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Section 2 Glomerular Filtration Section 2 Glomerular Filtration

Urine forming begins with filtration from the glomerular capillaries into Bowman`s capsule of a large amount of fluid that is virtually free of protein. Most substances in the plasma, except for proteins, are freely filtrated so that their concentration in the glomerular filtrate in Bowman`s capsule are almost the same as in the plasma

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Section 2 Glomerular Filtration Section 2 Glomerular Filtration

Protein

Glucose

RBC

H2O

Minerals

Urea

glomerular filtration

reabsorption

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Test of Filtration FluidTest of Filtration FluidIn the Bowman`s capsule Filtration is the

ultrafiltration fluid of palsma.

Composition Plasma (g%) Initial urine

Protein 70.0 micro amount

Glucose 1.0 1.0

Amino acids 0.5 0.5

Minerals 7.0 7.0

H2O 920 980

Urea 0.3 0.3

The Glomerular Gapillary The Glomerular Gapillary Membrane ( filtration membrane )Membrane ( filtration membrane ) Is similar to that of

other capillaries, excepted that it has three major layers forming the physical barrier.

Being negatively charged coated to forming the

electric barrier.

The Glomerular Gapillary Membrane The Glomerular Gapillary Membrane ( filtration membrane )( filtration membrane )

The physical barrier

The electric barrier

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The Physical BarrierThe Physical Barrier

The endothelium of the capillary: Perforated by thousands of small

holes called “ fenestrae ” , diameter is about 70-90 nm.

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The Physical BarrierThe Physical Barrier

Basement membrane:

Which consists of a meshwork of collagen and proteoglycan fibrillae, diameter is about 2-8 nm, however, diameter of plasma albumin is 3.6 nm.

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The Physical BarrierThe Physical Barrier

Footlike processes ( Podocytes ) :

The foot processes are separated by gaps called “ slit pores ”, diameter is about 4-11 nm.

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The Electric Barrier The Electric Barrier The endothelial pores, basement

membrane and surface coat of the podocytes contain negatively charged glycoproteins which impede the passage of negatively

charged plasma proteins.Such as plasma albumin.

Protein filtration:

influence of negative charge on glomerular wall

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Filtration Membrane Filtration Membrane Damaged or InjuryedDamaged or Injuryed

Acute glomerulonephritis

→ → obstruction of glomerular

capillary → → filtration

membrane area ↓ → →

amount of filtrate fluid ↓

→ → oliguria

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Two Important Concepts Two Important Concepts GFR: The quantity of glomerular filtrate formed

per minute by both kidneys. The GFR in an average-sized man is approximately 125 ml/min.

This value can

using calculates

the clearance of

inulin in kidneys,

which is the golden

standard, as we discuss later.

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The glomerular filtration fraction

The glomerular filtration rate

The total renal plasma flow

=

125ml/min

660ml/min≈ 20%

=

FF: The Glomerular Filtration Fraction

Fraction is the filtration rates as percentage of the total renal plasma flow that passes through both kidneys.

Effective Filtrated Pressure

Figure 26.10a, b

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Forces Controlling FiltrationForces Controlling FiltrationGlomerular capillary hydrostatic

pressure:PGC

Bowman's space hydrostatic

pressure:PBS

Glomerular capillary colloid osmotic pressure: GC

Effective filtration pressure=PGC – (PBS + GC )

Forces Controlling FiltrationForces Controlling Filtration

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Factors Influence GFR

(1)PGC is glomerular capillary hydrostatic pressure:

BP fluctuation between in 80~180mmHg →→ maintained the stable of RBF →→ the RBF does not vary in the denervated →→ GFR stabilize ( Autoregulation )

BP fluctuation between in 80~180mmHg →→ maintained the stable of RBF →→ the RBF does not vary in the denervated →→ GFR stabilize ( Autoregulation )

When BP the lowing 80mmHg in the hypovolumic shock, out the autoregulation , oliguria appearanced because of GFR decreasing.

But the lowing 40 ~ 50 mmHg , GFR is zeor ,anuria can be seen.

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Factors Influence GFR

(2)PBc is Bowman's space hydrostatic pressure

-It is increased by constriction of the ureters.

-Increases in PBc cause decreases in net filtration

pressure and GFR. - In normal condition , the Bowman’s space pressure (PBC) do not change significantly except in renal disease or obstruction of the urinary tract by urinary stone or tumor.

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Factors Influence GFR (3) πGC : glomerular capillary colloid osmotic

pressure. It normally increases along the length of the glomerular capillary because filtration of water increases the protein concentration of glomerular capillary blood. It is increased by increases in protein concentration. Increases in πGC cause decreases in net filtration A decrease in plasma protein concentration due to

intravenous infusion of a large volume of saline will decrease intracapillary osmotic pressure leading to an elevated GFR.

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Factors Influence GFR(4) Renal Blood Flow : change the “ balance point of filtration”

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Factors Influence GFR

(5) Kf: filtration coefficient

GFR

Kf = ———— is a constant

EFP (effective filtration pressure)

Related to filtration membrane’s permeability and area.