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PROSESPEMBENTUKAN URIN
Rahmatina B. HermanBagian Fisiologi
Fakultas Kedokteran Universitas Andalas
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Urine Formation
The urinary system forms the urine and carriesit to the outside that consists of:
- The kidneys as the urine forming organs
- The structures that carry urine from kidneysto the outside for eliminating from the body
Three basic processes in urine formation:
1. Filtrationby glomerolus
2. Reabsorptionby tubules
3. Secretionby tubules
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Filtration By Glomerolus
Glomerular capillaries:impermiabel to protein
Glomerular filtrates:- protein-free- concentration of materials that do not bind with protein
as same as in plasma
Filtration rate
of glomerular capillary >> other capillaries, because ofgreater in:
- hydrostatic pressure- glomerular filtration coefficient (Kf)
product of permeabilityand effective filtration surface
area of glomerular capillary
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..Filtration By Glomerolus
The factors governing filtration acrossglomerular capillaries (GC) are the same as all
other capillaries
For each nephron:
- Glomerular filtration coefficient (Kf)
- Mean hydrostatic pressure in GC (PGC)
- Mean hydrostatic pressure in Bowmans capsule (PT)- Colloid osmotic pressure of plasma in GC (GC)
- Colloid osmotic pressure of filtrate (T) protein
free
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PGC GC
PT
PGC : Mean hydrostatic pressure in GC : 60 mmHgGC : Colloid osmotic pressure of plasma in GC : 32 mmHg
PT : Mean hydrostatic pressure in Bowmans capsule : 18 mmHg
Net filtration pressure: 60-32-18= 10 mmHg
Glomerular capillary
Bowmans capsule
Net Filtration Pressure
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Is actual rate of filtration by glomerularcapillaries
Depends on:
- Net filtration pressure- Filtration coefficient (Kf)
In males: 125 mL/min (7.5 L/h or 180 L/d)
in females: 115 mL/min (6.9 L/h or 160 L/d)
Glomerular Filtration Rate (GFR)
GFR = (Kf) x Net filtration pressure
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Factors Affecting GFR
Changes in renal blood flowChanges in glomerular capillary hydrostatic pressure
- Changes in systemic blood pressure- Afferent or efferent arteriolar constriction
Changes in hydrostatic pressure in Bowmans capsule- Ureteral obstruction- Edema of kidney inside tight renal capsule
Changes in concentration of plasma proteins- Dehydration , hypoproteinemia, etc (minor factors)
Changes in Kf- Changes in glomerular capillary permeability- Changes in effective filtration surface area
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Filtration fraction:- Fraction of plasma flowing through glomeruli that
is filtered into tubules
- Ratio of GFR to renal plasma flow (RPF)
- Normal: 0,20it means: 20 % of plasma that enters glomeruli isfiltered by glomerular capillaries
- GFR varies less than RPFwhen there is fall in systemic blood pressure, GFR fallsless than RPF, because of efferent arteriole constriction
filtration fraction rises
GFRRPF
..Filtration
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Filterability of solutes is determined by:
- Size/ molecular weight (MW)
- Electrical charge:
Negative charge is more difficult than
positive charge, because basement
membran of glomerular capillary consists
proteoglican with negative charge
Filterability
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Filterability of substances by GC decreaseswith increases MW
Substance MW Filterability
Water
Sodium
Glucose
InulinMyoglobin
Albumin
18
23
180
5.50017.000
69.000
1,0
1,0
1,0
1,00,75
0,005
..Filterability
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Secretion and Reabsorption
Once the glomerular filtrate is formed, then thetubular cells will:
Increase the concentration of certain
substances in the filtrate by secretion
Reduce the concentration of certain substances
in the filtrate by reabsorption
Secretion or reabsorption rate depending on
the needs of the body of the material
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Basic Mechanism of Secretion and Reabsorption
Active transport:- primary active transport
- secondary active transport
- active transport mechanism for proteinreabsorption: pinocytosis (endocytosis)
Passive transport:
- through intercellular space
- using carrier
Osmosis: water
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Transport Maximum (Tm)
Limit of the rate at which the solute can betransported through active transport mechanism
Due to transport carrier system becomes saturated as
tubular load increases
Passive transport does not demonstrate Tm, because
the rate is determined by other factors:- Electrochemical gradient for diffusion
- Permeability of the membrane for the substance
- The time that the fluid containing the substance
remains within the tubule
This type of transport is referred to as gradient-time
transport
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Transport in Proximal Tubules
Proximal tubule epithelial cells are highly metabolicand have large numbers of mitochondriato supportpotent active transport processes
Proximal tubule epithelial cells have extensive brush
border on the luminal side and also extensivelabyrinth of intercellular and basal channelsextensive surface area for rapid transport
Epithelial brush border is loaded with protein carrier
molecules and a large number of sodium ionssecondary active transport (co-/ counter transport)
So, it is the most active reabsorption process
Water moves across membrane by osmosis
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Reabsorption in Proximal Tubule
In the first half of proximal tubule:- sodium is reabsorbed by co-transport along with
glucose, amino acids, and other solutes
- leaving behind solution that has higher chlorideconcentration flow to the second half of proximal
tubule
In the second half of proximal tubule:
- sodium is reabsorbed mainly with chloride ions
- little glucose and amino acids remain to be
reabsorbed
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Secretion in Proximal Tubule
Proximal tubule is important site for secretionof many substances that must be rapidlyremoved from body, such as:
- organic acids and bases
- end product of metabolism
- many potentially harmful drugs or toxin
- para-aminohippuric acid (PAH)
Normal person can clear 90 % of PAH from plasmaflowing through kidneys and excrete it into urine
So, PAH clearance can be used as index of renalplasma flow (RPF)
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Transport in Loop of Henle
Loop of Henle consists of 3 functionally distinctsegments:
- the descending thin segment
- the ascending thin segment
- the thick ascending segment
The thin segments have thin epithelial membranes
with no brush borders, few mitochondria, and minimal
levels of metabolic activityThe thick segmenthas thick epithelial cells that have
high metabolic activity and are capable of active
reabsorption of sodium, chloride, and potassium
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..Transport in Loop of Henle
The descending thin segment:- Highlypermeable to water
- Moderately permeable to most solutes, including urea
and sodium
The ascending thin segment:- impermeable to water- reabsorption capacity is very low
The thick ascending segment- impermeable to water- highly metabolic active reabsorption of Na, Cl, K (25%)- has Na-H counter transport mechanism
tubular fluid becomes very dilute
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Transport in Distal Tubules
The very first portion of distal tubule forms part ofjuxtaglomerular complex that provides feedback
control of GFR and blood flow in the same nephron
The next early part of distal tubule is highly convolutedand has many of the same reabsorptive characteristics
of the thick segment of ascending limb of loop of
Henle:
- avidly reabsorbs most of ions including Na, Cl, K- virtually impermeable to water and urea
Also dilutes the tubular fluid
T i L Di l T b l
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Transport in Late Distal Tubulesand Cortical Collecting Tubule
The second half of distal tubule and the subsequent corticalcollecting tubule have similar functional characteristics
Anatomically, composed of 2 distinct cell types:
>principal cells: reabsorb Na+& water, and secrete K+
> intercalated cells: reabsorb K+& HCO3-, and activelysecrete H+ play a key role in acid-base regulation
Almost completely impermeable to urea
Rate of Na+ reabsorption and K+ secretion is controlled by
aldosteroneand their concentration in body fluids
Permeability to water is controlled by ADH(vasopressin)
important mechanism for controlling the degree of dilution
or concentration of urine
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Transport in Medullary Collecting Duct
Play an extremely important role in determining thefinal urine output of water and solutes
Epithelial cells are nearly cuboidal with smooth
surfaces and relatively few mitochondria
Permeability to water is controlled by ADH
Permeable to urea reabsorbed into medullary
interstitium raise osmolality concentrated urine
Capable of secreting H+ also play key role in acid-
base regulation
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Tubule CharacteristicsUrine Concentration
Segment of Tubules
Active
NaCl
Transport
Permeability
H2O NaCl Urea
Thin descending limb 0 +++++ + +
Thin ascending limb 0 0 + +
Thick ascending limb +++++ 0 0 0
Distal tubule + + ADH 0 0
Cortical collecting tubule + + ADH 0 0
Inner medullary collecting tubule + + ADH 0 +++
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