The trachea, or windpipe, courses from the larynx into the thorax, where the trachea bifurcates into the right and left primary (principal or mainstem) bronchi at the T4–T5 vertebral level (Figure 3-2B). The right primary bronchus divides into superior, middle, and inferior secondary (lobar) bronchi, corresponding to superior, middle, and inferior lobes of the right lung, respectively. The left primary bronchus divides into superior and inferior secondary bronchi, corresponding to superior and inferior lobes of the left lung, respectively. Each secondary bronchus further divides into tertiary (segmental) bronchi, which further divide. The smallest bronchi give rise to bronchioles, which terminate in alveolar sacs where the exchange of gases occurs.