Osmoregulation and Excretion

1
Chapter 44
Osmoregulation and Excretion
2
Overview A Balancing Act

• Physiological systems of animals operate in a
  fluid environment
• Relative concentrations of water and solutes must
  be maintained within fairly narrow limits
• Osmoregulation regulates solute concentrations
  and balances the gain and loss of water

3

• Freshwater animals show adaptations that reduce
  water uptake and conserve solutes
• Desert and marine animals face desiccating
  environments that can quickly deplete body water
• Excretion gets rid of nitrogenous metabolites and
  other waste products

4
Figure 44.1
5
Concept 44.1 Osmoregulation balances the uptake
and loss of water and solutes

• Osmoregulation is based largely on controlled
  movement of solutes between internal fluids and
  the external environment

6
Osmosis and Osmolarity

• Cells require a balance between uptake and loss
  of water
• Osmolarity, the solute concentration of a
  solution, determines the movement of water across
  a selectively permeable membrane
• If two solutions are isoosmotic, the movement of
  water is equal in both directions
• If two solutions differ in osmolarity, the net
  flow of water is from the hypoosmotic to the
  hyperosmotic solution

7
Figure 44.2
Selectively permeablemembrane
Solutes
Water
Hyperosmotic side
Hypoosmotic side
Net water flow
8
Osmotic Challenges

• Osmoconformers, consisting only of some marine
  animals, are isoosmotic with their surroundings
  and do not regulate their osmolarity
• Osmoregulators expend energy to control water
  uptake and loss in a hyperosmotic or hypoosmotic
  environment

9

• Most animals are stenohaline they cannot
  tolerate substantial changes in external
  osmolarity
• Euryhaline animals can survive large fluctuations
  in external osmolarity

10
Marine Animals

• Most marine invertebrates are osmoconformers
• Most marine vertebrates and some invertebrates
  are osmoregulators
• Marine bony fishes are hypoosmotic to seawater
• They lose water by osmosis and gain salt by
  diffusion and from food
• They balance water loss by drinking seawater and
  excreting salts

11
Figure 44.3
(b) Osmoregulation in a freshwater fish
(a) Osmoregulation in a marine fish
Gain of waterand salt ionsfrom food
Excretionof salt ionsfrom gills
Osmotic waterloss through gillsand other
partsof body surface
Gain of waterand some ionsin food
Uptake ofsalt ionsby gills
Osmotic watergain throughgills and otherparts
of bodysurface
Key
Gain of waterand salt ionsfrom drinkingseawater
Excretion of salt ions andsmall amounts of water
inscanty urine from kidneys
Excretion of salt ions andlarge amounts of water
indilute urine from kidneys
Water
Salt
12
Figure 44.3a
(a) Osmoregulation in a marine fish
Gain of waterand salt ionsfrom food
Excretionof salt ionsfrom gills
Osmotic waterloss through gillsand other
partsof body surface
Key
Gain of waterand salt ionsfrom drinkingseawater
Excretion of salt ions andsmall amounts of water
inscanty urine from kidneys
Water
Salt
13
Freshwater Animals

• Freshwater animals constantly take in water by
  osmosis from their hypoosmotic environment
• They lose salts by diffusion and maintain water
  balance by excreting large amounts of dilute
  urine
• Salts lost by diffusion are replaced in foods and
  by uptake across the gills

14
Figure 44.3b
(b) Osmoregulation in a freshwater fish
Uptake ofsalt ionsby gills
Gain of waterand some ionsin food
Osmotic watergain throughgills and otherparts
of bodysurface
Key
Excretion of salt ions andlarge amounts of water
indilute urine from kidneys
Water
Salt
15
Figure 44.4
16
Animals That Live in Temporary Waters

• Some aquatic invertebrates in temporary ponds
  lose almost all their body water and survive in a
  dormant state
• This adaptation is called anhydrobiosis

17
Figure 44.5
50 ?m
50 ?m
(a) Hydrated tardigrade
18
Figure 44.5a
50 ?m
(a) Hydrated tardigrade
19
Figure 44.5b
50 ?m
20
Land Animals

• Adaptations to reduce water loss are key to
  survival on land
• Body coverings of most terrestrial animals help
  prevent dehydration
• Desert animals get major water savings from
  simple anatomical features and behaviors such as
  a nocturnal lifestyle
• Land animals maintain water balance by eating
  moist food and producing water metabolically
  through cellular respiration

21
Figure 44.6
Water balance ina kangaroo rat(2 mL/day)
Water balance ina human(2,500 mL/day)
Ingestedin food (750)
Ingestedin food (0.2)
Ingestedin liquid(1,500)
Watergain(mL)
Derived frommetabolism (1.8)
Derived frommetabolism (250)
Feces (100)
Feces (0.09)
Waterloss(mL)
Urine(0.45)
Urine(1,500)
Evaporation (900)
Evaporation (1.46)
22
Energetics of Osmoregulation

• Osmoregulators must expend energy to maintain
  osmotic gradients
• The amount of energy differs based on
• How different the animals osmolarity is from its
  surroundings
• How easily water and solutes move across the
  animals surface
• The work required to pump solutes across the
  membrane

23
Transport Epithelia in Osmoregulation

• Animals regulate the solute content of body fluid
  that bathes their cells
• Transport epithelia are epithelial cells that are
  specialized for moving solutes in specific
  directions
• They are typically arranged in complex tubular
  networks
• An example is in nasal glands of marine birds,
  which remove excess sodium chloride from the blood

24
Figure 44.7
Secretory cellof transportepithelium
Lumen ofsecretorytubule
Vein
Artery
Nasal saltgland
Ducts
Nasal gland
Saltions
Capillary
Secretory tubule
Transportepithelium
Nostril with saltsecretions
Salt secretion
Blood flow
Key
Salt movement
Blood flow
Central duct
25
Figure 44.7a
Nasal saltgland
Ducts
Nasal gland
Nostril with saltsecretions
(a)
Location of nasal glandsin a marine bird
26
Figure 44.7b
Vein
Artery
Nasal gland
Capillary
Secretory tubule
Transportepithelium
Key
Salt movement
Blood flow
Central duct
(b) Secretory tubules
27
Figure 44.7c
Secretory cellof transportepithelium
Lumen ofsecretorytubule
Saltions
Blood flow
Salt secretion
(c) Countercurrent exchange
28
Concept 44.2 An animals nitrogenous wastes
reflect its phylogeny and habitat

• The type and quantity of an animals waste
  products may greatly affect its water balance
• Among the most significant wastes are nitrogenous
  breakdown products of proteins and nucleic acids
• Some animals convert toxic ammonia (NH3) to less
  toxic compounds prior to excretion

29
Figure 44.8
Proteins
Nucleic acids
Aminoacids
Nitrogenousbases
NH2Amino groups
Mammals, mostamphibians, sharks,some bony fishes
Most aquaticanimals, includingmost bony fishes
Many reptiles(including birds),insects, land
snails
Ammonia
Urea
Uric acid
30
Figure 44.8a
Most aquaticanimals, includingmost bony fishes
Mammals, mostamphibians, sharks,some bony fishes
Many reptiles(including birds),insects, land
snails
Urea
Uric acid
Ammonia
31
Forms of Nitrogenous Wastes

• Animals excrete nitrogenous wastes in different
  forms ammonia, urea, or uric acid
• These differ in toxicity and the energy costs of
  producing them

32
Ammonia

• Animals that excrete nitrogenous wastes as
  ammonia need access to lots of water
• They release ammonia across the whole body
  surface or through gills

33
Urea

• The liver of mammals and most adult amphibians
  converts ammonia to the less toxic urea
• The circulatory system carries urea to the
  kidneys, where it is excreted
• Conversion of ammonia to urea is energetically
  expensive excretion of urea requires less water
  than ammonia

34
Uric Acid

• Insects, land snails, and many reptiles,
  including birds, mainly excrete uric acid
• Uric acid is relatively nontoxic and does not
  dissolve readily in water
• It can be secreted as a paste with little water
  loss
• Uric acid is more energetically expensive to
  produce than urea

35
Figure 44.9
36
The Influence of Evolution and Environment on
Nitrogenous Wastes

• The kinds of nitrogenous wastes excreted depend
  on an animals evolutionary history and habitat,
  especially water availability
• Another factor is the immediate environment of
  the animal egg
• The amount of nitrogenous waste is coupled to the
  animals energy budget

37
Concept 44.3 Diverse excretory systems are
variations on a tubular theme

• Excretory systems regulate solute movement
  between internal fluids and the external
  environment

38
Excretory Processes

• Most excretory systems produce urine by refining
  a filtrate derived from body fluids
• Key functions of most excretory systems
• Filtration Filtering of body fluids
• Reabsorption Reclaiming valuable solutes
• Secretion Adding nonessential solutes and wastes
  from the body fluids to the filtrate
• Excretion Processed filtrate containing
  nitrogenous wastes, released from the body

39
Figure 44.10
Filtration
Capillary
Filtrate
Excretorytubule
Reabsorption
Secretion
Urine
Excretion
40
Survey of Excretory Systems

• Systems that perform basic excretory functions
  vary widely among animal groups
• They usually involve a complex network of tubules

41
Protonephridia

• A protonephridium is a network of dead-end
  tubules connected to external openings
• The smallest branches of the network are capped
  by a cellular unit called a flame bulb
• These tubules excrete a dilute fluid and function
  in osmoregulation

42
Figure 44.11
Nucleusof cap cell
Flamebulb
Cilia
Interstitialfluid flow
Tubule
Opening inbody wall
Tubules ofprotonephridia
Tubulecell
43
Metanephridia

• Each segment of an earthworm has a pair of
  open-ended metanephridia
• Metanephridia consist of tubules that collect
  coelomic fluid and produce dilute urine for
  excretion

44
Figure 44.12
Coelom
Capillarynetwork
Components of a metanephridium
Collecting tubule
Internal opening
Bladder
External opening
45
Malpighian Tubules

• In insects and other terrestrial arthropods,
  Malpighian tubules remove nitrogenous wastes from
  hemolymph and function in osmoregulation
• Insects produce a relatively dry waste matter,
  mainly uric acid, an important adaptation to
  terrestrial life
• Some terrestrial insects can also take up water
  from the air

46
Figure 44.13
Digestive tract
Rectum
Hindgut
Intestine
Midgut(stomach)
Malpighiantubules
Salt, water, andnitrogenouswastes
Feces and urine
To anus
Malpighiantubule
Rectum
Reabsorption
HEMOLYMPH
47
Kidneys

• Kidneys, the excretory organs of vertebrates,
  function in both excretion and osmoregulation

48
Figure 44.14-a
Excretory Organs
Kidney Structure
Nephron Types
Juxtamedullary nephron
Cortical nephron
Renalcortex
Posteriorvena cava
Renalmedulla
Renal artery
Renalarteryand vein
Kidney
Renal vein
Renalcortex
Aorta
Ureter
Ureter
Renalmedulla
Urinarybladder
Urethra
Renal pelvis
49
Figure 44.14-b
Nephron Organization
Afferent arteriolefrom renal artery
Glomerulus
Bowmanscapsule
Proximaltubule
Peritubularcapillaries
Distaltubule
Efferentarteriolefrom glomerulus
Branch ofrenal vein
Descendinglimb
LoopofHenle
Vasarecta
Collectingduct
200 ?m
Ascendinglimb
Blood vessels from a humankidney. Arterioles and
peritubularcapillaries appear pink
glomeruliappear yellow.
50
Figure 44.14a
Excretory Organs
Posteriorvena cava
Renalarteryand vein
Kidney
Aorta
Ureter
Urinarybladder
Urethra
51
Figure 44.14b
Kidney Structure
Renalcortex
Renalmedulla
Renal artery
Renal vein
Ureter
Renal pelvis
52
Figure 44.14c
Nephron Types
Juxtamedullary nephron
Cortical nephron
Renalcortex
Renalmedulla
53
Figure 44.14d
Nephron Organization
Afferent arteriolefrom renal artery
Glomerulus
Bowmans capsule
Proximaltubule
Peritubularcapillaries
Distaltubule
Efferentarteriolefrom glomerulus
Branch ofrenal vein
Descendinglimb
LoopofHenle
Vasarecta
Collectingduct
Ascendinglimb
54
Figure 44.14e
200 ?m
Blood vessels from a human kidney. Arterioles
and peritubular capillaries appear pink
glomeruli appear yellow.
55
Concept 44.4 The nephron is organized for
stepwise processing of blood filtrate

• The filtrate produced in Bowmans capsule
  contains salts, glucose, amino acids, vitamins,
  nitrogenous wastes, and other small molecules

56
From Blood Filtrate to Urine A Closer Look

• Proximal Tubule
• Reabsorption of ions, water, and nutrients takes
  place in the proximal tubule
• Molecules are transported actively and passively
  from the filtrate into the interstitial fluid and
  then capillaries
• Some toxic materials are actively secreted into
  the filtrate
• As the filtrate passes through the proximal
  tubule, materials to be excreted become
  concentrated

Animation Bowmans Capsule and Proximal Tubule
57
Figure 44.15
Proximal tubule
Distal tubule
Nutrients
NaCl
H2O
HCO3?
H2O
K?
HCO3?
NaCl
NH3
H?
H?
K?
Filtrate
CORTEX
Loop ofHenle
NaCl
H2O
OUTERMEDULLA
NaCl
Collectingduct
Key
Urea
Active transport
NaCl
H2O
Passive transport
INNERMEDULLA
58

• Descending Limb of the Loop of Henle
• Reabsorption of water continues through channels
  formed by aquaporin proteins
• Movement is driven by the high osmolarity of the
  interstitial fluid, which is hyperosmotic to the
  filtrate
• The filtrate becomes increasingly concentrated

59

• Ascending Limb of the Loop of Henle
• In the ascending limb of the loop of Henle, salt
  but not water is able to diffuse from the tubule
  into the interstitial fluid
• The filtrate becomes increasingly dilute

60

• Distal Tubule
• The distal tubule regulates the K and NaCl
  concentrations of body fluids
• The controlled movement of ions contributes to pH
  regulation

Animation Loop of Henle and Distal Tubule
61

• Collecting Duct
• The collecting duct carries filtrate through the
  medulla to the renal pelvis
• One of the most important tasks is reabsorption
  of solutes and water
• Urine is hyperosmotic to body fluids

Animation Collecting Duct
62
Solute Gradients and Water Conservation

• The mammalian kidneys ability to conserve water
  is a key terrestrial adaptation
• Hyperosmotic urine can be produced only because
  considerable energy is expended to transport
  solutes against concentration gradients
• The two primary solutes affecting osmolarity are
  NaCl and urea

63
The Two-Solute Model

• In the proximal tubule, filtrate volume
  decreases, but its osmolarity remains the same
• The countercurrent multiplier system involving
  the loop of Henle maintains a high salt
  concentration in the kidney
• This system allows the vasa recta to supply the
  kidney with nutrients, without interfering with
  the osmolarity gradient
• Considerable energy is expended to maintain the
  osmotic gradient between the medulla and cortex

64

• The collecting duct conducts filtrate through the
  osmolarity gradient, and more water exits the
  filtrate by osmosis
• Urea diffuses out of the collecting duct as it
  traverses the inner medulla
• Urea and NaCl form the osmotic gradient that
  enables the kidney to produce urine that is
  hyperosmotic to the blood

65
Figure 44.16-1
Osmolarityof interstitialfluid(mOsm/L)
300
300
300
300
H2O
CORTEX
400
400
H2O
H2O
H2O
OUTERMEDULLA
600
600
H2O
H2O
900
900
Key
H2O
INNERMEDULLA
Activetransport
1,200
1,200
Passivetransport
66
Figure 44.16-2
Osmolarityof interstitialfluid(mOsm/L)
300
300
100
300
100
300
NaCl
H2O
CORTEX
400
200
400
H2O
NaCl
H2O
NaCl
H2O
NaCl
OUTERMEDULLA
600
600
400
NaCl
H2O
H2O
NaCl
900
700
900
Key
H2O
NaCl
INNERMEDULLA
Activetransport
1,200
1,200
Passivetransport
67
Figure 44.16-3
Osmolarityof interstitialfluid(mOsm/L)
300
300
100
300
100
300
300
H2O
NaCl
H2O
CORTEX
400
200
400
400
H2O
NaCl
H2O
NaCl
H2O
H2O
NaCl
NaCl
H2O
NaCl
H2O
OUTERMEDULLA
600
600
600
400
NaCl
H2O
H2O
Urea
H2O
H2O
NaCl
900
700
900
Urea
Key
H2O
NaCl
H2O
INNERMEDULLA
Urea
Activetransport
1,200
1,200
1,200
Passivetransport
68
Adaptations of the Vertebrate Kidney to Diverse
Environments

• The form and function of nephrons in various
  vertebrate classes are related to requirements
  for osmoregulation in the animals habitat

69
Mammals

• The juxtamedullary nephron is key to water
  conservation in terrestrial animals
• Mammals that inhabit dry environments have long
  loops of Henle, while those in fresh water have
  relatively short loops

70
Birds and Other Reptiles

• Birds have shorter loops of Henle but conserve
  water by excreting uric acid instead of urea
• Other reptiles have only cortical nephrons but
  also excrete nitrogenous waste as uric acid

71
Figure 44.17
72
Freshwater Fishes and Amphibians

• Freshwater fishes conserve salt in their distal
  tubules and excrete large volumes of dilute urine
• Kidney function in amphibians is similar to
  freshwater fishes
• Amphibians conserve water on land by reabsorbing
  water from the urinary bladder

73
Marine Bony Fishes

• Marine bony fishes are hypoosmotic compared with
  their environment
• Their kidneys have small glomeruli and some lack
  glomeruli entirely
• Filtration rates are low, and very little urine
  is excreted

74
Concept 44.5 Hormonal circuits link kidney
function, water balance, and blood pressure

• Mammals control the volume and osmolarity of
  urine
• The kidneys of the South American vampire bat can
  produce either very dilute or very concentrated
  urine
• This allows the bats to reduce their body weight
  rapidly or digest large amounts of protein while
  conserving water

75
Figure 44.18
76
Antidiuretic Hormone

• The osmolarity of the urine is regulated by
  nervous and hormonal control
• Antidiuretic hormone (ADH) makes the collecting
  duct epithelium more permeable to water
• An increase in osmolarity triggers the release of
  ADH, which helps to conserve water

Animation Effect of ADH
77
Figure 44.19-1
Osmoreceptors inhypothalamus triggerrelease of
ADH.
Thirst
Hypothalamus
ADH
Pituitarygland
STIMULUSIncrease in bloodosmolarity
(forinstance, aftersweating profusely)
HomeostasisBlood osmolarity(300 mOsm/L)
78
Figure 44.19-2
Osmoreceptors inhypothalamus triggerrelease of
ADH.
Thirst
Hypothalamus
Drinking reducesblood osmolarityto set point.
ADH
Pituitarygland
Increasedpermeability
Distaltubule
STIMULUSIncrease in bloodosmolarity
(forinstance, aftersweating profusely)
H2O reab-sorption helpsprevent
furtherosmolarityincrease.
Collecting duct
HomeostasisBlood osmolarity(300 mOsm/L)
79

• Binding of ADH to receptor molecules leads to a
  temporary increase in the number of aquaporin
  proteins in the membrane of collecting duct cells

80
Figure 44.20
ADHreceptor
LUMEN
Collectingduct
COLLECTINGDUCT CELL
ADH
cAMP
Second-messengersignaling molecule
Storagevesicle
Exocytosis
Aquaporinwater channel
H2O
H2O
81

• Mutation in ADH production causes severe
  dehydration and results in diabetes insipidus
• Alcohol is a diuretic as it inhibits the release
  of ADH

82
Figure 44.21
EXPERIMENT
Aquaporingene
Prepare copies of humanaquaporin genes
twomutants plus wild type.
Promoter
Mutant 2
Wild type
Mutant 1
Synthesize mRNA.
H2O(control)
Inject mRNA into frogoocytes.
Transfer to 10-mOsmsolution and observeresults.
Aquaporinproteins
RESULTS
Injected RNA
Permeability (?m/sec)
196
Wild-type aquaporin
20
None
17
Aquaporin mutant 1
18
Aquaporin mutant 2
83
Figure 44.21a
EXPERIMENT
Aquaporingene
Prepare copies of humanaquaporin genes
twomutants plus wild type.
Promoter
Mutant 1
Mutant 2
Wild type
Synthesize mRNA.
H2O(control)
Inject mRNA into frogoocytes.
Transfer to 10-mOsmsolution and observeresults.
Aquaporinproteins
84
Figure 44.21b
RESULTS
Injected RNA
Permeability (?m/sec)
196
Wild-type aquaporin
20
None
17
Aquaporin mutant 1
Aquaporin mutant 2
18
85
The Renin-Angiotensin-Aldosterone System

• The renin-angiotensin-aldosterone system (RAAS)
  is part of a complex feedback circuit that
  functions in homeostasis
• A drop in blood pressure near the glomerulus
  causes the juxtaglomerular apparatus (JGA) to
  release the enzyme renin
• Renin triggers the formation of the peptide
  angiotensin II

86

• Angiotensin II
• Raises blood pressure and decreases blood flow to
  the kidneys
• Stimulates the release of the hormone
  aldosterone, which increases blood volume and
  pressure

87
Figure 44.22-1
JGAreleasesrenin.
Distaltubule
Renin
Juxtaglomerularapparatus (JGA)
STIMULUSLow blood volumeor blood pressure(for
example, dueto dehydration orblood loss)
HomeostasisBlood pressure,volume
88
Figure 44.22-2
Angiotensinogen
Liver
JGAreleasesrenin.
Distaltubule
Renin
Angiotensin I
ACE
Angiotensin II
Juxtaglomerularapparatus (JGA)
STIMULUSLow blood volumeor blood pressure(for
example, dueto dehydration orblood loss)
HomeostasisBlood pressure,volume
89
Figure 44.22-3
Angiotensinogen
Liver
JGAreleasesrenin.
Distaltubule
Renin
Angiotensin I
ACE
Angiotensin II
Juxtaglomerularapparatus (JGA)
STIMULUSLow blood volumeor blood pressure(for
example, dueto dehydration orblood loss)
Adrenal gland
Arteriolesconstrict,increasingblood pressure.
Aldosterone
More Na? and H2Oare reabsorbed indistal
tubules,increasing blood volume.
HomeostasisBlood pressure,volume
90
Homeostatic Regulation of the Kidney

• ADH and RAAS both increase water reabsorption,
  but only RAAS will respond to a decrease in blood
  volume
• Another hormone, atrial natriuretic peptide
  (ANP), opposes the RAAS
• ANP is released in response to an increase in
  blood volume and pressure and inhibits the
  release of renin

91
Figure 44.UN01
Animal
Inflow/Outflow
Urine
Large volume of urine
Freshwaterfish. Lives inwater
lessconcentratedthan body fluids fishtends
to gainwater, lose salt
Does not drink water
Salt in(active trans-port by gills)
H2O in
Urine is lessconcentratedthan bodyfluids
Salt out
Marine bony fish. Lives inwater
moreconcentratedthan bodyfluids fishtends to
losewater, gain salt
Drinks water
Small volumeof urine
Salt in
H2O out
Urine isslightly lessconcentratedthan
bodyfluids
Salt out (activetransport by gills)
Terrestrialvertebrate.Terrestrialenvironmentt
ends to losebody waterto air
Moderatevolumeof urine
Drinks water
Salt in(by mouth)
Urine ismore concentratedthan bodyfluids
H2O andsalt out
92
Figure 44.UN01a
Animal
Inflow/Outflow
Urine
Large volume of urine
Does not drink water
Freshwaterfish. Lives inwater
lessconcentratedthan body fluids fishtends
to gainwater, lose salt
H2O in
Salt in(active trans-port by gills)
Urine is lessconcentratedthan bodyfluids
Salt out
93
Figure 44.UN01b
Animal
Inflow/Outflow
Urine
Marine bony fish. Lives inwater
moreconcentratedthan bodyfluids fishtends to
losewater, gain salt
Drinks water
Small volumeof urine
Salt in
H2O out
Urine isslightly lessconcentratedthan
bodyfluids
Salt out (activetransport by gills)
94
Figure 44.UN01c
Animal
Inflow/Outflow
Urine
Terrestrialvertebrate.Terrestrialenvironmentt
ends to losebody waterto air
Drinks water
Moderatevolumeof urine
Salt in(by mouth)
Urine ismore concentratedthan bodyfluids
H2O andsalt out
95
Figure 44.UN02