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Urinary System (Physiology of Kidneys, chapter
17) The urinary system two kidneys, two
ureters, one urinary bladder, and one urethra.
5 general functions 1, Maintain water balance
by excretion and reabsorption of water.
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2, Excretion of waste products urea, ammonia,
uric acid, creatinin, H, drugs (antibiotics)
and toxins 3, Maintain blood pH balance by a,
excreting waste products organic acids and
organic bases b, excreting H directly c,
reabsorb HCO3- to blood
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4. Regulating electrolyte metabolism by
controlling excretion and reabsorption of ions.
Ca, Na and K ions can be excreted or
reabsorbed by the tubular system of the
kidneys. 5, Synthesis of hormones erythropoitin
and renin, ...
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Gross Anatomy From outside to inside, a kidney
contains three layers of structure. Fig. 17.2c
on p527 Cortex - the outer layer of kidney
which extends into medulla
(columns) Medulla- the deeper region, contains
renal pyramids Cavity - the middle space is
divided into several portions minor calyx ?
major calyx ? renal pelvis ...
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Micro anatomy of kidneys Nephron - the
functional unit of kidney. Includes
glomerular capsule proximal convoluted
tubule distal convoluted tubule loop of
henle These small tubes are associated with
capillary vessel. Fluid filtered out of the
vessels and enter the tubes.
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Nephrons are mostly located in the cortex of the
kidney. Each kidney has more then one million
of nephrons. This number is more than enough to
carry out the functions of kidneys.
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The capillary vessels in a nepheron form a
spherical corpuscle called glomerulus.
Afferent arteriole - the small artery that
enters glomerulus Efferent arteriole the
one that runs out of the glomerulus is
called. The efferent arteriole will deliver the
blood to the next capillary bed, the peritubular
capillaries.
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• Renal (nepheron) tubules
• The tubular portion of the nepheron
• glomerular capsule
• proximal convoluted tubule
• descending limb of Henle
• ascending limb of Henle
• distal convoluted tubule.

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Glomerular capsule (Bowman's capsule) Fluid
filters out of the glomerular capillary and
enters glumerular capsule to form filtrate.
Proximal tubule has very permeable wall and
allows the reabsorption of water and ions back
into the blood stream. Fluid then enters the
loop of Henle.
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Loop of Henle located in the medulla contains a
descending lime, and an ascending limb and
connects with the distal convoluted tubule.
Distal convoluted tubule is the major site for
secretion to occur. Collecting duct collects
urine from distal tubules of several nephrons
and delivers it to minor calyx.
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Kidney cortex is basically formed by nephrons.
And medulla is formed by collecting ducts and
loops of Henle.
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Urine formation contains three steps 1.Glomerula
r filtration The glomerular capillary walls
are very permeable, about 100 - 400 times more
permeable than capillary vessel elsewhere.
The high permeability allows water and other
soluble substances to be filtered out into
Bowman's capsule.
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The total volume of the filtrate is very high.
The glomerular filtration rate (GFR) in is 115
ml/ min (women ) to 125 ml/min (man) ( 180 L
per day). The total blood volume in a person
is 5 L. This means that blood passes through
the kidney numerous times a day, and most of the
glomerular filtrate must be reabsorbed.
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The main driving force for glomerular filtration
is glomerular hydrostatic pressure or glomerular
blood pressure. Glomerular filtrate contains
water, glucose, a.a., ions, urea, uric acid,
ammonia, vitamins, etc... Pretty much every
thing in the blood except for blood cells and
proteins. Glomerular filtration is based on
sizes of the molecules and is nonspecific.
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Inflammation in glomeruli can damage the
capillary vessel wall and make the membrane
"leaky". So proteins and red blood cells can
be found in urine, and GFR can become higher
than normal. If the condition is not
controlled, scar can form from inflammation, GFR
can become very low and renal failure may occur.

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Four forces involved in glomerular
filtration GHP - glomerular hydrostatic
pressure (50 mmHg) major force to drive
filtration BCOP - blood colloid osmotic pressure
(25 mmHg), a counter force for filtration.
GFHP - glomerular filtrate hydrostatic pressure
(15 mmHg), caused by the resistance of urine
draining. CCOP - capsule colloid osmotic
pressure. Under normal conditions, this force
can be ignored.
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The net force that drive filtration in
glomerulus is called net filtration pressure or
NFP NFP (GHP- GFHP) - (BCOP - CCOP) (50
15) (25 0) 10 mmHg This value favors
filtration to occur. NFP lower than 10 can
cause hypofiltration. If higher than 10, hyper
filtration.
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2, Tubular Reabsorption If compare glomerular
filtrate and the final urine Glucose in the
filtrate is absent in urine, whereas urea and
uric acid are more concentrated in urine than in
glomerular filtrate. - Some thing has happened
after glomerular filtration.
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Blood flow in the kidneys After coming out of
the glomerulus, efferent artery continues into
peritubular capillary which runs along with
proximal and distal tubules and lead to vasa
recta ? ? renal vein.
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During reabsorption, useful materials that leaked
out during filtration are selective taken back,
and are transported into peritubular
capillaries. Waste products retained in blood
are further excreted from peritubular
capillaries into proximal or distal tubules and
loops.
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Tubular reabsorption mostly happens in proximal
tubules. Water, glucose, a.a., vitamins, ions
such as Ca, Na, K, Mg, PO4, HCO3- and Cl-
are reabsorbed. Active transport glucose,
a.a., Na, Ca, Mg, PO4, HCO3. Passive
transport water and some ions by
diffusion and osmosis.
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Glucose reabsorption (545-546) Glucose is
actively reabsorbed by carrier proteins. Normally
, glucose in filtrate is completely reabsorbed.

However, glucose reabsorption has certain limit
(threshold), When glucose concentration in
filtrate is above the threshold of reabsorption,
some glucose will be excreted in urine
glucoseurea.
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The membrane carrier protein for glucose can be
Saturated. The maximum transport rate that the
glucose carriers can achieve is called transport
maximum (Tm). Renal plasma threshold for
glucose is 180-200 mg/ml. In diabetes, blood
glucose levels are constantly higher than renal
plasma threshold ...
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Na and water reabsorption Fig. 17.11 About 65
of salt and water in filtrate is reabsorbed in
proximal tubule by sodium pump, which is an
energy consuming process. The reabsorbed Na
enters peritubular capillaries. When these
positively charged Na ions are pumped out of
filtrate, negative ions such as Cl, HCO3, PO4
will follow because of the charge attraction.
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The reabsorption of the ions causes the osmotic
pressure inside of blood vessels to increase...
So water is reabsorbed passively by osmosis.
90 of salt and water are reabsorbed before
the urine is excreted.
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Countercurrent Multiplication (Reabsorption of
salt and water in the loop of Henle) For water
to be reabsorbed, there must be a difference in
osmolarity between the two sides of the tubule
wall. The curved loop of Henle provide the
mechanism for this to happen. Note fluid
flows from descending to ascending limb.
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1, Na and Cl are pumped out of the ascending
limb which is not permeable to water. 2,
Reabsorption of ions elevates the osmotic
pressure in the peritubular areas that are
shared between ascending and descending limbs.
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The reabsorption of NaCl, makes the
interstitial fluid hypertonic and the fluid in
the descending limb hypotonic.
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3. In descending limb, the tube wall is
permeable to water but not permeable to salt.
Water diffuses out of the descending limb by
osmosis. This causes the solute concentration
in the descending limb to increase.
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4, When concentrated fluid in descending limb
flows into ascending limb, the concentration
gradient will accelerate the transport of Na and
Cl out of the tube into peritubular area.
H2O
NaCl
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This counter current multiplication works as a
positive feedback system to maximize the
reabsorption of water and salt.
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Ca reabsorption or excretion is controlled by
two hormones calcitonin and PTH. Calcitonin
increases Ca excretion and decreases Ca
reabsorption. PTH does opposite things.
When blood concentration of Ca is too high,
calcitonin is released by thyroid gland to lower
it. When Ca is low, PTH is released to increase
blood Ca.
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3, Tubular Secretion After glomerular
filtration, some undesirable substances still
remain in the blood stream. Tubular secretion
works as a back up of kidney excretion of
undesirable materials. Substances secreted in
the tubular system include H, K, creatinine,
uric acid, ammonia, drugs and toxins.
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Both H and K can be actively and
passively secreted into renal tubules. Na/K
exchange pump on the tubular membrane pumps 3
Na out of the tubules and 2 K into the tubules
by hydrolyzing one ATP molecule.
Also, when Na is reabsorbed, electrical charge
inside of the tubules will become more negative
K can diffuse (passively) into the tubules
due to charge attraction.
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H can also be actively and passively secreted
into tubules in the same way as K. Na
retention is always associated with secretion of
H and K.
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Uric acid is a metabolic product of protein and
RNA metabolism. Over production or poor
excretion of uric acid in the kidneys can cause
elevated blood levels Of uric acid. Some uric
acid can precipitate in joints to cause pain and
joint deformation gout.
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Creatinine is a waste product from skeletal
muscle metabolism. To evaluate a persons
kidney clearence function, blood and urine
levels of creatinin levels can be tested and
compared. If the blood creatinin is higher
than normal and urine creatinin is low, the
persons kidney function would be bad.
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Regulation of urine volume Urine volume is
largely determined by kidney water reabsorption,
which is controlled by two hormones, ADH and
aldosterone. ADH targets on the distal
convoluted tubule and collecting duct and
increases their water permeability, so more
water can move out of the urine duct by osmosis.
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Under normal effects of ADH, 27 liters of
water Is reabsorbed each day. ADH also causes
vasoconstriction. With more water reabsorption
and vasoconstric- tion, the net effect is
increased blood volume and blood pressure.
The stimulation signal for ADH release is
high blood osmolarity or low blood pressure.
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Aldosterone produced by adrenal cortex also acts
on DCT and collecting duct. It enhances Na
reabsorption and therefore enhances water
reabsorption. The secretion of aldosterone is
stimulated by decreased blood volume and
decreased Na concentration in the blood.
Aldosterone and ADH have additive effects on
water reabsorption and blood pressure
elevation.
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Why do hypertension patients need low Na diet?
High NaCl can increase blood osmolarity which
increases ADH release,
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Renal Acid-base Regulation (550-551) Kidney
help to maintain blood pH by excreting H and
reabsorb HCO3-. H can be actively and passively
secreted into proximal tubules as an charge
exchange with Na (during Na reabsorption).
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HCO3- reabsorption The reabsorption of HCO3-
occurs indirectly, because the tubule wall is
not permeable to HCO3-. When the urine is
acidic, HCO3- combines with H ? H2CO3 ? CO2
H2O. CO2 then diffuses into the tubule cells,
where CO2 H2O ? H2CO3 --gt HCO3- H.
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Under normal conditions, HCO3- is reabsorbed
back to blood, and H can stay in the cell or to
be pumped out into the filtrate and excreted.
When there is acidosis, almost all H will
go to filtrate to help the reabsorption of HCO3-.
During alkalosis, less H will be excreted.
Without binding with H in the tubule, less
HCO3- will be reabsorbed, which helps to bring
down the pH.
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The excretion of H in urine helps to maintain
blood pH balance and also can acidify urine.
The acidic urine can eliminate bacteria growth
in the urinary system. Infection can be
eliminated by frequent urination.
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Micturation (uriation) When the urinary bladder
is filled with urine, the stretch receptors will
be stimulated. The impulses are sent to the
micturition reflex center in the spinal cord.
A parasympathetic motor neuron carries
impulses to bladder wall to cause the smooth
muscle contraction. In the mean time, impulses
are sent to internal and external urethral
sphincters ...
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This is a spinal reflex. Normally,
messages are also relayed to the brain. The
descending impulse can either inhibit or
stimulate the spinal reflex depending on the
situation. Urinary bladder can hold 600 ml of
urine. The stretch receptors will be stimulated
when there is more than 150 ml of fluid.