The endolymph in the membranous labyrinth has an ionic composition similar to which fluid, and the perilymph is similar to which fluid?

• A Endolymph is similar to plasma (high Na+, low K+); perilymph is similar to intracellular fluid
• B Endolymph is similar to intracellular fluid (high K+, low Na+); perilymph is similar to extracellular/CSF (high Na+, low K+) ✓
• C Both endolymph and perilymph have identical ionic compositions similar to CSF
• D Endolymph has high Na+ and high protein; perilymph has high K+ and low protein

Explanation

Endolymph is unique among extracellular fluids: it has high K+ (~150 mEq/L) and very low Na+ (~1 mEq/L), similar to intracellular fluid, and generates the endocochlear potential (+80 mV) by the stria vascularis. Perilymph has high Na+ (~140 mEq/L) and low K+ (~5 mEq/L), resembling CSF/extracellular fluid. This ionic difference is essential for hair cell mechanotransduction — K+ entry through stereocilia mechanotransduction channels depolarizes hair cells. In Meniere's disease, endolymphatic hydrops (excess endolymph) disrupts this balance.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.