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Department
Physiology
Course
Physiology 2130
Professor
Anita Woods
Semester
Winter

Description
Module  11  –  Renal  System     By  the  end  of  this  section,  you  should  be  able  to:   • Describe  homeostasis  as  it  applie s  to  body  fluids  and  osmolarity   • List  the  functions  of  the  kidney.   • Draw  a  diagram  of  the  kidney  and  label  the  major  components.   • Draw  a  diagram  of  the  kidney  blood  flow.   • Draw  a  diagram  of  a  nephron  and  add  arrows  showing  the  direction  for  filtration,   reabsorption,  secretion,  and  excretion.   • Distinguish  between  filtration,  reabsorption,  secretion,  and  excretion.   • Describe  and  distinguish  between  renal  b lood  flow  and  renal  plasma  flow  and  give   values  for  each.   • Explain  glomerular  filtration  and  list  the  factors  that  affect  it.   • Explain  Starling  Forces  and  list  the  four  Starling  Forces.   • Describe  filtered  load  and  distinguish  it  from  glomerular  filtration.   • List  and  describe  all  the  tubular  transport  mechanisms  involved  in  the  movement  of  ions   and  fluid  along  the  nephron.   • Describe  the  reabsorption  of  sodium,  glucose,  amino  acids,  potassium,  chloride  and   water  and  the  secretion  of  hydrogen  ions  in  the  proximal  t ubule.   • Describe  the  reabsorption  of  water  in  the  descending  limb  and  the  reabsorption  of   sodium,  chloride,  potassium  and  water  in  the  ascending  limb  of  the  loop  of  Henle.   • Describe  the  reabsorption  of  sodium  and  water  and  the  secretion  of  potassium  in  the   distal  tubule  of  the  nephron.   • Describe  the  reabsorption  of  sodium  and  water  and  the  secretion  of  potassium  in  the   collecting  duct.   • Describe  the  concept  of  water  balance  and  how  the  kidneys  regulate  it.   • Describe  the  control  and  release  of  antidiuretic  hormon e  (ADH)  and  its  effect  on  the   kidneys.   • Describe  the  renin-­‐angiotensin  system  and  the  production  of  angiotensin  II  and  its   effects  on  the  kidneys.   • Describe  the  filtration,  reabsorption,  and  secretion  of  potassium  by  the  kidneys.                                   Introduction     • The  renal  system  includes  the  kidneys,  ureters,  bladder,   and  urethra.   • The  principal  function  of  the  kidneys  is  the  regulation  of   water  balance,  electrolyte   levels,  pH  of  the  blood,  and  the  long-­‐term  regulation  of  arterial  pressure.     Functions  of  the  Kidneys     • The  basic  function  of  the  kidneys  is  to   remove  nonessential  substances  from  the   plasma,  including  waste  metabolites,  excess  water,  and  electrolytes  and  to   recover  any   essential  substance  like  glucose.     • In  doing  so,  the  kidneys  play  a  major  role  in  regulating  the  water  levels,  the  chemical   concentration  of  the  body  fluid  compartments,  and  pH  (or  acidity)  of  the  blood.   • It  is  important  to  understand  that  the  kidneys  do  NOT  produce  water  or  electrolytes  but   only  CONSERVE  them  by  reducing  the  amount  rem oved  from  the  body.   • The  elimination  of  waste  or  foreign  substances  is  an  important  function  of  the  kidneys.   This  includes  the  removal  of  drugs,  food  additive,  and  vitamin  that  are  excreted  in  urine.   • The  kidneys  also  act  as  an   endocrine  gland,   producing  hormones  or  components  of   hormonal  systems  such  as  erythropoietin,   renin,  vitamin  D,  and  stanniocalcin.     Anatomy  of  the  Kidneys     • The  kidneys  are  roughly  the  size  of   a  fist.   • They  consist  of  an  outer   renal  cortex,  a  middle   renal  medulla,  and  inner  calyces  that  drain  into   a  central  renal  pelvis.   • The  renal  pelvis  then  drains  into  the   ureter.     • Located  within  the  renal  pyramids  are  the   functional  units  of  the  kidneys  –  the  nephrons.   Each  nephron  drains  through  a   collecting  duct   into  a  calyx.       Anatomy  –  Blood  Supply  of  the  Kidneys     • Blood  flows  to  the  kidneys  through  the   renal  artery.   This  large  artery  branches  into  several   interlobar   arteries  that,  in  turn,  branch  into  arcuate  arteries.     • The  blood  in  the  arcuate  arteries  flows  through  the   interlobular  arteries  to  supply  the  nephron.  The  blood   supply  to  the  nephron  drains  into  the   interlobular  vein,   the  arcuate  vein,  the  interlobar  vein,  and  then  into  the   renal  vein.             Anatomy  –  The  Nephron     • The  nephron  is  the  functional  unit  of  the   kidneys.   • There  are  over  1  million  nephrons  in  each   kidney  whose  function  is  to  filter  the  blood,   reabsorb  essential  substances,  and  excrete   nonessential  molecules  and  waste.   • Each  nephron  is  composed  of  a  highly  coiled   hollow  tube  surrounded  by  a  complex  blood   supply.   • The  Glomerular  Capsule  (also  called  Bowman’s   Capsule)  surrounds  a  very  small,  highly   permeable  capillary  bed  called  the   Glomerulus.   • These  structures  are  often  collectively  referred   to  as  the  Renal  Corpuscle.     • The  tubular  portion  of  the  nephron  consists  of   the  following  structures  in  order:  Proximal   Convoluted  Tubule  (a  highly  coiled  portion  of   the  nephron),  Descending  and  Ascending  Limb   of  the  Loop  of  Henle,  Distal  Convoluted   Tubule,  and  Collecting  Duct.     Anatomy  –  Blood  Supply  of  the  Nephron     • The  blood  supply  is  very  complex   • Blood  from  the  renal  artery  eventually   reaches  the  interlobular  artery  that   drains  into  the  afferent  arteriole.   • The  afferent  arteriole  gives  rise  to  the   glomerulus  (where  filtration  takes  place).   • The  blood  from  the  glomerulus  enters  the   efferent  arteriole  and  then  enters  the   peritubular  capillaries  (a  dense  network   of  capillaries  surrounding  the  tubes  of  the   nephron),  which  then  drains  into  the   interlobular  vein  and  eventually  back  to   the  renal  vein.                           The  Renal  Corpuscle     • The  renal  corpuscle  is  made  up  of  the  glomerular   capsule  (Bowman’s  Capsule)  and  glomerulus.     • This  is  the  site  where  the  blood  is  filtered   –  a   process  called  Glomerular  Filtration .   • The  fluid  that  is  filtered  from  the  blood   that  enters   the  glomerular  capsule  (or  capsular  space)  is  called   the  Filtrate.     • Glomerular  filtration,  as  we  will  see,  is   facilitated  by  a  highly  permeable  capillary   endothelium  that  is  surrounded  by  podocytes.     • The  larger  the  diameter  afferent  arteriole  and   smaller  diameter  efferent  arteriole  also   enhance  glomerular  filtration.     Processes  along  the  Nephron     • Each  section  of  the  nephron  has  different  functions.   These  functions  can  be  broken  into   Filtration,   Reabsorption,  and  Secretion.   • Important  Terms:   o Filtration:  Is  the  movement  of  fluid  through  the  glomerular  capillary  due  to   hydrostatic  pressures.     o Filtrate:  Is  the  solution  created  by  filtration.  The  filtrate  is  generally  composed   of  water  plus  all  the  dissolved   solutes  in  the  blood  (except  for  large  proteins   that  are  too  big  to  be  filtered).     o Reabsorption:  Is  defined  as  the  movement  of  a  substance  from  the  lumen  of  the   nephron  back  into  the  blood.   o Secretion:  Is  the  movement  of  a  substance  from  the  blood  into  the   lumen  of  the   nephron.   o Excretion:  Is  the  removal  of  a  substance  from  the  body   • Excretion  =  Filtration  +  Secretion  –  Reabsorption       Glomerular  Filtration     • Glomerular  Filtration  is  the  bulk  flow  of  fluid  from  the  blood   into  the  glomerular  capsule.  This  fluid,  called  the  filtrate,   contains  the  same  substances  as  plasma  with  the  exception  of   large  proteins  and  RBCs.     • Glomerular  Filtration  is  affected  by  the   extremely  permeable   capillaries,  which  make  up  the  glomerulus,  and  Starling   Forces.   • Special  epithelial  cells  called   Podocytes  surround  the  capillaries.  The  podocytes  have   large  Filtration  Slits  that  are  formed  between  Pedicles.   • These  structural  features  incre ase  filtration  at  the  glomerulus.     Glomerular  Filtration   –  Starling  Forces     • Starling  forces  cause  the  bulk  of  movement  of  fluid  across   capillaries  due  to  a  combination  of  hydrostatic  and  colloid   osmotic  forces.  In  glomerular  filtration,  the  pressure  of  each   force  is  different.     • Blood  Hydrostatic  Pressure  =  60  mmHg   o Almost  twice  that  in  a  regular  capillary,  causing  filtration  of  fluid  into  the   glomerular  capsule.   o This  pressure  is  principally  due  to  the  difference  in  diameter  between  the   afferent  (large)  and  efferent  (small)  arterioles.   • Colloid  Osmotic  Pressure  =  32  mmHg   o Due  to  Plasma  Proteins   o This  pressure  value  causes  the  reabsorption  of  fluid  into  the  plasma.   • Capsular  Hydrostatic  Pressure  =  18  mmHg   o Causes  the  reabsorption  of  fluid.     • There  is  no  Colloid  Osmotic  Force  in  the   Glom.   Cap.  Since  very  few  proteins  are  filtered.   • Therefore,  the  resulting   Net  Filtration  Pressure   is  10  mmHg  OUT  of  the  glomerulus  into  the   capsular  space.       Glomerular  Filtration  Rate  (GFR)  and  Filtered  Load     • The  kidneys  filter  a  tremendous  amount  of  fluid  each  day   –  roughly  180  L/day.   • This  Glomerular  Filtration  Rate  (GFR)  is  the  volume  of  fluid  that  is  filtered  by  the   glomerulus  during  a  certain  time  period.   • Since  the  kidneys  filter  many  other  substances,  it  is  important  to  be  able  to  calculate   the   amount  of  these  substances  filtered  by  the  kidneys  per   day;  this  is  called  the  Filtered   Load.  Equation  1:  Filtered  Load  =  (GFR)  x  (Plasma  Concentration  of  the  Substance)   • It  is  important  also  to  calculate  the  Urine  Concentration   of  a  substance  and  the   Amount   of  Solute  Excreted.     o Urine  Concentration   is  the   amount  of  the  solute  that  is   excreted  per  unit  volume  of  urine   (g/L).   o The  Amount  of  Solute  Excreted  is   the  actual  amount  (in  grams)  of   solute  that  is  excreted  in  the   urine  and  can  be  calculated  using   equation  2.   • These  values  also  tell  the  physician   important  information  concerning  the   health  and  functioning  of  the  kidneys.   • The  Amount  Reabsorbed  is  amount  of  filtered  substance  that  is  taken  back   up   (reabsorbed)  by  the  kidneys  and  can  be  calculated  using  equation  3.   • The  Fraction  Excreted  is  calculated  using  equation  4.     • You've  been  given  the  following  information:   o Glomerular  Filtration  Rate  (GFR)  =  180  L/day     o Na+  plasma  concentration  =  5  g/L     o Amount  of  Na+  reabsorbed  =  626  g/day     o Amount  of  water  excreted  =  1.8  L   • You  want  to  find  out  the  c oncentration  of  Na+  in  the  urine —essentially,  how  much  Na+   is  being  excreted?    Always  remember  the  following  (from  section  11.10):   o 1.  Amt.  Excreted  =  Amt.  Filtered  (or  Filtered  Load)  +  Amt.  Secreted   –  Amt.   Reabsorbed   § Since  Na+  is  not  secreted  in  this   example,  we  can  dispense  with  that   part  of  the  equation  and  the  result  will  be  as  follows:   o 2.  Amt.  Excreted  =  Filtered  Load   –  Amt.  Reabsorbed   § You  are  given  the  amount  of  Na+  reabsorbed  but  not  the  filtered  load.   Filtered  load  is  calculated  as  follows:   o 3.  Filtered  Load  =  GFR  x  Plasma  Concentration   § Putting  in  the  numbers  will  give  you  a   filtered  load  of  900  g.  This  is  what   is  being  filtered  at  the  glomerulus.   • Inserting  this  number  into  equation  2  gives  you  the  following:    Amt.  of  Na+  excreted  =   900  g  –  626  g    Amt.  of  Na+  excreted  =  274  g   o This  is  the  amount  of  Na+  excreted  in  the  urine.  However,  you  want  to  know  the   concentration  of  the  urine!   • Use  the  following:   o Amt.  of  Na+  Excreted  =  Urine  Concentration   x  Amt.  of  Water  Excreted     4.  Urine   Concentration  of  Na+  =  Amt.  of  Na+  excreted  /  Amt.  of  Water  Excreted   • Finally,  insert  the  numbers  to  get  the  answer  as  follows:   o Urine  Concentration  of  Na+  =  152.2  g  of  Na+/L  of  Urine     Tubular  Transport  Mechanisms   –  Introduction     • It’s  time  t  look  at  the  rest  of  the  nephron  –   from  the  proximal  convoluted  tubule  to  the   collecting  duct.   • Throughout  these  sections  of  the  nephron,   reabsorption  and  secretion  occur  to  different   substances  in  a  regulated  or  non-­‐regulated   manner.   • Reabsorption  and  secretion  involve  a  number   of  transport  mechanisms  including:   Active   Transport,  Secondary  Active  Transport,   Facilitated  Diffusion,  Simple  Diffusion,   and   Osmosis  (in  the  case  of  water).     Tubular  Transport  Mechanisms   –  Reabsorption     • Over  99%  of  the  substances  filtered  in  the  glomerulus  are   reabsorbed  back  into  the   circulation  at  different  sites  along  the  nephron.   • When  molecules  are  reabsorbed  from  the  filtrate  back  into  the  circulation,  there  are  2   transport  routes  that  can  be  taken:  Paracellular  Transport,   and/or  Transcellular   Transport.   • The  tubular  cells  are  joined  together  by   Tight  Junctions.     • Generally,  these  tight  junctions  don’t  allow  substances  to  cross  between  the  cells.   However,  along  the  nephron,  these  tight  junctions  vary  and  can  be  quite  leaky.   As  a   result,  some  substances  can  diffuse  between  the  tubular  cells  by  a  process  called   Paracellular  Transport.     o Paracellular  Transport  is  generally   non-­‐regulated,  occurring  without  any   hormone  control.   • Other  substances  are  transported  across  the  tubular  cell  membrane  from  the  lumen   into  the  cell,  then  into  the  interstitial  fluid  and  into  the  blood.  This  form  of  reabsorption   is  called  Transcellular  (Transepithelial)  Transport.     o In  many  cases,  Transcellular  Transport  can  be  regulated  (increased  or   decreased)  by  Hormones   o However,  most  cases  of  Transcellular  Transport  are   non-­‐regulated  and  occur   without  any  hormonal  control.     Tubular  Transport  Mechanism   –  Reabsorption:  The  Na/K  Pump     • Many  of  the  transport  mechanisms  along  the  nephron  rely  upon   the  Na+/K+  Pump   • This  pump  is  an  Active  Transport  Mechanism  because  it  requires   ATP  in  order  to  move  3  Na  OUT  of  the  cell  and  2  K  INTO  the  cell.   • As  a  result,  it  helps  establish  a  concentration  gradient  for  both   + ions  across  the  cell  membrane  –  a  HIGH  concentration  of  Na   OUTSIDE  the  cell,  and  a  HIGH  concentration  of  K  INSIDE  the  cell.     • This  pump  can  power  other  transport  mechanisms  by  a  process   called  Secondary  Active  Transport.       Tubular  Transport  Mechanisms   –  Reabsorption:  Secondary  Active  Transport     + • In  Secondary  Active  Transport,  the  Na  concentration  gradient  that  is  established  by  the   Na/K  pump  is  used  to  power  other  transporters.     + • As  Na  moves  into  the  cell  DOWN  it’s  concentration  gradient,  other  substances  will   either  move  in  with  the  Na  or  will  move  out  in  exchange  with  the  incoming  Na.   + + + • Secondary  Active  Transporters  include  the  Na /Glucose  co-­‐transporter  and  the  Na /H   exchanger.   o The  Na /Glucose  Co-­‐Transporter  is  located  on  the  luminal  side  of  the  tubule   cells.  With  this  transporter,  as  each  Na  diffuses  into  the  cell,  a  single  glucose   molecule  is  carried  in  along  with  it.   + + o The  Na /H  Exchanger  on  the  other  hand,  moves  one  H  out  of  he  cell  for  every   Na  that  diffuses  in.  It  is  also  located  on  the  luminal  side  of  the  cells.               Tubular  Transport  Mechanisms   –  Secretion     • Secretion  is  the  process  by  which  the  kidneys  remove  unwanted  substances  from  the   blood  into  the  lumen  of  the  nephron.   • Secretion  is  generally  a  hormonally  regulated  process  but,  in  some  cases,  it  can  occur   without  any  hormone  control  (non -­‐regulated).     • Most  substances  that  are  secreted  are  eventually  secreted  in  the  urine.   • Secreted  substances  include  H  and  K .   • This  process  does  rely  on  the  presence  of  the  Na+/K+  pump.       EVERYTHING  UNDER  TUBULAR  TRANSPORT  BEFORE  THIS  WAS  THE  INTRODUCTION     Tubular  Transport  Mechanisms     • Keep  in  mind  the  following:   o Na  Reabsorption  takes  place  in  the  proximal  tubule,  ascending  limb  of  the  Loop   of  Henle,  and  early  distal  tubule  –  mostly  non-­‐regulated  mechanisms.  However,   Na+  reabsorption  in  the  proximal  tubule  can  also  be  regulated  by  the  hormone   Angiotensin  II,  and  by  the  hormone  Aldosterone  in  the  late  distal  tubule  and   collecting  duct.     o In  a  healthy  individual,  all  of  the  glucose  that  is  filtered  at  the  glomerulus  is   reabsorbed  in  the  proximal  tubule.  Amino  Acids,  the  building  blocks  of  proteins,   are  also  reabsorbed  in  the  proximal  tubule.     o Water  Reabsorption  takes  place  in  the  proximal  tubule  and   descending  limb  of   the  loop  of  Henle  through  non-­‐regulated  mechanism.  No  water  is  reabsorbed  in   the  ascending  limb.  Water  absorption  is  regulated  by  Antidiuretic  Hormone   (ADH)  in  the  late  distal  tubule  and  collecting  duct.     o K+  Absorption  takes  place  in  the  proximal  tubule  and  the  ascending  limb  of  the   loop  of  Henle.  Secretion  of  K+  occurs   in  small  amounts  in  the  ascending   limb.  Larger  amounts  of  this  ion  are   secreted  in  the  late  section  of  the   distal  tubule  and  collecting  duct   under  the  influence  of  the  hormone   Aldosterone.     o H+  Secretion  occurs  in  the  proximal   tubule  and  the  ascending  limb  of  the   loop  of  Henle  –  it  can  be  BOTH   regulated  and  non-­‐regulated.   Hydrogen  ions  are  also  secreted  in   the  late  distal  tubule  and  collecting   duct  but  this  complex  mechanism  will   be  not  covered.           Proximal  Convoluted  tubule   –  Reabsorption  of  Na+,  Glucose,  and  Amino  Acids     • The  PCT  reabsorbs  roughly  66%  of  the  total  filtrate.   • Due  to  the  Na+  concentration  gradient  e stablished  by  the  Na/K  pump,  Na+  can  be   reabsorbed  into  the  tubule  cells  by  Simple  Diffusion,  the  Na+/Glucose  Co-­‐transporter,   or  the  Na+/H+  Exchanger.     • We  will  later  see  that  the  Na/H  exchanger  can  be  regulated  by  the  hormone   Angiotensin  II.   • Amino  acids  are  also  reabsorbed  along  this  section  of  the  nephron  by  a   Na+/Amino  Acid   Co-­‐transporter  (Similar  to  the  Na/Glucose  co-­‐transporter).   • In  a  healthy  individual,  the  Na/glucose  co-­‐transporter  reabsorbs  all  of  the  glucose  in  the   filtrate.  This  is  not  the  case  in  people  with  diabetes  mellitus .     o Diabetes  Mellitus  is  a  disease  that  affects  the  pancreas’  ability  to  produce  the   hormone  Insulin.  Insulin  is  essential  for  cells  to  take  up  and  store   glucose  after  a   meal.  Therefore,  without  insulin,   Glucose  Concentrations  build  UP  in  the  blood.   o Large  quantities  of  glucose  are  filtered  by  the  glomerulus,  and  as  a  result,  the   Na+/glucose  co-­‐transporters  cannot  reabsorb  all  of  it.  Consequently,  some  is   excreted  in  the  urine.  This  is  one  of  the  important  symptoms  of  diabetes  – glucose  in  the  urine.     o Since  this  co-­‐transport  system  is  a  form  of  Secondary  Active  Transport,  it  can  be   saturated.  So,  like  a  single  postal  worker  tying  to  sort  all  of  the  mail  coming   down  a  fast  conveyor  belt,  a  lot  of  glucose  will  get  by  t he  pumps.       Proximal  Convoluted  Tubule   –  Reabsorption  of  Water     • Now  that  Na+,  glucose,  and  amino  acids  have  been  reabsorbed,  the   filtrate  will  have  a   LOWER  solute  concentration  (and  HIGHER  water  concentration)  compared  to  the  cell   and  interstitial  fluid.   • With  this  osmotic  gradient  and  the  presence  of  special  water  channels  (called   Aquaporins),  the  water  will  move  down  its  concentration  gradient  by  osmosis  causing  it   to  be  reabsorbed.     • Water  will  be  reabsorbed  by  both   Paracellular  Transport  between  the  cells,  and  by   Transcellular  Transport  across  the  cells.     • It  is  extremely  important  to  note  that  water  reabsorption  takes  place  only  AFTER   solutes  have  reabsorbed  –  particularly  Na+.       Proximal  Convoluted  Tubule   –  Potassium  and  Chloride     • 65%  of  all  the  filtered  K+  and  Cl-­‐  are  reabsorbed  in  the   proximal  tubule.  All  of  this  reabsorption  is  through   2  types  of   Paracellular  Transport   –  both  of  which  are  NOT  regulated.     • Recall  that  tubule  cells  are  joined  together  by  tight  junctions   and  that  these  junctions  can  be  leaky  –  particularly  in  the   proximal  tubule.     • Cl-­‐  ions  are  also  reabsorbed  in  the  proximal  tubule  by   Transcellular  transport  that  uses  a  complex  mechanism  that   won’t  be  covered.     Proximal  Convoluted  Tubule   –  Potassium  and  Chloride  Reabsorption       • Reabsorption  of  K+  and  Cl -­‐  occurs  by  2  Paracellular  Mechanisms   –  Solvent  Drag  and   Simple  Diffusion.   o Solvent  Drag  involves  the  reabsorption  of  K+  with  the  movement  of  water.  We   have  already  seen  that  water  is  reabsorbed  by  osmosis  between  the  tubule   cells.  This  water,  as  it  moves  between  the  cells,  also  carries  with  it  some  of  the   dissolved  substances  in  the  filtrate  –  including  K+.  This  movement  of  a  solute   (the  K+)  in  a  solvent  (the  water)  is  called   solvent  drag.     o The  other  Paracellular  route  of  K+  reabsorption  is  by   Simple  Diffusion  down  its   concentration  gradient.  Recall  that  water  is  reabsorbed  by  osmosis  by  both   Paracellular  and  Transcellular  mechanisms.  As  water  is  reabsorbed  through  the   tubule  cells,  K+  remain  in  the  filtrate.  As  more  water  is  reabsorbed  alone,  the   filtrate  becomes  more  and  more  concentrated  with  K+.  As  a  result,  the  K+   concentration  gradient  increases  to  the  point  where  L+  can  move  into  the   interstitial  space  b/w  the  tubule  cells  (Paracellular  route)  via  simple  diffusion.       Reabsorption  of  Filtrate  Back  into  Circulation     • How  is  all  this  material  returned  to  circulation?   • This  material  must  first  leave  the  cells  and  enter  the  interstitial  space.   Na+  will  leave  the   tubule  cells  by  the  Na/K  pump.  Glucose  and  Amino  Acids  are  transported  across  the   basal  membrane  of  the  cells  by   Specific  Facilitated  Diffusion  Transporters.   • Remember,  K+  are  reabsorbed   by  Paracellular  transport,  so  they  are  already  in  the   interstitial  fluid.     • Once  in  the  interstitial  fluid,  all  these  substances  are   reabsorbed  into  circulation  by  Starling  Forces.       Reabsorption  of  Filtrate  Back  into  the  Circulation     • Reabsorption  of  water  and  all  dissolved  molecules  from   the  interstitial  space  back  into  the  peritubular  capillaries   occurs  due  to  Starling  Forces.   • The  pressure  for  these  Starling  Forces  around  the  tubule   cells  are  much  different  than  those  around  a  typical   capillary.     • In  the  kidney,  the  capillary:   o Hydrostatic  Force  (P ) C  =  13  mmHg   o Interstitial  (P IF  =  6  mmHg   o Osmotic  Force  due  to  Proteins  in  Plasma  Pπ )  =  32  mmHg   o Interstitial  Osmotic  Force  (π )IF  =  15  mmHg   • As  a  result,  the  Net  Filtration  Pressure  moving  the  fluid  is  calculated  to  be   -­‐10  mmHg   (10mmHg  back  into  the  capillary).   o Net  = C(P IF  P )P  – IF (π  –  π )     NOTE:  The  very  large  osmotic  pressure  is  caused  by  the  plasma  proteins .  This  pressure  is  large   because  during  the  glomerular  filtration,  almost  all  substances  were  filtered  except  large   proteins.     Proximal  Convoluted  Tubule   –  Concentration
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