GFR and Starling forces at the glomerulus
Three pressures across the capillary wall set the net filtration pressure, and GFR is Kf times that number. Use them to predict why a kidney stone lowers filtration.
Chapters (10)
Explain how glomerular hydrostatic and oncotic pressures and Bowman's space pressure set GFR, and predict GFR changes when each force changes.
The short version
- Net filtration pressure = glomerular capillary pressure − Bowman's space pressure − glomerular capillary oncotic pressure.
- GFR = Kf × net filtration pressure, so a falling Kf lowers GFR even when every pressure is unchanged.
- In the textbook model the net pressure is about 10 mm Hg (55 − 15 − 30) and falls along the capillary as oncotic pressure rises.
- Ureteral obstruction raises Bowman's space pressure, lowers the net pressure and lowers GFR.
- Dehydration lowers GFR; low plasma albumin, on its own, raises net pressure and filtration.
A stone in the ureter: what changed?
A stone lodges in a ureter, and that kidney's filtration drops. The glomerulus itself is undamaged. So what did the stone change?[3]
This lesson explains GFR and the Starling forces at the glomerulus: how the pressures in and around the glomerular capillary set the glomerular filtration rate, and how to predict what happens when each one moves.[1][3]
The mental model: three Starling forces across the capillary wall
Filtration is a tug of war across the capillary wall. One force pushes fluid out and two push back.[1][4]
| Force | Direction | What it is |
|---|---|---|
| Glomerular capillary hydrostatic pressure | Pushes fluid out | The pressure inside the glomerular capillary |
| Bowman's space hydrostatic pressure | Pushes back | The pressure of the fluid already filtered |
| Glomerular capillary oncotic pressure | Pushes back | Plasma proteins, mainly albumin, hold water inside the capillary |
Oncotic pressure in Bowman's space is about zero, because healthy glomeruli keep protein out of the filtrate.[1][3]
Net filtration pressure and GFR = Kf × NFP
π = oncotic pressure; Bowman's space oncotic pressure is about 0, so it drops out
Kf = filtration coefficient
Kf reflects how much filtering surface there is and how leaky it is.[3][6] So GFR depends on two things: the net pressure, and Kf.[1]
The textbook numbers: 55, 15 and 30
These come from a common textbook model; published values vary between sources. The glomerular capillary pressure is about 55 mm Hg, Bowman's space pressure about 15, and the capillary oncotic pressure at the start of the capillary about 30.[2]
- 55 − 15 − 30
- = 10 mm Hg, the net pressure pushing fluid out
Why the net pressure falls along the capillary
The oncotic pressure does not stay at 30. Water leaves, protein stays behind, so plasma protein concentrates as blood moves along the capillary.[1][5]
In the usual teaching model, the oncotic pressure reaches around 35 by the efferent end, and the net pressure shrinks to about 5 (55 − 15 − 35). The model treats the capillary hydrostatic pressure as roughly constant along the way, so the net pressure declines. The 35 is a teaching value, not a measurement.[1][5]
In rat micropuncture studies the net pressure can reach about zero before the end of the capillary. This is called filtration equilibrium.[5] In humans the pressure profile is not directly measured, so the rat result is an inference. For exams, remember only that net pressure falls along the capillary.
Predicting GFR when one force changes
Change one force and hold the rest.
| Change | What happens to the forces | GFR |
|---|---|---|
| Ureteral obstruction | Urine backs up, so Bowman's space pressure rises and the net pressure falls | Falls |
| Dehydration | Lower blood volume lowers capillary pressure, and concentrated plasma raises oncotic pressure | Falls |
| Low plasma albumin | Oncotic pressure falls, so by the formula the net pressure rises on its own | Rises |
| Glomerular disease that destroys filtering surface | Kf falls, even if every pressure is unchanged | Falls |
Take the stone first. Urine backs up behind it, so the pressure in Bowman's space rises. That is a bigger subtraction, so the net pressure falls and so does the filtration rate. The glomerulus is undamaged; one of the forces around it has moved.[3]
Dehydration, in this simple model, works through two forces at once. Lower blood volume lowers the capillary pressure, and concentrated plasma raises the oncotic pressure. Both lower the filtration rate.[3]
Low plasma albumin lowers the oncotic pressure, which is one of the opposing forces. By the formula, on its own that raises the net pressure and filtration. The words on its own matter: the rule changes one force and holds the rest.[3]
Loss of filtering surface is different, because it changes Kf and not a pressure. Glomerular disease that destroys filtering surface lowers Kf, so the filtration rate falls even if every pressure is unchanged.[6]
- Capillary pressure 60, Bowman's space pressure 10, oncotic pressure 32
- 60 − 10 = 50; 50 − 32 = 18 mm Hg
- Raise Bowman's space pressure to 20: 60 − 20 − 32 = 8 mm Hg
- The net pressure drops from 18 to 8
Three things to keep
- Net filtration pressure is capillary pressure, minus Bowman's space pressure, minus oncotic pressure.
- Filtration rate is Kf times net pressure, so a falling Kf lowers it even with normal pressures.
- Obstruction raises Bowman's space pressure, and that lowers filtration.
The next lesson looks at how the afferent and efferent arterioles change filtration rate, plasma flow and filtration fraction.
Practice question
Original · USMLE styleA 58-year-old man comes to the emergency department because of severe left flank pain and vomiting that began 6 hours ago. Temperature is 37.1 C (98.8 F), pulse is 98/min, and blood pressure is 134/82 mm Hg. Ultrasonography shows left hydronephrosis, and a stone is seen in the proximal left ureter. In a physiology teaching model of his left kidney, the following pressures are estimated at the afferent end of a glomerular capillary:
| Glomerular capillary hydrostatic pressure | 52 mm Hg |
|---|---|
| Bowman's space hydrostatic pressure | 22 mm Hg |
| Glomerular capillary oncotic pressure | 26 mm Hg |
| Bowman's space oncotic pressure | 0 mm Hg |
Common questions
What are the Starling forces at the glomerulus?
Glomerular capillary hydrostatic pressure pushes fluid out. Bowman's space hydrostatic pressure and glomerular capillary oncotic pressure push back.
How is net filtration pressure calculated?
Net filtration pressure = glomerular capillary pressure − Bowman's space pressure − glomerular capillary oncotic pressure. In the textbook model it is 55 − 15 − 30 = 10 mm Hg.
What is the formula for GFR?
GFR = Kf × net filtration pressure. Kf reflects how much filtering surface there is and how leaky it is.
Why does a ureteral stone lower GFR?
Urine backs up, so Bowman's space pressure rises. The net filtration pressure falls, and so does the filtration rate.
What is filtration equilibrium?
In rat micropuncture studies the net pressure can reach about zero before the end of the capillary. In humans the pressure profile is not directly measured, so the rat result is an inference.
Transcript
Read the full narration, by chapter686 words
0:00A stone in the ureter: what changed?
A stone lodges in a ureter, and that kidney's filtration drops. The glomerulus itself is undamaged. So what did the stone change? Today: how pressures in and around the glomerulus set the glomerular filtration rate, and how to predict what happens when each one moves. First, the mental model.
0:20The mental model: three forces across the capillary wall
Filtration is a tug of war across the capillary wall. Pushing fluid out is the hydrostatic pressure in the glomerular capillary. Pushing back are two forces. One is the hydrostatic pressure in Bowman's space, the fluid already filtered. The other is the oncotic pressure of plasma proteins, mainly albumin, which hold water inside the capillary. Oncotic pressure in Bowman's space is about zero, because healthy glomeruli keep protein out of the filtrate. Put them together and you get net filtration
0:53Net filtration pressure and GFR = Kf x NFP
pressure. Glomerular capillary hydrostatic pressure, minus Bowman's space hydrostatic pressure, minus glomerular capillary oncotic pressure. Glomerular filtration rate equals the filtration coefficient, called K f, times that net pressure. K f reflects how much filtering surface there is and how leaky it is. Now the numbers, from a common textbook model.
1:19The textbook numbers: 55, 15 and 30
The glomerular capillary pressure is about 55 mm Hg. Bowman's space pressure is about 15. At the start of the capillary the oncotic pressure is about 30. Fifty-five, minus 15, minus 30, leaves 10 mm Hg, the net pressure pushing fluid out. But the oncotic pressure does not stay at
1:43Net pressure falls along the capillary
30. Water leaves, protein stays behind, so plasma protein concentrates as blood moves along the capillary. By the efferent end the oncotic pressure may be around 35, and the net pressure shrinks to about five. The capillary hydrostatic pressure falls only slightly along the way, so the net pressure declines. In rat micropuncture studies the net pressure can reach about zero before the end of the capillary. That is called filtration equilibrium. In humans we cannot measure it directly, so for exams just remember that net pressure falls along the capillary. Now predict.
2:25Predict: obstruction, dehydration, low albumin, lower Kf
Change one force, and hold the rest. Ureteral obstruction: urine backs up, Bowman's space pressure rises, net pressure falls, and filtration rate falls. That is our stone. Dehydration: lower blood volume lowers capillary pressure, and concentrated plasma raises oncotic pressure. Both lower filtration rate. Low plasma albumin: oncotic pressure falls, so on its own it raises net pressure and filtration. Glomerular disease that destroys filtering surface lowers K f, so filtration falls even if every pressure is unchanged. A quick check with a new set of numbers.
3:06Quick check with new numbers
Capillary pressure 60, Bowman's space pressure 10, oncotic pressure 32. Sixty minus 10 is 50, minus 32 is 18. Now raise Bowman's space pressure to 20. Net pressure drops to eight. Your turn.
3:25Practice question
A 58-year-old man has severe left flank pain and vomiting. Ultrasound shows left hydronephrosis from a ureteral stone. In the left kidney, glomerular capillary hydrostatic pressure is 52 mm Hg, Bowman's space pressure is 22, and the capillary oncotic pressure at the afferent end is 26. Oncotic pressure in Bowman's space is zero. What is the net filtration pressure at the afferent end? A, minus four. B, four. C, 26. D, 48. E, 56. Pause here and decide.
4:09Answer: wrong options first, then the correct one
The correct setup is capillary pressure, minus Bowman's space pressure, minus oncotic pressure. E adds the oncotic pressure, which is a force that opposes filtration, so E is out. D adds Bowman's space pressure instead of subtracting it, so D is out. C leaves out Bowman's space pressure, which is the very thing the stone raised, so C is out. A reverses the subtraction. Filtration is not driven backwards here, so A is out. The answer is B: 52 minus 22 minus 26 is four. Still positive, so the kidney filters, but far less than with a normal Bowman's space
4:52High-yield recap and next lesson
pressure. Three things to keep. One: net filtration pressure is capillary pressure, minus Bowman's space pressure, minus oncotic pressure. Two: filtration rate is K f times net pressure, so a falling K f lowers it even with normal pressures. Three: obstruction raises Bowman's space pressure, and that lowers filtration. Next up: how the afferent and efferent arterioles change filtration rate, plasma flow and filtration fraction.
Sources
Numbers in the text, e.g. [1], point here- Part One: Glomerular Filtration and Tubular Function. LITFL. partone.litfl.com/glomerular_filtration_and_tubular_function.html
- OpenStax Anatomy and Physiology 2e, 25.5 Physiology of Urine Formation. openstax.org/books/anatomy-and-physiology-2e/pages/25-5-physiology-of-urine-formation
- Regulation of Glomerular Filtration Rate (24.3C). LibreTexts, Anatomy and Physiology (Boundless). med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Anatomy_and_Physiology_(Boundless)/24%3A__Urinary_System/24.3%3A_Physiology_of_the_Kidneys/24.3C%3A_Regulation_of_Glomerular_Filtration_Rate
- Glomerular Filtration Pressure. University of Nottingham. nottingham.ac.uk/nmp/sonet/rlos/bioproc/gfr/page_six.html
- Deen WM, Robertson CR, Brenner BM. Dynamics of glomerular ultrafiltration in the rat. J Clin Invest. jci.org/articles/view/107324/pdf
- Brenner and glomerular haemodynamics (memoriam). PMC11543014. pmc.ncbi.nlm.nih.gov/articles/PMC11543014
The pressures used here are a common textbook model; published values vary between sources. Filtration equilibrium is shown in rat micropuncture studies and cannot be measured directly in humans.
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Kidney plate in the opening scene: Servier Medical Art (smart.servier.com), licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/); adapted (recoloured, outlined, resized). All other diagrams are original. Example pressures are teaching values.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-REN-023.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01GFR and Starling forces at the glomerulus5:25
- EP 02Afferent vs efferent arteriole: GFR, RPF and filtration fraction5:16
- EP 03Renal clearance: inulin, creatinine and PAH5:20
- EP 04Glucose transport maximum, splay and SGLT24:55
- EP 05Countercurrent multiplier: how the kidney concentrates urine5:02
- EP 06Aldosterone and ADH in the distal nephron5:20
- EP 07Diuretics by nephron segment: loop, thiazide and more5:53
- EP 08RAAS explained: renin, angiotensin II and aldosterone5:23