Final answer:
Endolymph fills the cochlear duct in the cochlea, with high potassium and low sodium ion concentrations, playing a key role in hearing.
Step-by-step explanation:
Endolymph is the fluid that fills the cochlear duct in the cochlea of the inner ear, containing high K+ (potassium ions) and low Na+ (sodium ions) concentrations. The cochlear duct is a key structure within the auditory portion of the inner ear, housing the organ of Corti, which is crucial for the transduction of sound waves into electrical signals that can be interpreted by the brain. Fluid movements within the cochlea, involving the endolymph in the cochlear duct as well as the perilymph in the adjacent scala tympani and scala vestibuli, are essential for the stimulation of the mechanoreceptor hair cells located in the organ of Corti.