Equine Lower Respiratory Tract - Horse Anatomy

Bronchi and Bronchioli

Bronchi divide, becoming smaller in diameter and forming what is known as the 'bronchial tree'. The right and left principle/primary bronchi arise from the trachea and enter the right and left lung, respectively at the hilus. Within the lungs, they divide into lobar/secondary bronchi which supply each lung lobe; they are named according to the lobe supplied. These subsequently further divide into segmental/tertiary bronchi which ventilate each bronchopulmonary segment within the lobe. Bronchopulmonary segments are cone-shaped sections of pulmonary tissue, with the apex pointing towards the bronchus and the base towards the free surface of the lung, within each lung lobe. The bronchi are lined with respiratory mucosa and are supported by hyaline cartilage.

Bronchioli arise from the segmental bronchi, they are smaller in diameter and have no cartilagenous support. True bronchioli branch to form terminal bronchioli, which end in alveoli.

Lungs

The left and right lungs lie within their pleural sac and are only attached by their roots, to the mediastinum, so they are fairly free within the thoracic cavity.

The lungs are divided into lobes by the bronchial tree:

• Left lung
  • Cranial and caudal lobes, the cranial lobe is not subdivided into cranial and caudal parts
• Right Lung
  • Cranial, caudal and accessory lobes. The middle lobe is absent.

The right lung is larger than the left, as it includes the accessory lobe. Although lobulated, the lobes are not divided by deep fissures in the horse; making the lobulation indistinct. The cranial and caudal lobes are only separated by the cardiac notch. Part of mediastinal surface of both lungs fuses caudal to hilus.