Trachea Divisions
The trachea bifurcates at the carina (T4/T5, angle of Louis) into the two main bronchi. Their asymmetry in angle, length, and width determines which lung is preferentially involved in aspiration and foreign body inhalation.
✦ The Mnemonic
"Right Makes Very Short Wide Angles"
Right main bronchus: more vertical, shorter, wider than left — foreign bodies go right
Clinical Breakdown
The trachea extends from C6 (lower border of cricoid cartilage) to T4/T5 (carina). It is 10–12 cm long and 2 cm wide, supported by 16–20 C-shaped cartilaginous rings (open posteriorly — the trachealis muscle spans the gap). The carina is the most sensitive area for triggering the cough reflex.
The right main bronchus is the preferential route for inhaled foreign bodies in adults (more vertical) and for endotracheal tube over-insertion (the ETT tip advances into the right bronchus if too far — causing left lung collapse). Always confirm ETT position on CXR — tip should be 2–4 cm above the carina.
The carina is at the level of the angle of Louis (sternal angle) — also the level of: T4/T5 disc, aortic arch beginning and end, azygos vein entering the SVC, and the 2nd rib. This is one of the most clinically significant surface landmarks.
⭐ Clinical Pearl
Widened carina angle on CXR (>70°) suggests subcarinal lymphadenopathy — most commonly from lung cancer, lymphoma, or sarcoidosis. The normal carina angle is 55–65°. This is assessed on a PA CXR at the bifurcation of the main bronchi, which overlies the cardiac shadow.