Ultrafiltration
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of the kidney and nephron structure. You can test your knowledge below.
What is the functional unit of the kidney?
The nephron.
What is the knot of capillaries in the Bowman's capsule called?
The glomerulus.
What blood vessel takes blood into the glomerulus?
The afferent arteriole.
Topic Explainer Video
What Is Ultrafiltration?
- Ultrafiltration is the first step in the formation of urine.
- It occurs in the glomerulus + Bowman's capsule.
- It’s a non-selective process that removes small molecules (e.g. water, glucose, urea and amino acids) from the blood under high pressure.
Structure Involved:
1. Glomerulus
- A bundle of capillaries inside the Bowman's capsule.
- Blood enters via a wide afferent arteriole and leaves via a narrow efferent arteriole → builds up high hydrostatic pressure.
2. Bowman’s Capsule
- A cup-shaped structure that collects the filtrate.
- Lined by podocytes (specialised epithelial cells with gaps in between).
Process of Ultrafiltration – Step-by-Step
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Blood enters the glomerulus at high hydrostatic pressure via the afferent arteriole.
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High hydrostatic pressure forces small molecules (such as glucose, amino acids, water and urea) out of the blood across three layers:
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Capillary endothelium (has pores called fenestrations between the cells).
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Basement membrane (acts as the selective filter, blocks large proteins and blood cells).
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Podocyte layer (epithelial cells with pores in between that line the bowman’s capsule).
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Filtrate enters the Bowman's capsule and passes into the nephron tubule.
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Large molecules (e.g. proteins, red blood cells) remain in the blood as they can’t pass through the basement membrane.
Filtrate Composition (vs Blood)
| Substance | Found in Filtrate? | Found in Blood? |
| Glucose | ✓ Yes | ✓ Yes |
| Urea | ✓ Yes | ✓ Yes |
| Water | ✓ Yes | ✓ Yes |
| Proteins | X No | ✓ Yes |
| Red Blood cells | X No | ✓ Yes |
Adaptations of the Glomerulus and Bowman’s Capsule
| Structure | Adaptation for Ultrafiltration |
| Afferent arteriole | Wider than efferent → increases hydrostatic pressure in the glomerulus. |
| Glomerular capillaries | Fenestrated (have pores) → allow small molecules through. |
| Basement membrane | Acts as molecular sieve → blocks proteins/red blood cells. |
| Podocytes | Have pores → allow filtrate to enter capsule. |
| Capillary network | Increases surface area for filtration. |
Key Terms
- Ultrafiltration: The process by which small molecules are forced from the blood into the nephron under high pressure.
- Podocytes: Specialised cells in the Bowman's capsule with.
- Basement membrane: A selective barrier that prevents large molecules like proteins from entering the filtrate.
- Afferent arteriole: Brings blood into the glomerulus.
- Efferent arteriole: Takes blood away from the glomerulus (narrower to maintain pressure).
No answer provided.
Exam Tip
Name the structures involved in filtration (e.g. the fenestrations in the capillary endothelium and the basement membrane), and always relate to high hydrostatic pressure.
Always name two or three substances that are small enough to pass through the filter.
No answer provided.
Describe how ultrafiltration produces glomerular filtrate. (4 marks)
1. High blood/hydrostatic pressure
2. Two named small substances pass out e.g. water, glucose, ions, urea
Accept mineral ions/minerals
Accept amino acids/small proteins
3. Through gaps/pores/fenestrations in capillary endothelium
4. Through basement membrane
5. Large proteins remain in blood.
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
If you want to try out another one, check this video out and see how you do!