Subjects and lessons to explore.

Law

Human Physiology II · Systems Physiology

Glomerular Filtration

7 min read
13 key termsReport an error
Want it in plain words first? Jump to Eli explains — the same idea, no jargon.
On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

Glomerular filtration is the first step of urine formation: blood pressure forces plasma through a three-layer barrier—, , and podocyte slit diaphragms—that blocks cells and most proteins while passing water and small solutes. Net filtration pressure is the balance of glomerular capillary hydrostatic pressure against the opposing oncotic and capsular pressures. The kidneys keep nearly constant across a wide blood-pressure range through the myogenic reflex and , and GFR can be measured with or estimated with .

Why this matters

GFR is the single most important measure of kidney function in the clinic. A 24-hour urine collection for creatinine clearance, or estimated GFR (eGFR) computed from a serum creatinine value, is used to stage chronic kidney disease and to dose drugs that the kidneys eliminate. A low can indicate reduced glomerular pressure, while glomerular damage that lets albumin into the urine (albuminuria) is an early sign of kidney disease. This is educational context, not guidance: laboratory reference ranges, staging criteria, and medication dosing vary by institution and jurisdiction and must be interpreted by qualified clinicians.

The college version

1. The Filtration Barrier

Three layers separate glomerular blood from Bowman's capsule. The fenestrated endothelium has pores that let water and small solutes pass but block blood cells. The basement membrane (glomerular basement membrane) is a negatively charged mesh of collagen and glycoproteins that blocks most plasma proteins, partly by size and partly by charge. The outermost layer, the , are epithelial cells whose foot processes interlock with thin slit diaphragms between them; these slits are the final size-and-charge screen. Together they allow molecules smaller than ~7 kDa through freely and almost entirely exclude albumin and larger proteins.

2. Starling Forces in the Glomerulus

Filtration is driven by the same physical forces that govern capillaries everywhere, but the glomerulus is unusual because its hydrostatic pressure is high and nearly constant along its length. The forces are: glomerular capillary hydrostatic pressure (PGC, ~55 mmHg, pushing fluid out), Bowman's capsule hydrostatic pressure (PBS, ~15 mmHg, opposing), glomerular capillary oncotic pressure (πGC, rising from ~0 to ~30 mmHg as filtration concentrates proteins, opposing), and Bowman's space oncotic pressure (πBS, ~0 mmHg, negligible). Net filtration pressure is NFP = PGC - PBS - πGC, about 10 mmHg at the afferent end, falling toward zero at the efferent end (filtration equilibrium).

3. GFR, Filtration Fraction, and Autoregulation

Glomerular filtration rate (GFR) is the volume of filtrate formed per minute (~125 mL/min, ~180 L/day). Filtration fraction is the fraction of renal plasma flow that becomes filtrate: FF = GFR / RPF ≈ 0.2. GFR is held steady by : the (vascular smooth muscle contracts when stretched by higher pressure, and relaxes when pressure drops) and tubuloglomerular feedback (the macula densa senses NaCl delivery; high NaCl triggers afferent arteriolar constriction, low NaCl triggers dilation). These keep GFR stable over mean arterial pressures of roughly 80–180 mmHg.

How it works

  1. Cardiac output delivers blood to the glomerulus at high pressure.
  2. Favorable Starling forces push plasma through the endothelium, basement membrane, and podocyte slits.
  3. A protein-free filtrate collects in Bowman's capsule and flows into the tubule.
  4. Myogenic and tubuloglomerular feedback adjust afferent (and efferent) arteriolar tone to keep GFR constant.
  5. Clearance of inulin (or creatinine) is used to quantify GFR in the laboratory.

Common confusions

Do not confuseWithDifference
FiltrationReabsorption / secretionFiltration is bulk, nonselective movement into the tubule; reabsorption returns to blood; secretion adds to tubule
GFRRenal plasma flowGFR is the filtered portion; RPF is the total plasma flow (GFR is only ~20% of RPF)
Filtration fractionFiltration coefficientFF is a ratio (GFR/RPF); Kf is a permeability×area property of the barrier
Myogenic mechanismTubuloglomerular feedbackMyogenic is intrinsic stretch response; TGF uses the macula densa signal
InulinCreatinineBoth estimate GFR, but inulin is inert while creatinine is slightly secreted

Memory aids

"P.B.P." — Pressure drives it, Barrier blocks it, Podocytes polish it. Net filtration = Push (capillary pressure) minus Pull (oncotic) minus Back (capsule pressure): "Push minus Pull minus Back = Net."

Quick review

Topic Recap

Glomerular filtration produces a cell- and protein-free ultrafiltrate driven by glomerular hydrostatic pressure against oncotic and capsular pressures, through a three-layer barrier. GFR, about 125 mL/min, is defended by myogenic autoregulation and tubuloglomerular feedback, and is measured clinically with creatinine clearance or estimated from serum creatinine. Filtration fraction ties GFR to renal plasma flow, linking the filter's output to the blood supply feeding it.

Knowledge Check

  1. What are the three layers of the glomerular filtration barrier, in order from blood to capsule?
  2. Which Starling force is high and nearly constant in the glomerulus (unusual for a capillary)?
  3. Define filtration fraction and give its typical value.
  4. How does the myogenic mechanism protect GFR when blood pressure rises?
  5. Why is inulin clearance equal to GFR?

Answers and Rationales

  1. Fenestrated endothelium → basement membrane → podocytes (with slit diaphragms)—each adds a size or charge barrier.
  2. Glomerular capillary hydrostatic pressure (~55 mmHg)—unlike most capillaries, it stays high along the whole length, favoring continuous filtration.
  3. Filtration fraction = GFR ÷ RPF ≈ 0.2—about one-fifth of renal plasma flow becomes filtrate.
  4. The myogenic mechanism makes the afferent arteriole constrict when stretched by higher pressure, reducing PGC and keeping GFR stable.
  5. Inulin is freely filtered but neither reabsorbed nor secreted, so the amount filtered equals the amount excreted—its clearance therefore equals GFR.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

The glomerulus is a very fine sieve with holes big enough for water, salts, glucose, and wastes to fall through but too small for blood cells and most proteins. Blood pressure is the push that squeezes this fluid out, while proteins left behind in the blood pull water back—a tug-of-war that decides how fast the sieve drips. The body works hard to keep that drip rate steady: if pressure rises, the kidney's own muscles tighten the entrance to the sieve so it doesn't leak too fast.

This is like a mesh bag under a running faucet—push more, flow more. It stops being exact because the glomerulus's "holes" are not static pores; they are living cells with a negative charge that actively repel many proteins, and the kidney can actively adjust the flow in either direction.

Simple Example

If your blood pressure jumps after exercise, a kitchen strainer would pour faster, but a healthy kidney barely changes its output—GFR stays around 125 mL/min. It "turns down" the faucet itself instead of overflowing.

Worked example

  1. Blood enters the glomerulus through the afferent arteriole; the narrow efferent arteriole downstream keeps glomerular capillary hydrostatic pressure high.
  2. High PGC pushes water and small solutes through the three-layer barrier; cells and most proteins stay behind, so πGC climbs along the capillary.
  3. Net filtration pressure is the driving gradient: GFR = Kf × NFP, where Kf is the filtration coefficient (the product of capillary surface area and hydraulic permeability) and NFP = (PGC - PBS) - (πGC - πBS).
  4. When blood pressure rises, the myogenic reflex constricts the afferent arteriole, and when more NaCl reaches the macula densa it reinforces that constriction (tubuloglomerular feedback), lowering PGC back toward normal.
  5. When blood pressure falls, these mechanisms reverse—the afferent arteriole dilates and the efferent arteriole may constrict—restoring PGC and protecting GFR.

Clearance estimates how fast the kidneys clear a substance from plasma: Cx = Ux × VPx, where Ux is urine concentration of the substance, V is urine flow rate (mL/min), and Px is plasma concentration. Inulin, a plant polysaccharide, is freely filtered and neither reabsorbed nor secreted, so its clearance equals GFR. Creatinine, a muscle-metabolism waste product, is freely filtered and only slightly secreted, so creatinine clearance closely approximates GFR and is used clinically.

Key takeaways

  • High yield: GFR ≈ 125 mL/min ≈ 180 L/day, but over 99% is reabsorbed—typical urine output is only 1–2 L/day.
  • High yield: The filtration barrier is three-layered: endothelium, basement membrane, podocytes (slit diaphragms).
  • High yield: The glomerulus is hydrostatically dominated—high, constant PGC unlike other capillaries where oncotic forces dominate.
  • GFR rises with increased afferent dilation or efferent constriction, and falls with the reverse.
  • Autoregulation holds GFR near-constant from ~80 to 180 mmHg mean arterial pressure.
  • Inulin clearance = GFR (gold standard); creatinine clearance ≈ GFR (practical surrogate).

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Describe the three layers of the glomerular filtration barrier and what each layer excludes.
  • Explain the Starling forces across the glomerular capillary and how they determine net filtration pressure.
  • Define glomerular filtration rate (GFR) and filtration fraction, and relate them to renal plasma flow.
  • Contrast the myogenic mechanism and tubuloglomerular feedback in autoregulating GFR, and explain how inulin and creatinine clearance estimate GFR.

Key vocabulary

Fenestrated endothelium
Inner capillary layer with pores
Basement membrane
Negatively charged mesh under the endothelium
Podocytes
Epithelial cells with interlocking foot processes
Slit diaphragm
Thin bridge between podocyte foot processes
Net filtration pressure (NFP)
Net force pushing fluid out of the capillary
Glomerular filtration rate (GFR)
Volume filtered per minute (~125 mL/min)
Filtration fraction
Fraction of renal plasma flow filtered
Autoregulation
Kidney's ability to hold GFR steady
Myogenic mechanism
Stretch-driven arteriolar constriction
Tubuloglomerular feedback
Macula densa sensing NaCl to adjust afferent tone
Clearance
Volume of plasma cleared of a substance per minute
Inulin
Freely filtered, inert polysaccharide
Creatinine
Muscle-derived waste product

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.