By the end of this lesson
- Explain the pressure basis of ultrafiltration.
- Describe the filtration barrier.
- Compare blood plasma with glomerular filtrate.
BIG QUESTION
How can pressure separate small plasma solutes from cells and proteins without choosing each molecule individually?
A high-pressure capillary bed and size-selective barrier produce filtrate; selectivity by usefulness happens later.
The glomerulus operates at high pressure
Blood enters through an afferent arteriole and leaves through a narrower efferent arteriole, helping maintain high hydrostatic pressurePressure exerted by a liquid, such as blood pressure in the glomerulus, which forces small molecules out of the capillaries. in glomerular capillaries.
This pressure drives fluid and small solutes out of blood; it is not active transport.
A filtration barrier excludes large components
Fenestrated capillary walls, a basement membrane and podocyteA cell of the inner wall of the Bowman's capsule with foot-like extensions and gaps between them that let filtrate pass through. filtration slits form a size-selective barrier. Water, ions, glucose, amino acids and urea pass into Bowman’s capsule.
Blood cells and most large plasma proteins remain in circulation.
Filtrate is not yet urine
glomerular filtrateThe liquid filtered out of the blood into the Bowman's capsule, containing water, glucose, salts and urea but not cells or large proteins. resembles plasma without cells and most proteins. It contains useful substances as well as waste.
Selective reabsorption later returns required molecules and much water to blood; final urine therefore has a different composition.
Blood, filtrate and urine
Presence or absence reveals where each process acts.
| Component | Glomerular filtrate | Normal final urine |
|---|---|---|
| Blood cells | Absent. | Absent. |
| Large plasma proteins | Mostly absent. | Absent/trace. |
| Glucose | Present. | Normally absent. |
| Urea | Present. | Present and concentrated. |
| Water and ions | Present. | Present in variable amounts. |
Language that earns precision
- ultrafiltration
- Pressure-driven filtration of small substances from glomerular blood.
- hydrostatic pressure
- Pressure exerted by a fluid against a surface.
- podocyte
- Specialised Bowman’s-capsule cell forming filtration slits.
- glomerular filtrate
- Fluid entering Bowman’s capsule after ultrafiltration.
Common misconceptions
Claim: The glomerulus actively chooses useful molecules.
Correction: Initial filtration is mainly size/pressure based; selective recovery happens in tubules.
Claim: Normal filtrate contains no glucose.
Correction: Glucose is small enough to filter and is later reabsorbed.
Claim: High pressure forces blood cells into the capsule.
Correction: The intact filtration barrier retains cells and large proteins.
Match the answer to the command word
Pressure → filtration barrier → small molecules pass, cells/proteins remain.
Do not call filtrate urine before reabsorption.
Recall, apply, then justify
Explain ultrafiltration in Bowman’s capsule.
Hint
Pressure, barrier and size.
Suggested marking points
- Glomerular blood has high hydrostatic pressure.
- Pressure forces fluid across the filtration barrier.
- Water and small solutes enter Bowman’s capsule.
- Blood cells remain in capillaries.
- Most large plasma proteins remain in blood.
State four substances normally found in glomerular filtrate.
Hint
Include useful solutes and waste.
Suggested marking points
- Water.
- Mineral ions.
- Glucose or amino acids.
- Urea.
Predict and explain the effect of damage that makes the glomerular barrier permeable to albumin.
Planning hint
Track protein into filtrate and water balance.
What a strong design includes
- Albumin would enter glomerular filtrate.
- Protein could appear in urine if not fully reabsorbed.
- Plasma protein concentration/oncotic pull could fall.
- More fluid may remain in tissues, contributing to oedema.
- Urinary protein can therefore indicate filtration-barrier damage.
- The prediction does not identify a single cause without clinical evidence.