Transcript Formation of Urine

Formation of Urine
1. Glomerular Forced
Filtration / Pressure Filtration
Recall,
 The renal artery splits into
numerous arterioles, each
feeding a nephron.
 The arteriole splits into
numerous capillaries, which
form a knot called a
glomerulus. The glomerulus
is enclosed by the
Bowman’s capsule (or
sometimes called the renal
capsule)- the first part of the
nephron.
1. Glomerular Forced
Filtration / Pressure Filtration
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The blood pressure in the
capillaries of the glomerulus
forces plasma out of the blood
by forced ultrafiltration.
The pressure in the glomurulus is
approx. 60 mm Hg and small
molecules move from the
glomerulus to the inside of
Bowman’s capsule across the
thin walls of each. Both the
capillary walls and the capsule
walls are formed from a single
layer of flattened cells with
gaps between them.
The molecules that are squeezed
out of the blood form a filtrate in
the renal capsule. Only blood
cells and large plasma proteins
remain in the blood.
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Filterable Blood Components
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Water
Nitrogenous Wastes
Nutrients
Ions (salts)
Nonfilterable Blood Components
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Formed elements (blood cells and platelets)
Proteins
Comparison…
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Solutes found in the
Glomerulus:
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Water
Sodium chloride
Glucose
Amino acids
Hydrogen ions
Erythrocytes (blood cells)
Platelets
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Solutes found in the
Bowman’s capsule:
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Water
Sodium chloride
Glucose
Amino acids
Hydrogen ions
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**If the composition of urine had the same
composition as the glomerular filtrate, the
body would continually lose nutrients, water
and salts.
RESULT?? Death would quickly follow from
dehydration and starvation.
CONCLUSION: The fluid must be altered
throughout its journey in the remaining
tubule.
2. Proximal Convoluted Tubule
– Reabsorption
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Responsible for a major
part of the reabsorption of
water and solutes in the
glomerulus filtrate
The cells of this section
are:
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Cuboidal (cube shaped)
Lined with thousands of
microvilli, which increase
surface area for absorption
Contain many mitochondria
that indicate they are
biochemically active
2. Proximal Convoluted Tubule
– Reabsorption
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NaCl and other solutes,
such as glucose and
amino acids are actively
reabsorbed by the cells of
the proximal tubule
Due to the movement of
these molecules a
hypertonic condition is
created in comparison to
the tubular fluid and
therefore water follows
these molecules.
2. Proximal Convoluted Tubule
– Reabsorption
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Some urea is reabsorbed to the
blood by diffusion. Urea is a
small, uncharged molecule, so
it can pass through
membranes by lipid diffusion
and there is not much the
kidney can do about it. Since
this is a passive process, urea
diffuses down its concentration
gradient until the concentrations
of urea in the filtrate and blood
are equal.
So in each pass through the
kidneys half the urea is
removed from the blood and
half remains in the blood.
2. Proximal Convoluted Tubule
– Reabsorption
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The water and solutes that
are reabsorbed are taken
up by the peritubular
capillaries, therefore
returning to the blood and
out the kidney.
Over 80% of the filtrate is
reabsorbed into the tissue
fluid and then to the blood.
3. Loop of Henle – Formation
of a ‘Salt Bath’
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The cells of the
descending limb and the
initial cells of the
ascending limb are flat,
with no microvilli and few
mitochondria. (They are
not specialized for
transport)
Partway up the ascending
limb, they become
specialized for transport.
(They are cuboidal with
many mitochondria.)
3. Loop of Henle – Formation
of a ‘Salt Bath’
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The job of the loop of Henle
is to make the tissue fluid in
the medulla hypertonic
compared to the filtrate in
the nephron.
The purpose of this "salt
bath" is to reabsorb water
as explained below (step 5).
The loop of Henle does this
by pumping sodium and
chloride ions out of the
filtrate into the tissue fluid.
3. Loop of Henle – Formation
of a ‘Salt Bath’
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The first part of the loop
(the descending limb) is
impermeable to ions,
but some water leaves
by osmosis. This
makes the filtrate more
concentrated as it
descends.
3. Loop of Henle – Formation
of a ‘Salt Bath’
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The second part of the loop
(the ascending limb)
contains a Na+ and a Clpump, so these ions are
actively transported out of
the filtrate into the
surrounding tissue fluid.
Water would follow by
osmosis, but it can’t,
because the ascending limb
is impermeable to water. So
the tissue fluid becomes
more salty (hypertonic) and
the filtrate becomes less
salty (hypotonic).
3. Loop of Henle – Formation
of a ‘Salt Bath’
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Since the filtrate is most
concentrated at the
base of the loop, the
tissue fluid is also more
concentrated at the
base of the medulla,
where it is three times
more concentrated than
seawater.
4. Distal Convoluted tubule –
Homeostasis and Secretion.
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The distal convoluted tubule
is relatively short and has a
brush border (i.e. microvilli)
with numerous membrane
pumps for active transport.
Final Na+ reabsorption
occurs and the process of
water reabsorption
explained next in step 5
also takes place to a degree
in the distal convoluted
tubule
5. Collecting Duct –
Concentration
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As the collecting duct
passes through the
hypertonic salt bath in the
medulla,
water leaves the filtrate by
osmosis, concentrating the
urine and conserving water.
The water leaves through
special water channels in
the cell membrane called
aquaporins.
5. Collecting Duct –
Concentration
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These aquaporin channels
can be controlled by the
hormone ADH, (antidiuretic
hormone) so allowing the
amount of water in the urine
to be controlled.
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More ADH opens the
channels  more water is
conserved in the body, and
more concentrated urine is
produced.
This is described in more
detail in water homeostasis
later.