---
title: "Inner Ear fluids - Perilymph and Endolymph"
description: "Quick notes on the fluids of the inner ear: endolymph and perilymph - useful for medical students and ENT residents."
source: "The ENT Resident"
canonical_url: "https://www.theentresident.com/ent-notes-lectures/perilymph-endolymph"
categories: ["Otology", "Differences Tables in ENT"]
video: "https://www.youtube.com/watch?v=4l18KLA0jS8"
image: "https://media.theentresident.com/og-images/perilymph-endolymph.png"
---

# Inner Ear fluids - Perilymph and Endolymph

Quick notes on the fluids of the inner ear: endolymph and perilymph - useful for medical students and ENT residents. Watch the full video lecture on YouTube: https://www.youtube.com/watch?v=4l18KLA0jS8

## 💧 Inner Ear Fluids — Endolymph & Perilymph

In this post, we'll cover a short but important topic — **the two fluids of the inner ear**, their **differences**, **composition**, and **how they are formed and absorbed**.

### 🔹 Overview

The **inner ear** contains two main fluids:

- **Perilymph**
- **Endolymph**

They differ in **composition**, **location**, and **origin**.

---

### 🌀 Perilymph

- **Composition**: Resembles **extracellular fluid** → *Rich in sodium ions (Na⁺)*  

- **Location**: Fills the space **between the bony and membranous labyrinth**

- **Connection with CSF**:

  - Communicates with **cerebrospinal fluid (CSF)** via the **aqueduct of cochlea**
  - This duct opens into the **scala tympani**, near the round window
  - It contains **connective tissue** resembling arachnoid, through which perilymph percolates

**💭 Theories of Formation**

1. **Filtrate of Blood Serum**  

   → Formed by **capillaries of the spiral ligament**

2. **Direct Continuation of CSF**  

   → Reaches the labyrinth via the **aqueduct of cochlea**

---

### 🌊 Endolymph

- **Composition**: Resembles **intracellular fluid**  
  → *Rich in potassium ions (K⁺)*

- **Location**: Fills the **entire membranous labyrinth**

**🧪 Secreted By:**

- **Stria vascularis** of the cochlea

- **Dark cells** of:
  - **Utricle**
  - **Ampullated ends** of semicircular ducts

**💭 Theories of Flow**

1. **Longitudinal Flow**  

   - Endolymph from cochlea travels through **ductus reuniens → saccule → utricle → endolymphatic duct**

   - Absorbed by the **endolymphatic sac** in the **subdural space**

2. **Radial Flow**  

   - Endolymph is secreted and absorbed by the **stria vascularis** itself

   - Assumes that the **endolymphatic sac is vestigial** in humans and plays no role in absorption

⚠️ *Note: These two views are still debated — both are important to know.*

---

### 📊 Comparison Table

| Feature | Endolymph | Perilymph |
| --- | --- | --- |
| Location | Inside membranous labyrinth | Between bony and membranous labyrinth |
| Ionic Composition | High K⁺ (Potassium) | High Na⁺ (Sodium) |
| Resembles | Intracellular fluid | Extracellular fluid / CSF |
| Protein Content | Lower | Higher |
| Glucose Content | Lower | Higher |
| Formation | Secreted by stria vascularis & dark cells | Either from blood (spiral ligament) or CSF |
| Absorption | By endolymphatic sac or stria vascularis | Percolates through cochlear aqueduct |

---

### 📌 Summary

- **Perilymph**: Sodium-rich, surrounds membranous labyrinth, resembles CSF  
- **Endolymph**: Potassium-rich, fills membranous labyrinth, essential for hair cell function  
- Both fluids have **distinct origins and roles** in hearing and balance

---

## About This Note

This note on Inner Ear fluids - Perilymph and Endolymph is one of the free ENT study notes published by The ENT Resident. It is free to read on the website, and it is paired with a full video lecture on YouTube.

This note is categorized under [Otology](https://www.theentresident.com/ent-notes-lectures/category/otology.md) and [Differences Tables in ENT](https://www.theentresident.com/ent-notes-lectures/category/differences-tables-in-ent.md). Each category page lists all the free ENT notes on that topic.

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