What happens to the horses trachea when it gallops?

It dilates in response to chemicals released during exercise, relaxing the smooth muscle layer of the tracheal wall, and allowing expansion with more air voume. The walls of the trachea also contain C-shaped rings of cartilage which prevent it from collapsing. So, the trachea remains open and acts as a pageway for air between the larynx and the bronchi of the lungs. The bronchi also dilate during galloping, also in response to chemicals secreted when autonomic nervous system signals ociated with the "fight or flight" stress response are activated.