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FORMATION OF URINE

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FORMATION OF URINE The formation of urine occurs in three separate steps. http://www.mhhe.com/biosci/ap/dynamichuman2/content/gifs/0178A.gif FILTRATION The movement ... – PowerPoint PPT presentation

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Title: 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
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