Kuliah 1. Proses Pembentukan Urin(Print Hal 8,13,25)

8/13/2019 Kuliah 1. Proses Pembentukan Urin(Print Hal 8,13,25)

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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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Kuliah 1. Proses Pembentukan Urin(Print Hal 8,13,25)

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