Which statement best describes the ionic composition of cochlear fluids?
- A Endolymph resembles extracellular fluid, high sodium and low potassium
- B Endolymph has high potassium and low sodium, similar to intracellular fluid ✓
- C Perilymph has high potassium and low calcium, similar to endolymph
- D Endolymph and perilymph have identical electrolyte compositions
Correct answer: B. Endolymph has high potassium and low sodium, similar to intracellular fluid
Explanation
Cochlear endolymph, secreted by the stria vascularis, is the only extracellular fluid in the body with an intracellular-like ionic profile: approximately 150 mmol/L potassium and very low sodium. This produces the endocochlear potential of about +80 mV in scala media, which drives current through hair cell transducer channels. Perilymph, filling scala vestibuli and scala tympani, is ordinary extracellular fluid rich in sodium, which eliminates option A.
Reference: Ganong Review of Medical Physiology, 26th ed.
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Written and medically reviewed by the StethoPrep medical team.