Inner Ear fluids - Perilymph and Endolymph

💧 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

📌 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