Basic Science Part 2: Starling Forces

Starling equation for GFR

GFR = Kf × [(P_GC − P_BS) − (π_GC − π_BS)]

Which Starling force FAVORS filtration?

Glomerular capillary hydrostatic pressure (P_GC) — the only favoring force

Which Starling forces OPPOSE filtration?

Bowman's hydrostatic pressure (P_BS) and glomerular oncotic pressure (π_GC)

Value of Bowman's space oncotic pressure (π_BS) and why

≈ 0, because essentially no protein is filtered → nothing to osmotically pull water

P_GC increases → GFR?

GFR ↑ (stronger outward push into Bowman's space)

P_BS increases → GFR?

GFR ↓ (back-pressure opposes filtration) — NOT reverse flow, just counter-pressure

π_GC (plasma oncotic) increases → GFR?

GFR ↓ (proteins pull/hold water back in the capillary)

π_GC (plasma oncotic) DECREASES → GFR?

GFR ↑ — less protein = weaker inward pull on water = more filtered (INVERSE relationship)

What does oncotic pressure actually do? (common trap)

Proteins (albumin) osmotically PULL WATER toward them / hold it in the capillary — it does NOT "block solutes from crossing"

Which protein is mainly responsible for plasma oncotic pressure?

Albumin (most abundant plasma protein)

NEPHROTIC SYNDROME: full Starling chain

Damaged barrier → albumin lost in urine → plasma albumin ↓ → π_GC ↓ → GFR ↑ (also: systemic edema)

Nephrotic syndrome: which force changes and GFR direction?

π_GC decreases → GFR INCREASES (Practice Q3)

Nephrotic syndrome: why edema? (separate effect)

Low plasma albumin → low oncotic pressure in SYSTEMIC capillaries → fluid leaks into tissues → generalized edema

URETERAL STONE: which force changes and GFR direction?

Urine backs up → P_BS increases → GFR DECREASES

Where is a kidney stone located (relative to nephron)?

Downstream in the urine outflow tract (ureter/pelvis/calyces) — NOT in the glomerulus (that's a blood vessel)

Efferent constriction (Ang II): which Starling force changes?

↑ P_GC (blood backs up in glomerular capillary = "kink in the hose") → GFR ↑

Does raising GFR raise blood pressure? (key distinction)

NO. GFR ↑ preserves FILTRATION. BP is raised separately by vasoconstriction + aldosterone/ADH salt & water RETENTION

Kf definition and what changes it

Filtration coefficient (permeability × surface area); DECREASED by disease (diabetic nephropathy, glomerular damage) → GFR ↓

3 layers of the filtration barrier (in order)

1) Fenestrated endothelium, 2) Basement membrane, 3) Filtration slits (podocyte foot processes)

Two "gates" of glomerular filtration

Size (basement membrane blocks large molecules) + Charge (negative glycoproteins repel negative albumin)

Why does albumin normally stay OUT of urine?

It's negatively charged and repelled by the negatively charged barrier (plus it's large)

Are small solutes (Na, K, glucose) affected by the barrier's charge?

No — too small; freely filtered regardless of charge. Charge only matters for large molecules like albumin

Filtration Fraction equation and normal value

FF = GFR / RPF ≈ 20%

What does FF = 20% mean?

Only 20% of plasma entering the glomerulus is filtered; 80% continues to peritubular capillaries

Consequence of INCREASED FF

More water filtered out → blood leaving glomerulus more protein-concentrated → ↑ oncotic pressure in peritubular capillaries → ↑ Na/water reabsorption in PCT

Which change increases FF?

Efferent constriction (GFR ↑ AND RPF ↓ — the only clean opposite-direction split)

Afferent constriction effect on FF

No change (GFR and RPF both fall together)