Past Papers · SAQ

Renal Tubular Physiology — Glomerular Filtration

Current · V5 (2025) → E1.iv Historical · V4 (2023) → H1.iii 3 exam appearances

2016A Q04

Exam question

Discuss the factors that influence filtration across the glomerular basement membrane.

CICMWrecks answer

Master answer

GFR

GFR:

“The amount of glomerular ultrafiltrate formed divided by the time of filtration”
Normal value is approx 125 mL/min or 180 L/day
Renal blood flow 1.25l/min in 70kg male → Filtration fraction 0.2

Functional anatomy:
3 Distinct Layers:

glomerular capillary endothelium
- highly specialised endothelium with fenestrations to ↓ filter thickness
- prevents cellular components of blood from coming into contact with BM
glomerular BM
- made of CT; -vely charged
- acts as filter
bowmans epithelial cells (podocytes)
- epithelial cells with foot processes → large SA
- negatively charged
- maintain BM + phagocytic functions

Direct GFR Determinants

The GFR (Net flux across the membrane) is the balance of hydrostatic pressure and oncotic pressure, as defined by the Classic Starling Equation.

GFR \; = \; K_f \; \times \; NFP

NFP ={ [(P_{GC} – P_T) – \sigma \; (\pi_{GC} – \pi_T)]}

where
K_f = Filtration coefficient
P = hydrostatic pressure
π = oncotic pressure
σ = Staverman’s reflection coefficient ie. Permeability of membrane to protein

Normally NFP = 17mmHg
Tubular oncotic pressure = zero throughout
GC oncotic pressure varies from 21 to 33 mmHg as filtrate is removed.
Thus less GFR produced at distal end of tubule.

	Afferent end of Glomerular Capillary (mmHg)	Efferent end of Glomerular Capillary (mmHg)
P_GC	60	58
P_T	15	15
π_GC	21	33
NFP	24	10

K_f	Filtration coefficient	= LpS = Hydraulic conductivity x Surface Area Glomerular surface area = 0.8m² • Altered by Mesangial cell contraction (see circulating factors e.g. Angiotensin II → ↓SA → ↓GFR) Patency of the normal capillary wall structure (ie in tubular dysfunction Kf ↑’s ↑GFR)
P_GC	hydrostatic pressure in capillary	relates to • RBF which is autoregulated for MAP 70-170mmHg • relative afferent/efferent arteriolar tone Affected by • Catecholamines • Local autoregulation -> Myogenic -> Tubuloglomerular feedback -> Hormones (see below)
P_T	hydrostatic pressure in tubule	relates to • obstruction to urinary flow (usually pathological) • ↑P_T → ↓ GFR (ie post renal obstructiion causing renal failure)
π_GC	oncotic pressure in capillary	relates to • plasma protein concentration (incr in dehydration, decreased in heart failure) • ↑Systemic plasma oncotic pressure → ↑π_GC → ↓GFR • ↓Renal plasma flow → ↑π_GC → ↓GFR
π_T	oncotic pressure in tubule	• usually zero, but can increase in renal failure/proteinuria • ↑’d π_T → ↑GFR
σ	Staverman’s reflection coefficient	• Permeability of membrane to protein usually 1 (no protein leak) • can decrease with nephritis/proteinuria

Factors influencing GFR

Circulating Factors

Prostaglandins
- PGI₂ and PGE₂ → vasodilates → ↑ renal blood flow → ↑ GFR
- ↓ PGE2, PGI2 and NO levels → inhibits afferent arteriolar vasodilation → ↓ GFR
Long term → ↓ renin release → ↓ ATII-induced efferent arteriolar vasoconstriction → ↓ GFR
Noradrenaline / Adrenaline
- constrict renal afferent and efferent arterioles → ↓ renal blood flow and reducing filtration → ↓GFR
- Constrict mesangial cells to → ↓ GFR.
Mesangial cell tone:
- Angiotensin 2, Endothelin, vasopressin → ↓ glomerular surface area and GFR
- ANP, PGE2, Dopamine and cAMP → all increase GFR

Solute factors

Size
- < 7 kDa (Eg. glucose, ions, urea, H2O) filtered freely,
- > 70 kDa (Eg. albumin) are not filtered;
- neutral particles < 4 mm diameter filtered freely, > 8 mm are excluded
Charge
- Negatively charge (Anionic) particles repulsed
- Cationic substances filtered more readily
Protein binding
- Albumin excluded

Disease Factors

Filtration decreases
- Shock → decreased glomerular pressure
- Obstruction → increased bowman’s capsule hydrostatic pressure
- Hypoproteinaemia → hepatic failure, nephrotic syndrome

JC / Gladwin / Sakurai / Kerr 2020

Reusable content

Formulae used in this answer

GFR — filtration coefficient relationship

GFR = Kf × net filtration pressure

CWF-0110

GFR \; = \; K_f \; \times \; NFP

Net filtration pressure

CWF-0111

NFP ={ [(P_{GC} - P_T) - \sigma \; (\pi_{GC} - \pi_T)]}

Answer by Gladwin, JC, Kerr, Sakurai

Past papers

Exam appearances

3 appearances

Exam	Exact exam wording	Candidate success
2016A Q04	Discuss the factors that influence filtration across the glomerular basement membrane.	38%
2014A Q23	What is the Glomerular Filtration Rate (GFR)? Discuss the physiological factors that can influence it.	40%
2009A Q13	Outline the factors that determine the composition and volume of glomerular filtrate in a normal person.	30%