Lecture #11 – Animal Osmoregulation
and Excretion
1
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
2
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
3
Critical Thinking
• The cellular metabolism of _____________
will produce nitrogenous waste.
4
Critical Thinking
• The cellular metabolism of ___________
will produce nitrogenous waste.
5
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
6
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
7
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
8
extreme dehydration
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
9
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
10
Water Balance in a Marine
Environment
• Marine animals that regulate water balance
are hypotonic relative to salt water (less
salty)
• Where does water go???
11
Critical Thinking
• Marine animals that regulate water balance
are hypotonic relative to salt water – 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
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
15
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
16
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
17
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
18
Water Balance in a Freshwater
Environment
• All freshwater animals are regulators and
hypertonic relative to their environment
(more salty)
• Where does water go???
19
Critical Thinking
• All freshwater animals are regulators and
hypertonic relative to freshwater – where
does water go???
20
Critical Thinking
• All freshwater animals are regulators and
hypertonic relative to freshwater – where
does water go???
21
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)
22
Some animals have the ability to go dormant
by extreme dehydration
23
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
25
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
26
Critical Thinking
• Animals that live on land have adaptations
to reduce water loss – such as???
27
Critical Thinking
• Animals that live on land have adaptations
to reduce water loss – such as???
28
Solute Balance in a Terrestrial
Environment
• Solutes are regulated primarily by the
excretory system
More later
29
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???
32
Critical Thinking
• What kind of electrochemical gradient???
33
Critical Thinking
• What kind of electrochemical gradient???
34
The Na+/Cl-/K+ Cotransporter
• A cotransporter protein uses this gradient
to move sodium, chloride and potassium
into the cell
35
The Na+/Cl-/K+ Cotransporter
• Sodium is cycled back out
• Potassium and chloride accumulate inside
the cell
36
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
37
Additional Ion Channels
• In some cases sodium also diffuses
between the epithelial cells
Shark rectal glands
Marine bony fish gills
38
Additional Ion Channels
• In other animals, chloride pumps,
additional cotransporters and aquaporins
are important
Membrane structure reflects function
39
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
40
Critical Thinking
• Why does ammonia raise pH???
• Remember chemistry……
41
Critical Thinking
• Why does ammonia raise pH???
• Remember chemistry..…
42
Critical Thinking
• Why does ammonia raise pH???
• Remember chemistry..…
43
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
44
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
45
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
46
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
47
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
48
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
49
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
50
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
51
Metanephridia in the earthworms
• Vascularized
tubules reabsorb
solutes and
maintain water
balance
• N waste is excreted
in dilute urine
52
Critical Thinking
• Earthworms are terrestrial – why would
they have to get rid of excess water by
producing dilute urine???
53
Critical Thinking
• Earthworms are terrestrial – why would
they have to get rid of excess water by
producing dilute urine???
54
Malphigian tubules in insects and
other terrestrial arthropods
• System of closed
tubules uses ATPpowered 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.
55
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
56
Malphigian tubules in insects and
other terrestrial arthropods
• Solutes and water
are reabsorbed in
the rectum
Again, using ATPpowered pumps
57
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
58
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
59
Critical Thinking
• Which of the world’s environments has
produced the most concentrated urine???
60
Critical Thinking
• Which of the world’s environments has
produced the most concentrated urine???
61
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
63
Critical Thinking
• Each nephron is vascularized…..
• What exactly does that mean???
64
Critical Thinking
• Each nephron is
vascularized…..
• What exactly does that
mean???
65
Nephron Structure
• Each nephron starts at a
cup-shaped closed end
Corpuscle
Site of filtration
Diagram of nephron
structure
• Next is the proximal
convoluted tubule in the
outer region of the
kidney (cortex)
66
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
67
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!!!
68
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
69
Filtration in the Corpuscle
Diagram of renal corpuscle
70
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
71
Epithelial cells lining Bowman’s Capsule have
extensions that make filtration slits – podocytes!
Diagram of podocytes and porous capillary
72
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
73
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)
74
Stepwise – From Filtrate to Urine
Diagram showing overview of urine production
75
The Proximal Tubule
• Secretion – substances are transported
from the blood into the tubule
• Reabsorption – substances are
transported from the filtrate back into the
blood
76
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
77
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
79
Critical Thinking
• What’s driving water transport???
80
Critical Thinking
• What’s driving water transport???
81
The Loop of Henle
• Differences in membrane permeability set
up osmotic gradients that recover water and
salts and concentrate the urine
82
Three Regions
Diagram of Loop of Henle. This diagram is used in the next 3 slides
83
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
84
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
85
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
86
The Distal Tubule
• Filtrate entering the distal
tubule contains mostly urea
and other wastes
• Na+, Cl- and water continue
to be reabsorbed
Diagram of the distal
tubule and collecting
duct. This diagram is
used in the next 2
slides.
The amount depends on body
condition
Hormone activity maintains
Na+ homeostasis
• Some secretion also occurs
87
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
88
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
89
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
90
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
91