# Glomerulus and Filtration

> In the kidney, blood is filtered in a ball of capillaries called the glomerulus. Water and small solutes are forced through a three-layer filter into Bowman's capsule, while blood cells and large proteins stay behind.

Web page: https://cellnaut.com/topics/nephron-glomerulus-filtration

![Teaching illustration: Glomerulus and Filtration](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/nephron-glomerulus-filtration-teaching.jpg)

## Labelled parts

1. **Afferent arteriole**: Brings blood into the glomerulus.
2. **Efferent arteriole**: Carries filtered blood away; being narrower, it keeps pressure high inside the glomerulus.
3. **Bowman's capsule**: A cup-shaped capsule that surrounds the glomerulus and collects the filtrate.
4. **Glomerular capillary tuft**: A ball of leaky capillaries where blood is filtered under pressure.
5. **Podocyte**: A specialized cell that wraps around the capillaries.
6. **Foot processes and filtration slits**: Interlocking foot processes leave narrow slits that block large proteins.
7. **Red blood cell**: Too large to pass the filter, so normally stays in the blood.
8. **Bowman's space**: The space where the filtrate collects.
9. **Start of proximal tubule**: The filtrate flows on here, where most water and nutrients are reabsorbed.

## Key points

- The efferent arteriole is narrower than the afferent one, which keeps the pressure high enough to filter.
- Podocyte foot processes leave filtration slits that act as the final sieve.
- The filtrate then flows to the proximal tubule, where most water and nutrients are reabsorbed.

## Check yourself

**1. Why do red blood cells normally stay in the blood instead of entering the filtrate?**

- A. They are too large to pass the filtration barrier
- B. They are destroyed in the glomerulus
- C. They are attracted to the podocytes
- D. There is no pressure

**Answer:** A. They are too large to pass the filtration barrier - Blood cells and large proteins cannot fit through the slits.

**2. What structure forms the filtration slits?**

- A. Red blood cells
- B. Podocyte foot processes
- C. The proximal tubule
- D. The afferent arteriole

**Answer:** B. Podocyte foot processes - Neighbouring foot processes interlock and leave slits between them.

## Related lessons

- [The Plasma Membrane and Transport](https://cellnaut.com/learn/plasma-membrane-and-transport)

## See it in 3D

- [Epithelial Cell](https://cellnaut.com/studio/epithelial)