L20: Glomerular Filtration And Tubular Transport

Question 1

Sieving mechanism of renal corpuscle

Answer 1

1. Size
2. Charge (-ve charge basement membrane repel -ve charge albumin protein)

—> only allow water, ions, small molecules but NOT large protein to pass through

Question 2

Proteinuria

Answer 2

Due to:

• Fusion / Collapse of podocyte foot processes
• Splitting of glomerular basement membrane

Potential problem: >150mg / 24hours, 10mg / 100ml
Nephrotic syndrome: Proteinuria >3.5g / 24 hours

Question 3

Net filtration pressure and factors causing changes

Answer 3

Glomerular capillary hydrostatic pressure - Bowman’s capsule hydrostatic pressure - Glomerular capillary oncotic pressure

Factors: (↑ Pa, ↑ Pv, ↓ Ra, ↑ Rv —> ↑ Net filtration pressure)

1. Vasodilation of afferent arteriole
2. Vasoconstriction of efferent arteriole

Question 4

Glomerular filtration rate (GFR)

Answer 4

***Total amount of filtrate formed per min (Normal: 90-140 ml/min)
Not applicable: proteinuria (high GFR but damaged kidney)

Depends on:

1. Net filtration pressure
2. Permeability
3. Surface area available for filtration

Question 5

Stages of kidney failure (GFR)

Answer 5

Normal (Stage 1): 90-140
Stage 2: 60-90
Stage 3: 30-60
Stage 4: 15-30
End stage: 0-15

Question 6

What is excretion

Answer 6

Excretion = Filtration + Secretion - Reabsoption

Question 7

Renal clearance

Answer 7

***Volume of plasma cleared of a substance in a given time period

Clearance = Excreted amount / concentration in body

Substance that is not reabsorbed nor secreted can be used to determine GFR:
1. Inulin: freely filtered, not reabsorbed, not secreted

2. Creatinine: freely filtered, not reabsorbed, slightly secreted —> over-estimation of GFR (實際filter的是較少) —> therefore called estimated GFR (eGFR)
   - also affected by age, muscle mass, food intake etc.

Question 8

Reabsorption pathways in renal tubules

Answer 8

1. Transcellular route (apical membrane —> cytoplasm —> basolateral membrane —> interstitium —> capillary):
   - Primary active transport
   - Secondary active transport
   - Facilitated diffusion
   - Osmosis
2. Paracellular route: leaky tight junction
   - Diffusion e.g. urea, Ca, K

Question 9

Transporter in tubules

Answer 9

Apical:

1. Co-transporter: SGLT2 (Na in + glucose in)
2. Exchanger: NHE3 (Na in / H out)
3. Exchanger: Pendrin (Cl in / HCO3 out)

Basolateral:

1. Uniporter: GLUT2 (glucose out)
2. ATPase: Na/K-ATPase (Na out/K in)
3. Channal: Cl channel (Cl out)

Question 10

Reabsorption direction of ions:

Answer 10

In:

• glucose
• Na
• Cl

Out:

• K
• H+
• HCO3-

Question 11

Tubular transport maximum

Answer 11

Maximum rate for reabsorption of substances via transporters (All available carriers are occupied)

For filtration: no upper limit (100% filtered)

e.g. Glycosuria: amount of glucose > transport maximum in proximal tubule
—> glucose in urine