The endolymph of the inner ear is secreted primarily by which structure, and what is its ionic composition compared to perilymph?

• A Stria vascularis; high K+, low Na+ (endolymph) vs low K+, high Na+ (perilymph) ✓
• B Reissner's membrane; high Na+, low K+ (endolymph) vs high K+, low Na+ (perilymph)
• C Spiral ligament; equal concentrations of K+ and Na+ in both fluids
• D Organ of Corti; high Na+, high K+ (endolymph) vs low Na+, low K+ (perilymph)

Explanation

Endolymph is secreted by the stria vascularis and has a unique ionic composition — high K+ (~150 mEq/L) and low Na+ (~1 mEq/L) — making it electrically positive (+80 mV, the endocochlear potential). Perilymph resembles extracellular fluid with high Na+ and low K+. This ionic gradient is essential for hair cell mechanotransduction; K+ flows into hair cells down its electrochemical gradient when stereocilia are deflected.

Reference: Dhingra Diseases of Ear, Nose and Throat, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.