Urinary System Physiology of Kidneys,

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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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Structure of the kidney From outside to
inside (Fig. 17.2c on p527) Cortex - the outer
layer of kidney which extends into medulla
(columns) Medulla- the deeper region, contains
renal pyramids Cavity where the urine is
formed.
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• Micro structure
• Nephron - the functional unit of kidney.
• glomerular capsule
• proximal convoluted tubule
• distal convoluted tubule
• loop of Henle
• (overhead)

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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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A vascular structure that is closely related to a
nepheron is glomerulus, which is a spherical
cluster of capillaries raped up by glomerular
capsule. 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 fluid filters out of the glomerular is called
glomerular filtrate, which contains waste
products urea, uric acid, ammonia, and also
useful materials Glomerular filtrate
urine
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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.
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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, 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.
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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 the opposite things.
When blood Ca is too high, calcitonin is
released to lower it. When Ca is low, PTH is
released to increase it.
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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 vasoconstriction, the net effect is increased
blood volume and blood pressure. The
stimulation signal for ADH release is highblood
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. When CO2 levels increase to certain level,
it diffuse into the tubule cells, where CO2
H2O ? H2CO3, which then dissociates into HCO3-
and 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
back 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. For the
summary of the functions of nephron Table 17.3
on p489.
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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.