FORMATION OF URINE

1
FORMATION OF URINE

• The formation of urine occurs in three separate
  steps.

http//www.mhhe.com/biosci/ap/dynamichuman2/conten
t/gifs/0178A.gif
2
FILTRATION

• The movement of fluids from the blood into the
  Bowman's capsule of the nephron.
• Blood moves from the afferent arteriole into the
  glomerulus (a high-powered filter, acting at 65
  mm of Hg, 1.5 times the normal pressure of a
  capillary).
• Dissolved solutes pass from the blood into the
  Bowman's capsule along the pressure gradient.
  Not all solutes make it through.
• DO DON'T (too
  large)
• Water -Plasma proteins
• Sodium chloride -Red blood cells
• Glucose -Platelets
• Amino acids
• protons

3
REABSORPTION

• The transfer of essential solutes and most water
  back into the surrounding cells and the blood
  stream.
• For every 120 mL of fluid (amount filtered by
  kidneys each minute), 1 mL of urine is formed,
  119 mL of fluid and solutes is reabsorbed.
• Occurs throughout the nephron, mainly in the
  proximal tubule and loop of Henle
• Occurs by active and passive transport.
• Passive
• water by osmosis (more on this on next slide),
• K by diffusion (proximal tubule)
• Active
• NaCl (Na is pumped from the nephron into the
  surrounding cells, Cl- follows by diffusion,
  major site of NaCl reabsorption is the loop of
  Henle, then the distal tubule and collecting
  ducts),
• HCO3- (passive initially in proximal tubule,
  active by the distal tubule)
• Glucose, Amino acids (proximal tubule)
• There is maximum amount that can be reabsorbed by
  active transport due to the amount of energy
  available, this is called the threshold level
  the rest will be excreted in the urine.
• NOT reabsorbed uric acid, urea, excess solutes
  above the threshold level.

4
WATER REABSORPTION

• Water is automatically reabsorbed back into the
  cells and bloodstream for 2 reasons
• The blood that left the glomerulus by the
  efferent arteriole travels through the
  peritubular capillaries. It has a very high
  concentration in plasma proteins and is very low
  in water, therefore it draws water from the
  nephron and surrounding cells into the blood.
• Then, as solutes (K, NaCl, etc) are drawn out of
  the nephron into the cells surrounding the
  nephron they create an osmotic gradient. Their
  concentration becomes high in the cells and low
  in the filtrate.
• Therefore, water moves out of the nephron into
  the surrounding cells once more, again by
  osmosis.
• As water gets reabsorbed, the solutes left in the
  nephron become quite concentrated and small
  amounts of some, like urea and uric acid will
  diffuse back into the blood, however most of the
  urea and uric acid will be left to form urine.
• Water being reabsorbed in the proximal tubule
  (65) and descending loop of Henle (25) is said
  to be obligatory because it is due to osmosis.
• Water reabsorption continues in the collecting
  duct and is called facultative because it is
  under control of the hormone ADH (next class).
• The stuff left in the nephron that is not
  reabsorbed is the filtrate.

5
SECRETION

• The release of substances into the filtrate from
  the blood and cells surrounding the nephron.
• Occurs mainly in the distal tubules and in the
  collecting duct, but some also occurs in the
  proximal tubule.
• Purpose
• to release any toxins and drugs that have not
  been filtered
• Maintain the electrolyte balance of the body (if
  positive sodium ions are reabsorbed then positive
  ions like potassium must be secreted to keep the
  balance even though reabsorbed intially).
• Acid-base balance (usually it is an acid being
  secreted, essentially a proton plus whatever it
  is attached to).
• The bicarbonate ion is never secreted since it is
  used as a buffer in the maintenance of our blood
  pH.
• Acidic juices like cranberry cause our urine to
  be quite acidic which helps protect against UTIs
  and prevent kidney stones.
• Hmwk Read section 7.5 and answer questions on p
  352 2-7