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The Kidney- The structure and function of the nephron

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					The Kidney


  The structure and function of the
  nephron
Structure of the nephron
 The nephron is the functional unit of
  the kidney
 There are about a million in each
  kidney.
 They consist of:
  a cup-shaped Bowman’s capsule
  immediately below the capsule a
  twisted region called the proximal
  convoluted tubule.
Structure of the nephron
 followed by the long hair-pin like loop
 of Henle, which runs deep into the
 medulla and then back into the cortex
 this is followed by another twisted
 region called the distil convoluted
 tubule
 This is joined to the collecting duct
 which carries urine through the medulla
 to the pelvis of the kidney
Glomerulus,
a knot of
capillaries
  afferent
  arteriole




Glomerulus,
a knot of
capillaries
  afferent
  arteriole

  efferent
  arteriole

Glomerulus,
a knot of
capillaries
  afferent
  arteriole

  efferent
  arteriole

Glomerulus,
a knot of
capillaries

  venuole
              Bowman’s capsule
  afferent
  arteriole

  efferent
  arteriole

Glomerulus,
a knot of
capillaries

  venuole
              Bowman’s capsule
  afferent
  arteriole
                                 proximal
  efferent                       tubule
  arteriole

Glomerulus,                      distil
a knot of
capillaries                      tubule

  venuole
                              Bowman’s capsule
  afferent
  arteriole
                                                 proximal
  efferent                                       tubule
  arteriole

Glomerulus,                                      distil
a knot of
capillaries                                      tubule

  venuole




              loop of Henle
                              Bowman’s capsule
  afferent
  arteriole
                                                   proximal
  efferent                                         tubule
  arteriole

Glomerulus,                                       distil
a knot of
capillaries                                       tubule

  venuole                                        ascending
                                                 loop
                                                 descending
                                                 loop




              loop of Henle
                              Bowman’s capsule
  afferent
  arteriole
                                                   proximal
  efferent                                         tubule
  arteriole

Glomerulus,                                       distil
a knot of
capillaries                                       tubule

  venuole                                        ascending
                                                 loop
                                                 descending
                                                 loop

                                                  Collecting
                                                  duct



              loop of Henle
                              Bowman’s capsule
  afferent
  arteriole
                                                   proximal
  efferent                                         tubule
  arteriole

Glomerulus,                                       distil
a knot of
capillaries                                       tubule

  venuole                                        ascending
                                                 loop
                                                 descending
                                                 loop

                                                  Collecting
                                                  duct



              loop of Henle         to renal
                                    pelvis
                              Bowman’s capsule
  afferent
  arteriole
                                                   proximal
  efferent                                         tubule
  arteriole

Glomerulus,                                       distil
a knot of
capillaries                                       tubule

  venuole                                        ascending
                                                 loop
                                                 descending
                                                 loop

 capillary                                        Collecting
 net                                              duct



              loop of Henle         to renal
                                    pelvis
Structure of the nephron
 Each nephron has a rich blood supply
 Each Bowman’s capsule is supplied
  with blood by an afferent arteriole
 This branches inside the Bowman’s
  capsule to form the glomerulus
 Taking blood away from the capsule
  is the efferent arteriole.
 The afferent arteriole is much wider
  than the efferent arteriole…..
Structure of the nephron
….. So there is more blood carried to the
  glomerulus than is carried away, and
  pressure in the glomerulus is high.
Ultrafiltration
 ..the filtering of small molecules
  (under pressure) out of the blood and
  into the Bowman’s capsule.
Bowman’s Capsule
 Bowman’s Capsule

glomerulus
 Bowman’s Capsule

glomerulus




afferent
arteriole
 Bowman’s Capsule

glomerulus

efferent
arteriole




afferent
arteriole
  Bowman’s Capsule

glomerulus

 efferent
 arteriole
distil
convoluted
tubule


 afferent
 arteriole
  Bowman’s Capsule
                     proximal
glomerulus           convoluted
                     tubule
 efferent
 arteriole
distil
convoluted
tubule


 afferent
 arteriole
  Bowman’s Capsule
                                proximal
glomerulus                      convoluted
                                tubule
 efferent
 arteriole
distil
convoluted
tubule


 afferent            capsular
 arteriole           space
Ultrafiltration
 The blood entering the glomerulus is
  separated from the space inside the
  Bowman’s capsule, by two cell layers
  and a basement membrane.
podocyte
cell
  podocyte
  cell




nucleus
  podocyte
  cell




nucleus




             Basement
             membrane
  podocyte
  cell




nucleus




             Basement
             membrane

              capillary
              endomethium
  podocyte
  cell




nucleus
                        Filtrate in
                        lumen of
                        capsule



             Basement
             membrane

              capillary
              endomethium
Ultra filtration
 The first layer is the wall or
  endothelium of the capillary. In the
  glomerulus this single layer of cells
  has thousands of gaps.
 The basement membrane between
  the two cell layers is composed of
  glycoprotein and collagen fibres. Its
  mesh-like structure acts as a filter
  during ultrafiltration.
Ultra filtration
 The second cell layer is the wall of the
  Bowman’s capsule
 The epithelial cells in this wall are
  called podocytes
 They have foot-like processes and do
  not fit tightly together so have gaps
  between them.
Ultra filtration
 Most molecules in the blood can pass
  through the gaps in he capillary and the
  podocyte.
 BUT the basement membrane acts as a
  filter preventing large molecules such as
  proteins and blood cells from passing
  through.
 Only the small soluble molecules can pass
  through due to the high pressure in the
  kidney
Reabsorbtion
Molecule or      Approx. concentrations / g
ion              dm-3
                    Plasma          Filtrate
water                900.0           900.0
protein               80.0            0.0
glucose                1.0            1.0
amino acids            0.5            0.5
urea                   0.3            0.3
inorganic ions         7.2            7.2
Reabsorbtion
 Up to 70% of water and solutes are
  removed from the plasma as it passes
  through the glomerulus
 These include some useful substances
  such as aa and glucose, these must
  be reabsorbed back into the body.
 This process is called selective
  reabsorbtion.
Reabsorbtion
 All the glucose, aa, vitamins and
  many Na and Cl ions are actively
  transported out of the proximal
  tubule and back into the blood.
Adaptations of the proximal tubule


 The cells possess microvilli to provide
  a large surface area for absorption
 Numerous mitochondria are present
  to provide ATP for active transport.
Reabsorbtion
 The uptake of these substances
  means that the blood in the
  capillaries surrounding the nephron
  now has a a relatively high solute
  concentration.
 So large amounts of water passes
  from the filtrate back to the blood in
  the proximal tubule by osmosis
The Loop of Henle
 Its function is to create an area of
  high solute concentration deep in the
  medulla.
 The collecting ducts of each nephron
  pass through this area and so a lot of
  water can be reabsorbed from the
  ducts by osmosis
 A concentrated urine can be produced
  as a result
Descending
limb
Descending
limb




             Ascending limb,
             impermeable to
             water but
             permeable to
             NaCl
Descending
limb




                      Ascending limb,
                      impermeable to
                      water but
       Descending     permeable to
       very           NaCl
       permeable to
       water
                      Ascending limb
                      – very thick

Descending
limb




                             Ascending limb,
                             impermeable to
                             water but
       Descending            permeable to
       very                  NaCl
       permeable to
       water
                      Ascending limb
                      – very thick

Descending
limb




Water moves out
tubule into back             Ascending limb,
into capillary               impermeable to
                             water but
       Descending            permeable to
       very                  NaCl
       permeable to
       water
                               Ascending limb
                               – very thick

Descending
limb




Water moves out
tubule into back                       Ascending limb,
into capillary                         impermeable to
                                       water but
       Descending                      permeable to
       very                            NaCl
       permeable to   Na and Cl ions
       water          move out of
                      tubule
                               Ascending limb
                               – very thick

Descending
limb




Water moves out
tubule into back                         Ascending limb,
into capillary                           impermeable to
                                         water but
       Descending                        permeable to
       very                              NaCl
       permeable to   Na and Cl ions
       water          move out of
                      tubule

                              Increasing salt
                              conc. in
                              interstitial fluid
                               Ascending limb
                               – very thick

Descending
limb                                       Decreasing
                                           salt conc. in
                                           interstitial
                                           fluid

Water moves out
tubule into back                         Ascending limb,
into capillary                           impermeable to
                                         water but
       Descending                        permeable to
       very                              NaCl
       permeable to   Na and Cl ions
       water          move out of
                      tubule

                              Increasing salt
                              conc. in
                              interstitial fluid
                                          Ascending limb
                                          – very thick

Descending
                      isotonic
limb                                                  Decreasing
                      filtrate                        salt conc. in
                                                      interstitial
                                                      fluid

Water moves out
tubule into back                                    Ascending limb,
into capillary                                      impermeable to
                                                    water but
       Descending                                   permeable to
       very                                         NaCl
       permeable to              Na and Cl ions
       water                     move out of
                                 tubule

                                         Increasing salt
                                         conc. in
                                         interstitial fluid
                                              Ascending limb
                                              – very thick

Descending
                          isotonic
limb                                                      Decreasing
                          filtrate                        salt conc. in
                                                          interstitial
                                                          fluid

Water moves out
tubule into back                                        Ascending limb,
into capillary                                          impermeable to
                                                        water but
       Descending                                       permeable to
       very                                             NaCl
       permeable to                  Na and Cl ions
       water                         move out of
                                     tubule
             hypertonic                      Increasing salt
                                             conc. in
             filtrate
                                             interstitial fluid
                                              Ascending limb
                                              – very thick

Descending
                          isotonic            hypotonic filtrate
limb                                                      Decreasing
                          filtrate                        salt conc. in
                                                          interstitial
                                                          fluid

Water moves out
tubule into back                                        Ascending limb,
into capillary                                          impermeable to
                                                        water but
       Descending                                       permeable to
       very                                             NaCl
       permeable to                  Na and Cl ions
       water                         move out of
                                     tubule
             hypertonic                      Increasing salt
                                             conc. in
             filtrate
                                             interstitial fluid
The loop of Henle
 The ascending limb is more permeable to
  salts and less permeable to water
 As filtrate moves up Na and Cl ions move
  out passively and then actively
 This cause water to move out of the
  descending limb by osmosis.
 Thus filtrate become more concentrated as
  it passes down the descending limb
The loop of Henle
 The solute conc. in any part of the
  loop is always lower in the ascending
  limb than the descending limb
 This mechanism is called the
  countercurrent mulitplier
  mechanism.
The loop of Henle
 As the collecting ducts pass through
  the medula to the pelvis they pass
  through this region of high solute
  concentration
 So water is also drawn out the
  collecting ducts due to osmosis
  resulting in a far more concentrated
  urine.