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Lecture #12 – 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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• All organismal systems exist within a water based environmentThe cell solution is water basedInterstitial fluid is water basedBlood and hemolymph are water based

• All metabolic processes produce wasteMetabolic processes that produce nitrogen

typically produce very toxic ammonia

Water and Metabolic Waste

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Critical Thinking

• The cellular metabolism of _____________ will produce nitrogenous waste.

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Critical Thinking

• The cellular metabolism of proteins, nucleic acids, and ATP 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 environmentBody fluids range from 2-3 orders of magnitude

more concentrated than freshwaterBody 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 osmoregulationMore in extreme environmentsLess 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???

• Remember water potential!

Ψ = P - s

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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

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Water Balance in a Marine Environment

• Marine animals that regulate water balance are hypotonic relative to salt waterThey dehydrate and must drink lots of waterMarine bony fish excrete very little urine

• Most marine invertebrates are osmoconformers that are isotonic to seawaterWater balance is in dynamic equilibrium with

surrounding seawater

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Solute Balance in a Marine Environment

• Marine osmoregulatorsGain solutes because of diffusion gradientExcess 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 osmoconformersActively regulate solute concentrations to

maintain homeostasis

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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+

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Solute Balance in a Marine Environment

• Marine osmoregulatorsGain solutes because of diffusion gradientExcess 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 osmoconformersActively 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???

• Solute concentration is much lower in the freshwater environment than in the cytoplasm

• Water is constantly moving by osmosis into the animal

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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 urineMost 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 solutesUrine is dilute, but there are still losses

• Active transport at gills replenishes some solutes

• Additional solutes acquired in food

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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

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Water Balance in a Terrestrial Environment

• Dehydration is a serious threatMost 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???

• 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

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Solute Balance in a Terrestrial Environment

• Solutes are regulated primarily by the excretory systemMore later

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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 animalsFirst modeled in sharks, later found in other animals

• Position of membrane proteins and the direction of transport determines regulatory functionVaries between different groups of animals

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The Pump

• Metabolic energy is used to transport K+ into the cell and Na+ outThis 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???• Two K+ in vs. 3 Na+ out…..

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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

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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 cellsShark rectal glandsMarine bony fish gills

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Additional Ion Channels

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

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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

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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..…ammonia is

NH3…..a base…..protons are abundant…..

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Critical Thinking

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

NH3…..a base…..protons are abundant…..• Ammonia readily acquires a proton to

become ammonium – NH4+

• This reduces proton concentration = raises pH

• Higher pH disrupts enzyme 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

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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 diffusionSponges, 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 environmentFiltrationSelective reabsorption and secretionExcretion

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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 cavityExcretion 1o maintains

water and solute balance• In other flatworms, the

protonephridia excrete nitrogenous waste

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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

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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???

• They are hypertonic to their moist environments and absorb excess water across their skin

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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

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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 rectumAgain, 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 environmentsMost adaptations alter the concentration and

volume of excreted urine

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Critical Thinking

• Which of the world’s environments has produced the most concentrated urine???

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Critical Thinking

• Which of the world’s environments has produced the most concentrated urine???

• Deserts – some desert animals almost never drink waterThey recycle metabolic water, absorb water

from their food, and produce extremely concentrated urine

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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

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Diagram of the human excretory system showing closeup of nephron

The Human Excretory System

• Each kidney is composed of nephronsThese are the functional sub-units of the kidney

• Each nephron is vascularized

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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???

• Each nephron is surrounded by a capillary bed where water and solutes are reabsorbed after filtration

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Diagram of nephron structure

Nephron Structure

• Each nephron starts at a cup-shaped closed endCorpuscleSite of filtration

• Next is the proximal convoluted tubule in the outer region of the kidney (cortex)

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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 ductAll 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

daily1,100 – 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 glomerulusA capillary bed with unusually porous epithelia

• Blood enters AND LEAVES the glomerulus under pressure

• Glomerulus is surrounded by Bowman’s CapsuleThe invaginated but closed end of the nephronThe enclosed space creates pressure

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Diagram of renal corpuscle

Filtration in the Corpuscle

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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

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Diagram of podocytes and porous capillary

Epithelial cells lining Bowman’s Capsule have extensions that make filtration slits – podocytes!

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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 Bowman’s

Capsule, passing then into the tubule

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Filtration in the Corpuscle

• Anything small enough to pass makes up the initial filtrateWaterUreaSolutesGlucoseAmino acidsVitamins…

• Filtration forced by blood pressure• Large volume of filtrate produced (180l/day)

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Diagram showing overview of urine production

Stepwise – From Filtrate to Urine

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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 bloodWater (99%)NaCl, K+

Glucose, amino acidsBicarbonateVitamins…

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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

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Critical Thinking

• What’s driving water transport???

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Critical Thinking

• What’s driving water transport???• The solute gradient produces lower ψ inside

epithelial cells – water follows the solutes

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

Three Regions

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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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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 reabsorbedThe amount depends on body

conditionHormone activity maintains

Na+ homeostasis• Some secretion also occurs

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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 kidneyWater 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 medullaHelps 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