Endolymph is a fluid found within the inner ear, specifically in the cochlea and the vestibular system. It plays a crucial role in hearing and balance by transmitting sound vibrations and helping the body detect changes in position and motion. This fluid is rich in potassium ions and low in sodium ions, which is essential for the proper functioning of sensory cells called hair cells. These hair cells convert mechanical stimuli into electrical signals, allowing the brain to interpret sounds and maintain equilibrium.