Osmoregulation Part Two

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Osmoregulation - Part Two

• Osmoregulatory Organs/Processes
• The Mammalian Kidney, its Anatomy and Physiology
  (Glomerular Filtration)

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Introduction

• Success of osmoregulation is highly dependent
  upon epithelial transport along the gills, skin,
  kidneys and gut
• Apical/mucosal/luminal (faces the external
  environment e.g. ocean vs. basalateral/serosal
  surface (faces internal environment/extracellular
  fluid)
• Cells and regulatory mechanisms are similar
  across these osmoregulatory organs but specific
  differences exist

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Remember

• Energy (ATP) is expended directly or indirectly
  in transporting ions across epithelia against a
  gradient
• The role of ATP varies with different types of
  pumps there are 3 classes of ion-motive
  ATPases/pumps

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3 Classes of Ion-motive ATPases or Pumps

• F-ATP synthases - found in mitochondria (
  chloroplasts) use proton electrochemical gradient
  to make ATP (generally H)
• V-ATPases (vacuolar) found mucosal side animal
  plasma membranes hydrolyze ATP to generate
  electrochemical gradient
• - remember gradients function to direct
  movement of ions through associated channels,
  symporters (carrier protein transfers 2 solutes
  same direction), and antiporters (carrier protein
  transfers 2 solutes opposite direction)

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Ion-motive pumps cont

• Na/K P-ATPases like V-ATPases (i.e. hydrolyse
  ATP to generate electrochemical gradient) except
  have a phosphorylated intermediate e.g. Na/K
  pump , Ca2 pump (involved in muscle contraction)
  H/K P-ATPase (involved in gastric
  acidification) found on serosal (basalateral)
  side of plasma membrane
• - Na/K pump (serosal) side of epithelial cells
  regulates intracellular Na levels cell volume
  by moving Na out of cell to the extracellular
  flluid

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ATPases cont

• Epithelial cells exhibit continuous,
  coordinated activity found in a combination of
  pumps, channels, etc on both sides p. 594-596
  movements may include
• If Kchannel (apical) excreting into
  extracellular space with electrochemical gradient
• If K channel (basolateral) cycling between cell
  and extracellular fluid driven by Na/K pump

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Ion-motive pumps cont

• If apical side contains a Na/glucose or
  Na/2Cl-/K symporter drive glucose, K or Cl-
  uptake (as Na/K ATPase directly or indirectly
  support movement of other substances e.g. glucose
• If above symporter on basolateral side Na/K
  pump drives Cl-uptake from extracellular fluid
  contains (Cl- channels on apical side efflux of
  accumulated Cl- also creates a transepithelial
  membrane potential that drives Na via
  paracellular path

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Ion-motive pumps cont

• If proton pump (H vATPase) apical side
  increases electrochemical gradient promotes Na
  uptake from dilute solutions (frog skin,
  freshwater fish gill couples Na uptake to acid
  excretion
• Carbonic anhydrase (catelyzes interconversion of
  CO2 and bicarbonate) co-localized with a proton
  maintains supply of protons

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Ion-motive pumps cont

• - bicarbonate accumulation thus apical proton
  pump coupled to a Cl- bicarbonate antiporter on
  basolateral membrane CO2 diffuses into a cell
  but leaves as acid across apical side with
  bicarbonate in exchange for Cl-
• - presence of proton pump associated with K/H
  antiporter secretion of K and uptake of acid
  proton gradient drives antiporter

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Other factors/points

• Presence of tight or gap junctions
• Influence of hormones e.g. aldosterone,
  epinephrine, etc.
• Organization of the osmoregulatory organ itself
  e.g. highly anatomical organization of mammalian
  kidney
• Depending on species and specialization of the
  osmoregulatory epithelial cells various
  combinations of channel, transports and ATPases
  may be present

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Mammalian Kidney

• Overview of anatomy
• Urine production
• Glomerular filtration
• Tubular filtration
• Tubular secretion
• Regulation of pH
• Urine concentration mechanism
• Control of water reabsorption

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Anatomy

• Pair each on dorsal side against inner surface of
  lower back outside of the peritoneum
• Significant blood flow (total volume through
  every 4-5 minutes)
• Tough CT capsule
• Cortex (outer) and medulla (inner sending
  papillae into renal pelvis (collecting space
  giving rise to the ureter which empties into the
  bladder micturation (urination) via urethra

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Anatomy cont

• Urine contains H2O metabolic by-products (urea,
  NaCl, KCl, phosphates, etc.)
• Volume composition of urine reflects volume of
  fluid taken in ( volume of water produced via
  metabolism minus water loss through lungs, skin
  feces) and amount/composition of ingested food

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Urination Introduction

• Contraction of smooth muscle of bladder wall
  relaxation of skeletal muscle sphincter around
  opening of bladder
• As bladder wall stretched, stretch receptors
  generate nerve impulses carried by sensory
  neurons to spinal cord/brain sensation of
  fullness
• Sphincter relaxed by inhibition of motor impulses
  smooth muscle of bladder wall contracts (ANS
  control) empties bladder controlled release
  not the slow dribble urine as from kidney to
  bladder

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Nephron functional unit of the kidney

• Epithelial tube closed at beginning/open at
  distal end
• Nephrons empty into collecting ducts
• Ducts combine to form papillary ducts which empty
  into renal pelvis
• Closed end is expanded to form cup-shaped
  Bowmans capsule and inside BC capillary
  complex glomerulus
• 1st step of urine formation occurs
  ultrafiltrate of blood moves from glomerulus to
  lumen of BC

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Renal tubule

• One epithelial cell layer thick
• This layer separates ultrafiltrate from
  interstitial fluid
• At certain spots, epithelial cells are
  morphologically specialized for transport dense
  pile of microvilli on apical surfaces and deep
  infoldings in basolateral membranes cells tied
  together by leaky tight junctions some
  paracelluar diffusion between lumen and
  interstitial space

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Closer look at the Nephron

• Several 100s lower vertebrates to several 1000s
  in small mammals to millions in humans/larger
  species
• 3 main regions
• Proximal nephron BC proximal tubule
• Loop of Henle hairpin loop descending limb
  and ascending limb
• Distal tubule ascending limb of LofH merges
  joins connecting duct of several nephrons

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Even closer look

• LofH found only in kidneys of birds and mammals
  (vertebrates without incapable of producing
  urine hyperosmotic to the blood)
• Glomeruli in cortex LofH in medulla
  radiating orientation
• 2 types of nephrons
• Juxtamedullary glomeruli inner part of cortex
  and LofH deep into medulla
• Cortical glomeruli in ourter cortex and short
  LofH extends only short distance into medulla

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Anatomy of Renal Circulation

• Renal artery divides into series short afferent
  arteriolies supply each nephron
• Glom. Capillaries subjected to higher pressures
  than other caps and come together to form
  efferent arteriolies
• In justamedullary nephrons efferents subdive to
  form 2nd series of capillaries surrounding LofH
  after join to form a vein these capalliaries
  also in a hairpin formation called vasa recta

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Urine Production

• Kidneys filter blood plasma reabsorb needed
  substances rest is excreted
• 3 processes define ultimate composition of urine
• Glomerular filtration (at BC)
• Tubular reabsorption (99 water and most salts
  from ultrafiltrate resulting in concentrated
  waste products e.g. urea
• Tubular secretion (generally by active transport)
• TR and TS occur along length of renal tubule

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Glomerular filtration

• 15-25 of water solutes removed from plasma
  much of ultrafiltrate is reabsorbed depends on
  3 factors
• Net hydrostatic pressure difference between lumen
  of glom caps and lumen of BC (favors filtration)
• Colloid osmotic pressure of plasma (opposes
  filtration -separation of proteins which remain
  in plasma)
• Hydraulic permeability (sievelike properties) of
  3-layered tissue separating the 2 compartments

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Net Pressure Gradient

• Results from sum of hydrostatic pressure
  difference between 2 compartments colloid
  osmotic pressure
• Glomulular filtration passive process due to
  net pressure gradient

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BC-glomeruli junction

• 3 layers to cross capillary wall, basement
  membrane of capillary and inner (visceral) layer
  of capsule
• Fenestrated capillaries (many large pores) 100
  x gt permeable than other capillaries
• Basement membrane collagen for structure and
  negatively charged glycoproteins repel albumin
  and other negatively charged proteins

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BC-glomeruli junction cont

• Viseral layer of BC
• - Filtration slits formed from cellular
  processes pedicels which extend from podocytes
  (foot cells)
• - filtrate driven by net pressure difference
  across the endothelium passes through pores in
  caps. and through the filtration slits into lumen
  of BC 3 layers sieve excludes almost all
  protein (size, shape and charge) - water with
  ions, glucose, urea and other small molecules pass

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Examples of factors affecting the pressure
differential

• Dehydration colloid osmotic pressure increases
• Kidney stones (obstructions) increase
  intracapsular pressure
• Both above result in glom. filtration rate
  decreasing
• Seepage of plasma through burned skin lower
  colloid osmotic pressure increases glom
  filtration rate

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Regulatory Processes

• To ensure changes in blood pressure and cardiac
  output have min. effect on glomerular filtration
  rate (GFR), control blood flow to kidney via
  modulating the resistance to flow in afferent
  arteriole other interrelated mechanism
  involving paracrine and endocrine secretions
  neuronal control

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Autoregulation of GFR intrinstic mechanisms

• Myogneic mechanism Increase blood pressure
  stretch afferent arteriole increases flow to
  glom wall responses by contracting reducing
  diameter or arteriole increasing resistance to
  flow
• Juxtaglomerular apparatus where distal tubule
  passes close to BC bet. Afferent efferent
  arterioles secrete subtances modulate renal
  blood flow 2 types of specialized cells
• Extrinsic neuronal control

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Juxtaglomerular apparatus specialized cells

• Macula densa modified distal-tubule cells
  monitor osmolarity and flow of fluid releases
  substances act in a paracrine fashion cause
  vasoconstriction or vasodilation in response to
  changes in flow
• Juxtaglomercular cells (granular) modified
  smooth-muscle cells located primarily in wall of
  afferent arteriole can release enzyme renin
  indirectly affects BP renal blood flow
• - 12 act together in feedback-control
  manner over wide range of BP

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Extrinsic Neuronal Control

• Afferent arterioles innervated by SNS SNS
  activation
• vasoconstriction of afferent arterioles
  reduction GF over-rides autoregulation occurs
  when sharp drop in BP (e.g. excessive blood loss)
  reduction in filtration helps restore blood
  volume pressure converse also true

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Extrinsic Neuronal Control cont

• Contractions of cells within glom closes
  portions of filtering caps. reduces area
  available for filtration podocytes also
  contractile no. filtration slits decreases
  contractionof either or both reduces hydralic
  permeability
• Red. Renal blood flow, fall in solute delievery
  or activation of SNS release of renin from
  granular cells leads to increased angiotensin
  II in blood

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Angiotensin II functions

• General constriction of arterioles throughout
  body raises BP increases renal blood flow and
  GFR effecent arterioles sensitive I.e. low
  levels cause constriction of efferents raises
  glom BP and increase filtration and high levels
  constrict both afferent and efferent reduce GF
• Stimulates release of steroid aldosterone from
  adrenal cortex and vasopressin from posterior
  pituitary (role in reabsorption of salts water)

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Tubular Reabsorption

• Human kidney produces 180 liters of
  filtrate/day but only 1 liter of urine gt99
  filtered water is reabsorbed
• 1800 g NaCl in original filtrate, only 10g (lt1)
  excreted in urine
• some substances are secreted into urine
• Renal clearance of plasma-borne substance
  volume of blood plasma from which the substance
  is cleared (removed) per unit time i.e. extent
  to which substance is reabsorbed or secreted
  mathematics on p. 603

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Renal Clearance

• When amount appearing in urine per minute
  amount removed from plasma GFR clearance 1
• Reabsorption reduces renal clearance below GFR
  whereas tubular secretion causes more of a
  substance to appear in urine than is carried into
  the tubule by GF

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Tubular Function

• Proximal tubule beginning of concentrating GF
  most NB in active reabsorption of salts
• 70 of Na removed from lumen by active transport
  proportional amount of H2O certain other
  solutes e.g. Cl- (passively) occurs at
  basolateral surface epithelial cells of proximal
  tubule (similar to frog skin and mammalian
  gallbladder epithelia)
• NaHCO3 reabsorbed proximally NaCl
  reabsorbed distally

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Tubular Function cont

• 75 of filtrate is reabsorbed before reaching
  LofH fluid is iso-osmotic to plasma and
  interstitial fluids
• At distal proximal tubule (joining descending
  LofH) GF reduced to ¼ of original volume
  substances not actively transported across tubule
  or not passively diffused are 4x gtconcentrated
  than original filtrate
• Microvilli (at luminal border tubular epithelial
  cells) brush border increase absorptive
  surface area of apical membrane promoting
  diffusion of salt and water from tubular lumen
  into epithelial cell

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Tubular Function cont

• Glucose aas reabsorbed at proximal tubule by
  carrier proteins on apical membrane
  co-transport Na and glucose or aas process is
  uphill for glucose and aas depends on Na
  electrochemical gradient created by Na/K pump
  in basolateral membrane once in tubular
  epithelial cell they diffuse into the blood
• Phosphates, Ca and other electrolytes reabsorbed
  to the amount body needs rest excreted

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Tubular Function cont

• Parathyroid hormone modulates reabsorption of
  phosphates and Ca
• Calcirtiol (active form of Vit D) is released
  into blood and stimulates CA reabsorption
  phosphate excretion ( Ca absorption form gut and
  its release from bone)
• Descending limb thin segment of ascending limb
  very thin cells containing few mitochondria and
  no brush border differing perm to NaCl and
  water I.e. descending low perm to NaCl, urea
  but perm to H2O

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Tubular Function cont

• Thick ascending limb LofH actively transport
  NaCl out from lumen to interstitial space but
  very little perm to H2O thus fluid reaching
  distal tubule is hypo-osmotic relative to
  interstitial fluid salt reabsorption by thick
  ascending limb NB in urine concentrating
• Distal tubule transport of K, H NH3 into
  lumen and Na, Cl- HCO3 out of lumen back into
  interstitial fluid (water follows passively)
  transport of salts under endocrine control is
  adjusted in response to osmotic conditions

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Collecting duct

• Perm to H2O H2O flows from lumen into
  interstitial fluid of renal medulla final step
  in productio of hyperosmotic urine water perm
  controlled by vasopressin (antidiuretic hormone
  ADH) feedback control (p. 605-606)
• Active NaCl transport
• Toward distal end highly perm to urea

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Tubular Secretion

• K, H, NH3, organic acids and organic bases (K
  reviewed in detail p. 608
• Organic anion secretion driven by Na/K pump
• Secretory mechanisms nonspecific dugs and
  toxins secreted also
• associated with liver physiology many
  substances (with normal metabolites) are
  conjugated with glucuronic acid or its sulfate in
  liver allows them to react with organic anionic
  and cationic transport systems excreted in urine