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TOPIC 3: OSMOREGULATION AND EXCRETION

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Title: TOPIC 3: OSMOREGULATION AND EXCRETION


1
TOPIC 3 OSMOREGULATION AND EXCRETION
  • Internal homeostasis
  • 1. Osmotic regulation
  • 2. Ionic regulation
  • 3. Excretion
  • 3 process are closely related
  • Problem maintain
  • internal environment

NaCl K Ca2
External Environment
2
Freshwater versus Seawater
3
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4
Functions of Major Solutes
  • Na
  • Major extracellular cation
  • K
  • Major cytoplasmic cation
  • Ca2
  • Mg2
  • Pi, HCO3-
  • NH4

5
Intracellular versus extracellular regulation
  • 1. Intracellular and extracellular
  • 2. Extracellular and external

6
Problems of Osmoregualtion
  • Water
  • entry routes
  • exit routes
  • entry exit regulation
  • Salt entry routes
  • exit routes
  • entry exit regulation
  • Regulation requires energy therefore some animals
    conform

7
  • Osmotic and ionic exchanges
  • 1. Obligatory exchanges
  • 2. Regulated exchanges
  • Factors affecting obligatory exchanges
  • Trans-epithelial gradients
  • Surface-to-volume ratios
  • Permeability
  • Feeding
  • Temperature, exercise, convection
  • Metabolism

8
Osmoregulators versus Osmoconformers
9
Osmoregulation in Aquatic Animals
  • Definitions
  • Euryhaline
  • Stenohaline
  • Hyperosmotic/hyperionic
  • Hypoosomotic.hypoionic
  • Isoosmotic/isoionic
  • Isotonic

10
Freshwater Animals
  • Body fluids are hypertonic therfore two problems
  • water influx
  • salt loss

NaCl lt 1 mmol L-1
O2, CO2
NaCl 150 mmol L-1
H2O
Na, Cl-
H2O, ions
11
Solutions
  • Freshwater fishes
  • 1. Teleosts
  • blood osmolarity 300 mosm
  • compromise between need for effective gas
    transfer and obligatory exchanges
  • reduce gill surface area
  • reduce permeability
  • dilute urine (lt10 mmol l-1)
  • do not drink (water)
  • active NaCl uptake by the gill

12
The Fish Gill
13
The Fish Gill
14
NaCl Uptake in Freshwater Teleosts
Na
H
Cl-
ATP
HCO3-
PVC
CC
15
  • 2. Elasmobranchs
  • Permanent or temporary FW dwellers
  • Low blood solute levels therefore less water
    influx
  • Less renal salt loss
  • 3. Amphibians
  • Similar to teleosts
  • Skin replaces gills
  • Active ion uptake
  • 4. Invertebrates
  • FW crayfish versus FW clam

16
  • FW crayfish
  • blood conc. 420 mM
  • Urine conc. 124 mM
  • low urine volume becuaew permeability is low
  • active NaCl uptake
  • FW clam
  • blood conc. 42 mM
  • urine conc. 24 mM
  • urine volume is high because permeability is high

17
Marine Animals
  • Body fluids hypotonic therefore 2 problems
  • Water loss
  • Salt gain
  • Marine invertebrates
  • Many are isoionic therefore isoosmotic
  • exceptions
  • Artemia
  • Isoosmotic vertebrates
  • Hagfish
  • Isoionic but divalents are regulated

18
  • Elasmobranchs
  • Isoosmotic but NOT isoionic
  • Urea and TMAO
  • Rectal gland, renal excretion
  • The coelocanth
  • Similar to elasmobranchs
  • Hypotonic vertebrates
  • Marine teleosts
  • Drink SW, absorb 70-80 of SW
  • Divalents are eliminated in faeces and by kidney
  • Monovalents excreted by the gill
  • Very low volume of isotonic urine

19
NaCl Excretion in SW Teleosts

-
Water
Blood
Na
K
Na
-

Cl-
Chloride cell
20
Osmoregulation in Terrestrial Environments
  • Problems
  • 1. Continuous dehydration therefore water
    conservation is primary concern
  • 2. Excretion of salts
  • Solutions
  • 1. Less permeable integuments
  • e.g. terrestrial arthropods - epicuticle
  • skin permeability related to temperature
    regulation

21
  • 2. Drinking of water
  • complex feedback system
  • freshwater versus seawater
  • 3. Extrarenal salt excretion
  • salt glands of marine or land birds and reptiles
  • Marine reptiles
  • marine lizards
  • marine turtles
  • sea snakes
  • marine birds
  • size and fluid composition related to habitat and
    diet
  • greater volumes than human kidney
  • 4. Reduction of glomerular filtration
  • Reduced capillary development in arid-dwelling
    snakes and lizards

22
  • Extreme reduction in bird kidney
  • Minimal filtration is unavoidable
  • Why are glomeruli retained?
  • Formation of hypertonic urine
  • Related to loop of Henle
  • Greatest in desert rodents (9000 mOsm)
  • Minimize faecal water loss
  • Absorption of faecal water
  • insects
  • desert rodents
  • Uricotelism
  • Requires least water of 3 nitrogenous wastes

23
  • Water absorption across body surfaces
  • Amphibians - pelvic patch
  • Terrestrial arthropods - absorption of water
    vapour
  • Storage of water
  • Urinary bladder of amphibians and reptiles
  • Correlation between available water storage and
    storage capacity
  • Behavioural responses
  • Avoidance of desiccating environments
  • Minimizing respiratory water losses
  • Internalization of respiratory surfaces
  • Temporal counter-current system

24
  • Hygroscopic materials
  • Tolerance to dehydration
  • Man versus spadefoot toad
  • Essential to maintain blood pressure
  • Production of metabolic water
  • Consequence of substrate oxidation
  • Glucose 0.69 g H2O
  • Protein 0.40 g H2O
  • Fat 1.07 g H2O
  • Starch 0.56 g H2O
  • Starch is most favourable substrate

25
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26
Water Balance in the Kangaroo Rat
  • One month..

27
THE VERTEBRATE KIDNEY
  • Paired, 1 of body weight YET 20-25 of cardiac
    output
  • Inner layer medulla
  • Outer layer cortex
  • Micturition not continuous due to sphincter
  • Stretch receptors

28
  • Paired, 1 of body weight YET 20-25 of cardiac
    output
  • Inner layer medulla
  • Outer layer cortex
  • Micturition not continuous due to sphincter
  • Stretch receptors

29
The Nephron
  • Functional unit of the kidney
  • Five regions
  • 1. Renal corpuscle
  • glomerulus
  • Bowmanss capsule
  • 2. Proximal tubule
  • proximal convoluted tubule
  • proximal straight tubule
  • 3. Loop of Henle

30
  • Descending limb
  • Ascending thin limb
  • Ascending thick limb
  • 4. Distal convoluted tubule
  • 5. Collecting duct system
  • Connecting tubule
  • Cortical collecting duct
  • Medullary collecting duct
  • Renal pelvis

31
Overview of Urine Formation
  • 3 processes involved
  • 1. Filtration
  • depends on molecular size
  • filtrate blood plasma
  • 15-25 of H2O solutes are filtered
  • rate of filtration GFR 125 ml/min 200 L/day
  • Most of the water and salts are reabsorbed
  • 2. Reabsorption
  • 99 of of H2O salts are reabsorbed

32
  • NaCl reabsorption involves active transport
  • H2O reabsorption is passive
  • 3. Secretion
  • some substances are secreted and thus more
    concentrated than initial filtrate
  • secretion is selctive and relies on active
    transport

1. Filtration 2. Secretion 3. Reabsorption
33
Quantification of Renal Function
  • The Renal Clearance Ratio
  • 1. Determine GFR
  • inject animal with inulin
  • inulin in filtrate inulin in urine
  • inulin in filtrate inulinf X Vf
  • inulin in urine inulin X Vu
  • If X Vf Iu X Vu
  • Vf Iu X Vu/If GFR

34
  • for inulin
  • the ratio Iu X UFR/Iplasma X GFR 1 renal
    clearance ratio
  • 2. Measure UFR, Xurine, Xplasma
  • RCR gt 1 net secretion
  • RCR lt1 net reabsorption

35
Details
  • 1.Glomerular filtration
  • 3 factors involved
  • 1. hydrostatic pressure differences
  • 2. colloid osmotic pressure differences
  • 3. permeability of the three-layered tissue
  • Net filtration pressure 5 - 25 mmHg

36
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37
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  • 2. Tubular reabsorption
  • Original filtrate is quickly modified
  • 99 of water reabsorbed
  • 99 of NaCl reabsorbed
  • All glucose is reabsorbed
  • Proximal convoluted tubule
  • 75 of NaCl and H2O are reabsorbed
  • 70 of filtrate is reabsorbed - isoosmotic
  • Descending limb
  • No active salt transport, low NaCl permeability
  • High water permeability water leaves
  • Thin ascending limb
  • No active salt transport

39
  • High NaCl permeability,
  • NaCl exits
  • Thick ascending limb
  • Low water permeability
  • Active NaCl transport
  • The filtrate becomes hypoosmotic
  • Distal convoluted tubule
  • Active NaCl transport
  • Water follows passively
  • Collecting duct
  • Variable permeability to water
  • Final step in formation of hyperosmotic urine

40
  • 3. Tubular secretion
  • K, H, NH3, acids and bases
  • Number of secretory pathways is small

41
Concentrating mechanism of the nephron
  • Osmotic removal of water from the collecting duct
  • due to formation of standing osmotic gradient
  • Corticomedullary conc. gradient
  • differential permeabilities
  • functional asymmetry
  • counter-current principle

42
Figure 14-33
43
Figure 14-34
44
Control of Kidney Function
  • Body fluid disturbances are minimised
  • Neural and/or endocrine
  • 3 methods of regulation
  • 1. Control of GFR
  • 2. Control of salt reabsorption
  • 3. Control of water reabsorption

45
Control of GFR
  • Important that GFR be maintained
  • Renal blood flow is fairly constant
  • Lower vertebrates
  • GFR can be adjusted

46
Control of Renal Na Reabsorption
  • The juxtaglomerular apparatus
  • Macula densa
  • juxtaglomerular cells
  • Stimulated by
  • sympathetic stimulation
  • reduced Na

47
  • Circulating catecholamines
  • Decreased blood pressure
  • Decreased plasma volume
  • Response
  • renin release into afferent arteriole
  • formation of angiotensin I
  • Angiotensin II
  • increased blood pressure
  • release of aldosterone
  • Aldosterone promotes Na reabsorption

48

49
  • Summary of renin release
  • Angiotensin II and aldosterone
  • Increased NaCl reabsorption
  • Increased water reabsorption
  • Increased plasma volume
  • Peripheral vasoconstriction
  • INCREASED BLOOD PRESSURE

50
Control of osmotic water retention
  • Depends on collecting duct permeability
  • Regulated by ADH
  • ADH increases water permeability
  • ADH secretion controlled by
  • plasma osmolality
  • blood pressure
  • ethanol

51
Nitrogen Excretion
  • Toxicity
  • Fish - 0.05-1.0 mmol L-1
  • Mammals - lt0.05 mmol L-1

52
  • Nitrogen end products

Uric acid
Ammonia
Urea
0.001 L g-1
0.4 L g-1
0.04 L g-1
much less toxic
highly toxic
less toxic
synthesis requires ATP
synthesis requires ATP
Aquatic invertebrates,bony fish, larval
amphibians
Birds, reptilesterrestrial insects
Mammals,adult amphibians
53
  • Ammonia-excreting animals
  • Aquatic invertebrates, bony fish, larval
    amphibians
  • NH3 synthesis
  • Excreted by diffusion across gills or body
    surface.
  • Cuttlefish
  • NH3 H2O NH4 OH-

NH4
cation (e.g. K)
NH3
54
  • Urea-excreting animals
  • Advantages and disadvantages
  • Some molluscs and annelids, mammals, adult
    amphibians, elasmobranch fish
  • Urea synthesis
  • Excreted by kidneys or gills.
  • Elasmobranchs??
  • Osmoregulatory strategy - urea plus
    trimethylamine oxide (TMAO)

urea 400 mM TMAO 70-100mM
55
  • Urea-excreting bony fish??

Toadfish
Lungfish drought
pulsatile
Urea excretion (µmol kg-1)
Time (h)
56
  • Amphibian metamorphosis

Xenopus
57
  • Uric acid-excreting animals
  • Advantages and disadvantages
  • Birds, reptiles, terrestrial insects, land
    snails
  • Crocodiles
  • Uric acid synthesis
  • Excretion by kidneys or Malpighian tubules
  • Cleidoic eggs

58
Linkage between NH3 and CO2 excretion
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