Step by Step
1
The Starling forces driving filtration
Filtration at the glomerulus is driven by a balance of pressures. Glomerular hydrostatic pressure (~55 mmHg) pushes fluid OUT. Capsular hydrostatic pressure (~15 mmHg) and blood oncotic pressure (~30 mmHg) push back IN. The net result is about 10 mmHg outward — driving filtration.
2
GFR itself
Glomerular filtration rate (GFR) is normally about 125 mL/minute, which adds up to roughly 180 liters filtered per day — yet only about 1.8 liters becomes actual urine, since about 99% of the filtrate is reabsorbed along the nephron.
3
What the filtrate contains
Filtrate composition is essentially plasma minus proteins and cells — it's protein-free, since the filtration barrier normally prevents large proteins and blood cells from passing through.
4
Autoregulation of GFR
Two mechanisms keep GFR stable despite changes in blood pressure: the myogenic reflex (increased blood pressure causes the afferent arteriole to constrict, protecting the glomerulus and maintaining GFR) and tubuloglomerular feedback (the macula densa senses sodium levels and signals the juxtaglomerular apparatus to adjust afferent arteriole tone accordingly).
Applied Walkthrough
1
In a healthy kidney, glomerular hydrostatic pressure (~55 mmHg) significantly exceeds the combined opposing pressures (capsular hydrostatic ~15 mmHg plus blood oncotic ~30 mmHg), leaving a net filtration pressure of about 10 mmHg driving fluid out of the glomerulus.
2
This produces a GFR of about 125 mL/minute — roughly 180 liters of fluid filtered per day, though the vast majority (about 99%) gets reabsorbed along the nephron, leaving only about 1.8 liters as final urine output.
3
If blood pressure suddenly rises, the myogenic reflex kicks in: the afferent arteriole constricts, protecting the glomerulus from the pressure spike and keeping GFR relatively stable despite the systemic change.
4
Separately, if the macula densa detects unusually high sodium delivery to the distal tubule, tubuloglomerular feedback signals the juxtaglomerular apparatus to adjust afferent arteriole tone — another way the kidney keeps GFR steady even as conditions fluctuate.
Exam Application
Exams test whether you can explain the Starling forces balance driving glomerular filtration, recall the key GFR numbers (125 mL/min, 180 L/day, 1.8 L urine), and distinguish the myogenic reflex from tubuloglomerular feedback as the two autoregulation mechanisms.
⚠ Common Trap
The most common trap is confusing the myogenic reflex with tubuloglomerular feedback — the myogenic reflex responds directly to blood pressure changes in the arteriole wall itself, while tubuloglomerular feedback responds to sodium levels detected by the macula densa — two distinct mechanisms achieving the same stabilizing goal.
✓ Quick Self-Check
1. What is the normal GFR, and how much fluid does this correspond to per day?
About 125 mL/minute, or roughly 180 liters per day.
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2. How much of the filtered fluid actually becomes urine, and what happens to the rest?
About 1.8 liters becomes urine; about 99% of the filtrate is reabsorbed.
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3. What is the net filtration pressure at the glomerulus, and what produces it?
About 10 mmHg outward, from glomerular hydrostatic pressure (55) minus capsular hydrostatic pressure (15) and blood oncotic pressure (30).
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4. What is the myogenic reflex?
A mechanism where increased blood pressure causes the afferent arteriole to constrict, protecting the glomerulus and maintaining stable GFR.
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5. What is tubuloglomerular feedback?
A mechanism where the macula densa senses sodium levels and signals the juxtaglomerular apparatus to adjust afferent arteriole tone.
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