The eustachian tube serves to make sure equal pressure on both sides of eardrum so it is not impeded in its movments. Leave the mechanical and enter the world of fluid dyanamics. Utricle is involved in vestibular function and will not be considered now. About pea size in humans. Coiled, triple chambered, tube.
Fluid is moved by action of stapes.
The round window bulges when stapes is moved. The tectorial membrane is moved by the vibrations going up and down the cochlea. As vibration of membrane occurs, the OHC wave in the wind, concentrating and amplifying the movements of the tectorial membrane. OHC have low connectivity to the optic nerve. IHC are different.