Final answer:
The three semicircular ducts, situated in three spatial planes, contain hair cells in the ampulla which respond to fluid movement caused by head rotation, providing our sense of balance by sending signals to the brain.
Step-by-step explanation:
The semicircular ducts are a critical component of the vestibular system that give us our sense of balance. There are three semicircular canals arranged orthogonally to each other, often described as being set in three spatial planes. One canal lies horizontally (the lateral or horizontal canal), and the other two (the anterior and posterior canals) are oriented roughly at 45-degree angles to the sagittal plane. The canals are fluid-filled and at the base of each canal, there is an expanded region called the ampulla which contains a cluster of hair cells. The hair cells have stereocilia that extend into a gelatinous structure known as the cupula. When the head moves, the fluid within the canals shifts, this causes the cupula to deflect and bend the stereocilia of the hair cells, sending signals to the brain indicating the direction and speed of the rotation.
Our brains interpret signals from all three canals to discern our orientation and movement within three-dimensional space, thus maintaining balance. The horizontal canal is primarily responsive to rotation of the head around a vertical axis, like when shaking the head 'no', while the anterior and posterior canals detect movements in other planes, such as nodding 'yes' or tilting the head to the side. This entire system allows the body to maintain equilibrium and coordinate movement based on changes in head and body position.