Tracheal bifurcation

The tracheal bifurcation is the point where the trachea divides into two main bronchi, marking the transition between the upper and lower respiratory tracts.

The tracheal bifurcation is the caudal division of the trachea into the right and left main bronchi. This division is characterized by an internal cartilaginous ridge called the carina, which is visible endoscopically. It is located at the level of the heart base, generally at the height of the 4th or 5th thoracic vertebra, although its exact position may vary slightly depending on the species and the phase of respiration.

The position and angle of the tracheal bifurcation can vary slightly between species:

• Dog and Cat (Carnivores): In carnivores, the tracheal bifurcation is generally located at the level of the 4th or 5th thoracic vertebra. The bifurcation angle is relatively acute. In dogs, there is often a tracheal bronchus (or right cranial lobar bronchus) that branches off the trachea before the main bifurcation, supplying the right cranial lung lobe. This tracheal bronchus is absent in cats.

• Ruminants (Cattle, Sheep): In ruminants, the tracheal bifurcation is also located caudally in the thorax. As in dogs, cattle and sheep possess a tracheal bronchus for the right cranial lobe, which branches off the trachea before the main bifurcation.

• Horse: In horses, the tracheal bifurcation is located more caudally than in carnivores, often at the level of the 6th thoracic vertebra. There is no tracheal bronchus in horses.

In humans, the tracheal bifurcation is generally located at the level of the 4th or 5th thoracic vertebra, similar to that of carnivores. The bifurcation angle is also relatively acute. Unlike some animal species (dogs, ruminants), humans do not have a tracheal bronchus branching off before the main bifurcation; the two main bronchi (right and left) are the first branches of the trachea. The human carina is also a very sensitive structure, playing a crucial role in the cough reflex.