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. The composition of endolymph is unique, containing high levels of potassium and low levels of sodium. This ionic balance is essential for the proper functioning of sensory cells, known as hair cells, which convert sound waves and movement into electrical signals that the brain interprets.