Air enters the respiratory system through the mouth and nose, where it is warmed and moistened.
Air breathed in through the nose is filtered by the coarse hairs that line the nostrils, which trap large dust particles. This mucus is continuously moved away by the beating of minute hair-like projections called cilia. At the entry to the windpipe is a flap, the epiglottis, which closes to prevent choking when food is swallowed. At its base, the windpipe divides into two tubes or bronchi, and one bronchus enters each lung. Both windpipe and bronchi are stiffened by rings of cartilage.