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Chapter  44  

Osmoregula1on  and  Excre1on  

BIOL  223  

A  Balancing  Act  •  Osmoregula+on  

•  Regulates  solute  concentra1ons    

•  balances  the  gain  and  loss  of  water    

•  must  be  maintained  within  fairly  narrow  limits  

•  Func1on  of  homeostasis  

•  Marine  organisms    

•  must  regulate  salt  uptake  and  conserve  water  

•  Freshwater  organisms    

•  must  regulate  water  uptake  and  conserve  solutes/salts  

•  Excre+on    

•  Gets  rid  of  nitrogenous  metabolites  and  other  waste  products  

•  from  breakdown  of  proteins  and  nucleic  acids  

A  Balancing  Act  

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Selectively permeable membrane

Net water flow

Hyperosmotic side

Hypoosmotic side

Water

Solutes

Osmosis  and  Osmolarity  •  Osmolarity  

•  Solute  concentra1on  of  a  solu1on  

•  Dictates  movement  of  water    

•  across  a  selec1vely  permeable  membrane  

•  Differs  from  tonicity  

•  Osmolarity  counts  penetra1ng  and  non-­‐penetra1ng  solutes  

•  Tonicity  only  counts  non-­‐penetra1ng  solutes    

•  If  two  solu1ons  differ  in  osmolarity  

•  Net  flow  of  water  is  from  the  hypoosmo1c  to  the  hyperosmo1c  solu1on    

Osmo1c  Challenges  •  Osmoconformers  

•  Isoosmo1c  with  their  surroundings    

•  do  not  regulate  their  osmolarity    

•  Only  some  marine  invertebrates  

•  Osmoregulators    

•  Expend  energy  to  control  water  uptake  and  loss    

•  in  a  hyperosmo1c  or  hypoosmo1c  environment  

•  Stenohaline  

•  Cannot  tolerate  substan1al  

changes  in  external  osmolarity    

•  Most  animals    

•  Euryhaline    

•  can  survive  large  fluctua1ons  in  

external  osmolarity  

Osmo1c  Challenges  

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Marine  Animals  •  Most  marine  vertebrates  and  some  

invertebrates  are  osmoregulators    

•  (Most  marine  invertebrates  are  osmoconformers)  

•  Marine  bony  fishes    

•  Are  hypoosmo1c  to  sea  water  

•  lose  water  by  osmosis    

•  gain  salt  by  diffusion  and  from  food  

•  balance  water  loss  by  drinking  seawater    

•  and  excre1ng  salts  

Excretion of salt ions from gills

Gain of water and salt ions from food

Osmotic water loss through gills and other parts of body surface

Excretion of salt ions and small amounts of water in scanty urine from kidneys

Gain of water and salt ions from drinking seawater

(a) Osmoregulation in a saltwater fish

Freshwater  Animals  

Uptake of water and some ions in food

Uptake of salt ions by gills

Osmotic water gain through gills and other parts of body surface

Excretion of large amounts of water in dilute urine from kidneys

(b) Osmoregulation in a freshwater fish

•  Freshwater  animals    

•  Constantly  take  in  water  by  osmosis    

•  from  their  hypoosmo1c  

environment  

•  Lose  salts  by  diffusion    

•  maintain  water  balance    by  

excre1ng  large  amounts  of  dilute  

urine  

•  Salts  lost  by  diffusion    

•  are  replaced  in  foods  and  by  

uptake  across  the  gills  

Animals  That  Live  in  Temporary  Waters  

•  Some  aqua1c  invertebrates  in  temporary  ponds      

•  Lose  almost  all  their  body  water    

•  and  survive  in  a  dormant  state  

•  called  anhydrobiosis  

(a) Hydrated tardigrade (b) Dehydrated tardigrade

100 µm

100 µm

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Land  Animals  

•  Land  animals    

•  Manage  water  budgets  by  drinking  

and  ea1ng  moist  foods    

•  and  using  metabolic  water  

•  Desert  animals    

•  Get  major  water  savings  from  simple  

anatomical  features    

•  and  behaviors  such  as  a  

nocturnal  life  style  

Transport  Epithelia  in  Osmoregula1on  

•  Transport  epithelia    

•  Specialized  epithelial  cells  that  regulate  solute  movement  

•  Arranged  in  complex  tubular  networks  

•  Example:  salt  glands  of  marine  birds  

•  remove  excess  sodium  chloride  from  the  blood  

Ducts

Nostril with salt secretions

Nasal salt gland

Fig.  44-­‐8  

Salt gland Secretory cell

Capillary

Secretory tubule

Transport epithelium

Direction of salt movement

Central duct

(a)

Blood flow

(b)

Secretory tubule

Artery Vein

NaCl

NaCl

Salt secretion

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Nitrogenous  Wastes:  Ammonia  

•  Ammonia  (NH3)  

•  Highly  toxic  

•  Animals  that  excrete  nitrogenous  wastes  as  ammonia  (most  aqua1c  organisms)    

•  Need  lots  of  water  

•  release  ammonia  across  the  whole  body  surface    

•  or  through  gills  

•  Via  diffusion  

•  In  fish,  much  of  ammonia  is  excreted  as  ammonium  (NH4)  

•  From  the  gills  

Nitrogenous  Wastes:  Urea  

•  The  liver  of  mammals  and  most  adult  

amphibians  

•  Converts  ammonia  to  less  toxic  urea  

•  The  circulatory  system    

•  Carries  urea  to  the  kidneys  

•  Where  it  is  excreted  

•  Conversion  of  ammonia  to  urea  is  

energe1cally  expensive,  but  

•  Excre1on  of  urea  requires  less  water  

than  ammonia  

Nitrogenous  Wastes:  Uric  Acid  •  Uric  acid  

•  Excreted  by  birds,  insects,  

land  snails,  and  many  

rep1les  

•  Largely  insoluble  in  water  

•  excreted  as  a  paste  

with  li^le  water  loss  

•  more  energe1cally  

expensive  to  produce  than  

urea  

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Excretory  Systems  

•  Excretory  systems    

•  Regulate  solute  movement  between  internal  fluids  and  the  external  

environment  

Capillary

Excretion

Secretion

Reabsorption

Excretory tubule

Filtration

Filtrate U

rine

Excretory  Processes  

•  Most  excretory  systems    

•  Produce  urine  by  refining  a  filtrate  

derived  from  body  fluids  

•  Key  func1ons  of  most  excretory  systems:  

•  Filtra+on:  pressure-­‐filtering  of  body  fluids  

•  Reabsorp+on:  reclaiming  valuable  solutes  

•  Secre+on:  adding  toxins  and  other  

solutes  from  the  body  fluids  to  the  filtrate  

•  Excre+on:  removing  the  filtrate  from  the  

system  

Survey  of  Excretory  Systems  

•  Systems  that  perform  basic  excretory  func1ons    

•  vary  widely  among  animal  groups  

•  They  usually  involve  a  complex  network  of  tubules  

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Protonephridia  •  Protonephridium  

•  Network  of  dead-­‐end  tubules  connected  to  external  openings  

•  Smallest  branches    

•  capped  by  flame  bulb  

•  Excrete  a  dilute  fluid  and  func1on  in  osmoregula1on  

Tubule

Tubules of protonephridia

Cilia

Interstitial fluid flow

Opening in body wall

Nucleus of cap cell

Flame bulb

Tubule cell

Metanephridia  

•  Metanephridia    

•  Tubules  collect  coelomic  fluid    

•  produce  dilute  urine  for  

excre1on  

•  Each  segment  of  an  earthworm    

•  has  a  pair  of  open-­‐ended  

metanephridia  

Capillary network

Components of a metanephridium:

External opening

Coelom

Collecting tubule

Internal opening

Bladder

Malpighian  Tubules  •  Malpighian  tubules    

•  Remove  nitrogenous  wastes  from  hemolymph    

•  and  func1on  in  osmoregula1on    

•  In  insects  and  other  terrestrial  arthropods  

•  No  filtra1on  occurs  

•  Instead  ac1ve  transport  by  tubule  cells  

•  Transport  epithelium  

•  Insects  produce  a  rela1vely  dry  waste  ma^er  

•  important  adapta1on  to  terrestrial  life  

Rectum

Digestive tract

Hindgut Intestine

Malpighian tubules

Rectum

Feces and urine

HEMOLYMPH

Reabsorption

Midgut (stomach)

Salt, water, and nitrogenous

wastes

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Structure  of  the  Mammalian  Excretory  System  

•  Mammalian  excretory  system    

•  Paired  kidneys  

•  Also  the  principal  site  of  water  balance    

•  and  salt  regula1on  

•  Supplied  with  blood  by  a  renal  artery    

•  drained  by  a  renal  vein  

•  Urine    

•  Exits  each  kidney  through  ureter  

•  drain  to  urinary  bladder  

•  urine  is  expelled  through  a  urethra  

Fig.  44-­‐14  

Posterior vena cava Renal artery and vein

Urinary bladder

Ureter

Aorta

Urethra

Kidney

(a) Excretory organs and major associated blood vessels

Cortical nephron

Juxtamedullary nephron

Collecting duct

(c) Nephron types

To renal pelvis

Renal medulla

Renal cortex

10 µm Afferent arteriole from renal artery

Efferent arteriole from glomerulus

SEM

Branch of renal vein

Descending limb

Ascending limb

Loop of Henle

(d) Filtrate and blood flow

Vasa recta

Collecting duct

Distal tubule

Peritubular capillaries

Proximal tubule Bowman’s

capsule

Glomerulus

Ureter

Renal medulla

Renal cortex

Renal pelvis

(b) Kidney structure Section of kidney from a rat 4 mm

•  The  mammalian  kidney  has  two  dis1nct  regions:    

•  Outer  renal  cortex    

•  Inner  renal  medulla  

Structure  of  the  Mammalian  Excretory  System  

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•  Nephron  

•  Func1onal  unit  of  the  vertebrate  kidney  

•  single  long  tubule  and  a  ball  of  capillaries    

•  called  the  glomerulus  

•  Glomerulus  

•  Loca1on  of  filtra1on  

•  Bowman’s  capsule    

•  Surrounds  and  receives  filtrate  from  the  glomerulus  

Structure  of  the  Mammalian  Excretory  System  

Fig.  44-­‐14cd  

Cortical nephron

Juxtamedullary nephron

Collecting duct

(c) Nephron types

To renal pelvis

Renal medulla

Renal cortex

10 µm Afferent arteriole from renal artery

Efferent arteriole from glomerulus

SEM

Branch of renal vein

Descending limb

Ascending limb

Loop of Henle

(d) Filtrate and blood flow

Vasa recta

Collecting duct

Distal tubule

Peritubular capillaries

Proximal tubule Bowman’s capsule

Glomerulus

Filtra1on  of  the  Blood  

•  Filtra1on    

•  blood  pressure  forces  fluid  from  

blood  in  glomerulus  

•  Into  the  lumen  of  Bowman’s  

capsule  

•  Filtra1on  of  small  molecules  is  

nonselec1ve  

•  filtrate  contains    

•  Salts,  glucose,  amino  acids,  

vitamins,  nitrogenous  wastes,  

and  other  small  molecules  

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Pathway  of  the  Filtrate  •  From  Bowman’s  capsule  

•  filtrate  passes  through  three  regions  of  nephron:    

•  Proximal  convoluted  tubule  (PCT)  

•  loop  of  Henle  (LoH)  

•  distal  convoluted  tubule  (DCT)  

•  Fluid  from  several  nephrons  flows    

•  Into  a  collec+ng  duct  

•  lead  to  renal  pelvis    

•  Drained  by  the  ureter  

•  Cor+cal  nephrons    

•  Mostly  confined  to  the  renal  cortex  

•  juxtamedullary  nephrons    

•  loops  of  Henle  descend  into  renal  medulla