The Lower Respiratory Tract

Hi :) In this post we'll take a look at the anatomy of the structures within the lower respiratory tract. This includes the trachea, lungs, pleura and diaphragm. 



Trachea

The trachea extends from the cricoid cartilage of the larynx to the tracheal bifurcation. It is supported by C-shaped rings of cartilage which prevent the trachea from collapsing and also allow the dilation of the oesophagus during deglutition. The rings open dorsally and the gap in the “c” shape is closed by the tracheal muscle. The rings of cartilage are connected by bands of fibroelastic tissue.

The trachea is lined by respiratory mucosa which is composed of pseudostratified ciliated epithelium. The cilia continually drive mucous and debris cranially towards the pharynx. 

Within the neck, the external aspect of the trachea is lined with loose connective tissue known as adventitia and this connects the trachea to surrounding structures. The caudal (recurrent) laryngeal nerve passes within this adventitia. The trachea passes ventral to the cervical spine and longus colli muscle until it reaches the thoracic inlet. This part of the trachea maintains a median position in relation to the oesophagus. Ventrally, it is connected to the long hyoid muscles, while the common carotid artery and vagosympathetic trunk pass on its lateral sides. 

Within the mediastinum of the thorax, the trachea deflects to the right where it crosses the aortic arch. Dorsal to this part of the trachea is the cranial vena cava, aortic arteries and oesophagus. The mediastinal lymph nodes and thymus (in young animals) lie ventrally.

At the fifth intercostal space, the trachea bifurcates to give off the primary bronchi. Three primary bronchi branch on the right hand side, while two branch in the left. This forms the pattern for the lobes of the lung.

The Lungs

Form

The lungs can be divided into lobes. In the dog, the right lung has cranial, middle, caudal and accessory lobes while the left lung has cranial and caudal lobes. However, in the left lung there are two portions to the cranial lobe: the cranial portion of the cranial lobe and the caudal portion of the cranial lobe.

There are differences in form between species, with the pig and ruminants having an extra branch of the trachea known as the tracheal bronchus. This branches to the right above the tracheal bifurcation and opens into the cranial lobe of the right lung. In addition, carnivores have deeper fissures in between their lobes while in species such as the horse and cow these fissures are quite shallow. This is due mainly to the flexibilty of the thorax in these animals.

Blood Supply

The pulmonary arteries enter the lungs at the tracheal bifurcation and generally follow the bronchi through the lungs. The pulmonary veins don’t always follow the bronchial tree and may run separately. The pulmonary arteries carry deoxygenated blood to the lungs while the pulmonary arteries carry oxygenated blood back to the heart. The lungs receive a nutritional blood supply from the broncho-oesophageal artery and vein.

Lymph Nodes

Lymph drains into the tracheobronchial lymph nodes which drains into the mediastinal lymph nodes which empty into the thoracic duct.

Innervation

Parasympathetic and sympathetic fibres innervate the lungs and are derived from the pulmonary plexus within the mediastinum. Efferent fibres supply the bronchial glands and muscles as well as the blood vessels. Afferent fibres originate from the mucosa and stretch receptors.  

Pleura

There are two pleural cavities in the thoracic cavity which are located on the right and left of the mediastinum. These cavities are lined with pleura which is a serous membrane. The pleura extends from the thoracic wall to cover the lungs and also lines the diaphragm and mediastinum. There are two main sections of pleura: parietal and visceral. The visceral pleura surround the lungs and may be called the pulmonary pleura. The parietal pleura surround the other structures, it includes costal pleura (which lines area formed by the ribs), the mediastinal pleura (covering the mediastinum), and the diaphragmatic pleura (lining the diaphragm).

Between the parietal and visceral pleura is a narrow space filled with serous fluid. This space has a negative pressure which keeps the lungs “stuck” to the walls of the thorax.

Diaphragm

The diaphragm separates the thoracic and abdominal cavities and is dome shaped. It has a triangle-shaped central tendon and a muscular periphery. The periphery includes the left and right crura which are connected to the ventral aspect of the first three or four lumbar vertebrae by tendons. The right crus is larger than the left crus. The costal part of the muscular periphery is much thinner.

The diaphragm has three main openings: the aortic, oesophageal hiatuses and the caval foramen. The aortic hiatus is the most dorsal and the aorta, azygous vein and thoracic duct pass through it. The oesophageal hiatus lies more ventrally and allows the oesophagus, the dorsal and ventral vagal trunks that accompany it and the vessels that supply the oesophagus into the abdominal cavity. The caval foramen lies within the central tendon and provides an entry point for the caudal vena cava into the abdominal cavity.