Which fluid fills the semicircular canals?

• A. Perilymph
• B. Synovial fluid
• C. Aqueous humor
• D. Endolymph

Answer: (D)

In the vestibular system, the semicircular canals are filled with endolymph, a fluid that lies inside the membranous labyrinth. When the head rotates, the endolymph moves and pushes on the cupula within the ampullae, bending the hair cells and sending signals that help detect angular motion. This fluid has a distinctive composition, with a high potassium concentration, which is important for the way hair cells transduce movement into neural signals.

For reference, perilymph fills the space between the membranous labyrinth and the surrounding bony labyrinth, aiding in mechanical transmission but not filling the semicircular canals themselves. Other bodily fluids like synovial fluid and aqueous humor live in joints and the eye, respectively, and aren’t involved in the function of the inner ear’s semicircular canals.