• All freshwater animals are regulators and
  hypertonic relative to freshwater where does
  water go???

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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)

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Some animals have the ability to go dormant by
extreme dehydration
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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

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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
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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

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Critical Thinking

• Animals that live on land have adaptations to
  reduce water loss such as???

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Critical Thinking

• Animals that live on land have adaptations to
  reduce water loss such as???

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Solute Balance in a Terrestrial Environment

• Solutes are regulated primarily by the excretory
  system
• More later

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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.
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The Pump

• Metabolic energy is used to transport K into the
  cell and Na out
• This produces an electrochemical gradient

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Critical Thinking

• What kind of electrochemical gradient???

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Critical Thinking

• What kind of electrochemical gradient???

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Critical Thinking

• What kind of electrochemical gradient???

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The Na/Cl-/K Cotransporter

• A cotransporter protein uses this gradient to
  move sodium, chloride and potassium into the cell

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The Na/Cl-/K Cotransporter

• Sodium is cycled back out
• Potassium and chloride accumulate inside the cell

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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

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Additional Ion Channels

• In some cases sodium also diffuses between the
  epithelial cells
• Shark rectal glands
• Marine bony fish gills

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Additional Ion Channels

• In other animals, chloride pumps, additional
  cotransporters and aquaporins are important
• Membrane structure reflects function

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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
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Critical Thinking

• Why does ammonia raise pH???
• Remember chemistry

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Critical Thinking

• Why does ammonia raise pH???
• Remember chemistry..

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Critical Thinking

• Why does ammonia raise pH???
• Remember chemistry..

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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

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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

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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

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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

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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

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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
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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

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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
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Metanephridia in the earthworms

• Vascularized tubules reabsorb solutes and
  maintain water balance
• N waste is excreted in dilute urine

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Critical Thinking

• Earthworms are terrestrial why would they have
  to get rid of excess water by producing dilute
  urine???

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Critical Thinking

• Earthworms are terrestrial why would they have
  to get rid of excess water by producing dilute
  urine???

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Malphigian tubules in insects and other
terrestrial arthropods

• System of closed tubules uses ATP-powered pumps
  to transport solutes from the hemolymph
• Water follows ? gradient into the tubules

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

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Malphigian tubules in insects and other
terrestrial arthropods

• Solutes and water are reabsorbed in the rectum
• Again, using ATP-powered pumps

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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

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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

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Critical Thinking

• Which of the worlds environments has produced
  the most concentrated urine???

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Critical Thinking

• Which of the worlds environments has produced
  the most concentrated urine???

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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
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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
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Critical Thinking

• Each nephron is vascularized..
• What exactly does that mean???

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Critical Thinking

• Each nephron is vascularized..
• What exactly does that mean???

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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
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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

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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!!!

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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 Bowmans Capsule
• The invaginated but closed end of the nephron
• The enclosed space creates pressure

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Filtration in the Corpuscle
Diagram of renal corpuscle
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Filtration in the Corpuscle

• The interior epithelium of Bowmans 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

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Epithelial cells lining Bowmans Capsule have
extensions that make filtration slits
podocytes!
Diagram of podocytes and porous capillary
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Materials are filtered through pores in the
capillary epithelium, across the basement
membrane and through filtration slits into the
lumen of Bowmans Capsule, passing then into the
tubule
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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)

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Stepwise From Filtrate to Urine
Diagram showing overview of urine production
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The Proximal Tubule

• Secretion substances are transported from the
  blood into the tubule
• Reabsorption substances are transported from
  the filtrate back into the blood

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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

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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

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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
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Critical Thinking

• Whats driving water transport???

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Critical Thinking

• Whats driving water transport???

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The Loop of Henle

• Differences in membrane permeability set up
  osmotic gradients that recover water and salts
  and concentrate the urine

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

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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

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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

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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.
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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

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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

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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

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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