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.


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.

Pleural Cavity and Membranes

Two pleural sacs line thoracic cavity, contacting in the medial plane to form the mediastinum. The pleura that covers the diaphragm (diaphragmatic pleura) reflects on itself to become the costal pleura that lines the ribs, this line of pleural reflection follows the attachment of the diaphragm. Craniodorsal to the line of pleural reflection the diaphragmatic and costal pleura are in contact, separated by potential space known as the costodiaphragmatic recess.


The mediastinum contains the heart and all other thoracic organs (heart, oesophagus, trachea, aorta, thymus) except the lungs (which are within the pleural sacs) and the caudal vena cava. In regions where the mediastinum contains no organs, it is thin and may be fenestrated. Fenestrations are mainly caudal to the heart. This part of the mediastinum is pushed to the left due to the mass of the larger right lung.


The diaphragm is a musculotendinous structure that separates the thoracic and abdominal cavities. Ii inserts on the thoracic wall in a curve from the 8th and 9th costal cartilages, across costochondral junctions of the 9th-15th ribs to the middle of the 18th rib; where it turns craniomedially to end at vertebral end of the last rib. The dome of the diaphragm bulges forward to 7th-8th intercostal space, thus is almost opposite the olecranon in the standing horse. There are three openings in the diaphragm as follows:

  • Aortic hiatus: Between the right and left crura, dorsal diaphragm. The aorta passes through this opening.
  • Oesophageal hiatus: Located in the right crus, the oesophagus passes through here.
  • Caval foramen: Located in the muscular centre of the diaphragm, the caudal vena cava passes through here.