Carina of trachea

The tracheal carina is an internal cartilaginous ridge located at the caudal end of the trachea, marking the precise point of its division into the two main bronchi.

The tracheal carina is a median sagittal ridge, formed by the last tracheal cartilaginous ring, which projects into the lumen of the trachea at the site of its bifurcation. It physically separates the openings of the right and the left main bronchus. This structure is composed of hyaline cartilage and is covered by the same respiratory mucosa as the rest of the trachea (pseudostratified ciliated columnar epithelium with goblet cells). Its position is generally at the level of the heart base, often between the 4th and 6th thoracic vertebra depending on the species.

The general morphology of the carina is conserved in most mammals, but its position and associated bronchial ramifications can vary:

• Dog and Cat (Carnivores): In carnivores, the carina is well-defined and generally located at the level of the 4th or 5th thoracic vertebra. In dogs, the presence of a tracheal bronchus (right cranial lobar bronchus) that branches off the trachea before the main carina is an important peculiarity. This bronchus is absent in cats, where the carina marks the division into only two main bronchi.

• Ruminants (Cattle, Sheep): As in dogs, ruminants also possess a tracheal bronchus for the right cranial lobe that emerges from the trachea before the carina. The carina itself is located more caudally in the thorax.

• Horse: In horses, the carina is located more caudally, often at the level of the 6th thoracic vertebra. There is no tracheal bronchus in horses; the carina marks the first division of the trachea into main bronchi.

In humans, the carina is a distinct anatomical structure very similar to that observed in carnivores (without a tracheal bronchus). It is generally located at the level of the 4th or 5th thoracic vertebra. As in animals, the human carina is a very sensitive area and an important reference point in bronchial endoscopy. It plays an indispensable role in the cough reflex and the distribution of air to the lungs. The absence of a tracheal bronchus in humans means that the carina is the only point of division into the right and left main bronchi.