These animals can have a variety of problems contributing to their breathing problems such as narrow nostrils, excessive soft tissue in the mouth and throat like the soft palate and relatively large tongues and they also tend to have very narrow tracheas (windpipes) compared to normal animals of a similar size. (vetsagainstbrachycephalism.com)
But it can persist much longer in flat - faced breeds or dogs with narrow tracheas (windpipes). (2ndchance.info)
These include narrow nostrils, excessive soft tissue in the mouth and throat, everted saccules in the larynx and a very narrow trachea or windpipe compared to animals of similar sizes. (k9magazine.com)