(EN-doh-lim-FA-tik sak)

A balloon-shaped structure filled with fluid in the inner ear. The fluid in the endolymphatic sac surrounds and protects the structures in the inner ear that affect hearing and balance. When pressure in the inner ear builds, the endolymphatic sac releases fluid to relieve the pressure. The endolymphatic sac also removes extra fluid and waste products from the inner ear and is involved in the inner ear’s immune response to foreign substances.