Urine Formation

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					Urine Formation
             Urine Formation
•   Urine Formation
    occurs as a series of
    three steps:

1) Filtration
2) Reabsorption
3) Secretion
• Filtration: the
  movement of fluids
  from the blood into
  the Bowman’s
  capsule of the

• Remembering that
  the glomerulus acts
  as a high-powered
  filter and has 1.5X the
  pressure of a normal
  capillary bed.
• Dissolved solutes
  pass from the blood
  into the Bowman’s
  capsule along the
  pressure gradient, but
  not all solutes make it
• Things that pass through the glomerulus include:
  ammonia, urea, water, sodium (Na+) and other
  cations, chloride (Cl-) and other anions, glucose,
  amino acids and protons.
• Things that DO NOT pass through the
  glomerulus include: blood plasma proteins, Red
  Blood Cells, White Blood Cells and platelets.
• Reabsorption is the transfer of essential solutes and most
  of the water back into the blood stream (peritubular

• For every 120 ml of fluid, the amount of fluid filtered by
  your kidneys each minute, 1 ml of urine is formed and 119
  ml of fluids and solutes are reabsorbed back into the
• It occurs by both passive and active transport.
• Numerous mitochondria are in the kidney to help
  provide energy, but the energy only lasts for so
• Threshold level: when the maximum amount of
  substances have been reabsorbed and the
  energy is spent.
• Any solute leftover in the nephron when the
  energy is spent will get excreted in the urine.
• Glucose and amino
  acids are actively
  reabsorbed from the
  proximal tubule.
• Water is passively
  reabsorbed from the
  proximal tubule.
• The cells surrounding
  the Loop of Henle
  have a high
  concentration of
  dissolved solutes
  (salt) which creates
  an osmotic gradient.
• Water will then move
  out of the Loop of
  Henle and into the
  cells/bloodstream to
  try to balance the
  solute concentration.
• Therefore more water
  is reabsorbed
  passively in the
  descending Loop of
 Counter-Current Multiplier System

• This refers to the process of concentrating
  the urine in the mammalian kidney.
• It is all based on the fact that the Loop of
  Henle consists of two parallel tubes and
  descends into the hypertonic medulla
  region of the kidney.
 Counter-Current Multiplier System
• Water flows out (reabsorbed) from the
  descending limb of the Loop of Henle into
  the medulla.
• The ascending limb is impermeable to
  water, but here NaCl (salt) is actively
  reabsorbed into the medulla, effectively
  diluting the fluid left in the nephron.
 Counter-Current Multiplier System

• This reabsorption of salt in the ascending limb,
  dilutes the urine and also allows for more water
  to be reabsorbed in the descending limb.
• This countercurrent flow multiplies the osmotic
  gradient between the fluid in the nephron and
  the medulla, thus also increasing the amount of
  water being reabsorbed.
• As water gets reabsorbed,
  the solutes in the nephron
  become quite
  concentrated and some,
  like urea, will diffuse back
  into the bloodstream… but
  this is so small an amount
  that it is not a worry.
• Most will be left in the
  nephron and leave the
  body in the urine.
• Water being absorbed in the proximal tubule (65%)
  and the descending Loop of Henle (25%) is said to
  be obligatory because it is due to osmosis.
• Water being reabsorbed in the collecting duct (10%)
  is said to be facultative because it is under the
  control of the anti-diuretic hormone (ADH).
• Secretion is the
  release of the
  substances into the
  filtrate (urine) from the
  cells surrounding the
• Occurs in the
  proximal and distal
  tubules as well as the
  collecting ducts.
• It is done to:
1) Release any toxins/drugs that have not been filtered
2) Maintain the electrolyte (Ion) balance of the body
   •   If certain cations are reabsorbed then others must be
       secreted in order to keep the balance
3) Balance the acidity/alkalinity of the blood.
   •   It is usually an acid being secreted (proton containing
       compound) therefore urine is usually slightly acidic.
   •   Bicarbonate ion (HCO3-) is never secreted as it is integral in
       maintenance of blood pH.
   •   Acidic juices like cranberry juice cause our urine to
       increase in acidity which helps protect us from UTIs and
       prevent kidney stones.
           The rest is Urine
• The remaining filtrate left in the collecting
  duct after filtration, reabsorption and
  secretion has occurred will travel to the
  renal pelvis, out the ureters and into the
  bladder for storage until the next urination

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