Endolymph, also known as Scarpa fluid, is a clear fluid that can found in the membranous labyrinth of the inner ear. It is unique in composition compared to other extracellular fluids in the body due to its high potassium ion concentration (140 mEq/L) and low sodium ion concentration (15 mEq/L). Many tissues play key roles in the production and maintenance of the ionic composition of endolymph, including the Reissner membrane, stria vascularis, and dark cells of the vestibular organs. Endolymph is separated from surrounding perilymph by the Reissner membrane that forms a barrier between the two fluids. The Reissner membrane allows for selective ion transport and ultimately the production of endolymph from perilymph. Another important tissue involved in the production of endolymph is the stria vascularis found lining the lateral wall of the cochlear duct. The cells found in this tissue help to maintain the high membrane potential and potassium ion concentration of endolymph. Dark cells present in the cristae ampullaris of the semi-circular canals make use of the sodium/potassium ATPase pump to actively pump potassium into the endolymphatic fluid. The ion concentration created and maintained by these various tissues results in endolymph having a high positive potential relative to perilymph. The potential gradient created between the two fluids allows for high sensitivity to sound waves that result in depolarization and nerve transmission to the brain for interpretation. Many tissues play a role in the regulation and reabsorption of endolymph; however, these processes are not entirely understood.