#5897 - Anatomy Of The Thoracic Cavity - Cardiorespiratory system

Anatomy of the Thoracic Cavity

Thoracic Cavity

The thoracic cavity is cone shaped, with the thoracic inlet situated at the apex. Its walls are formed by the thoracic vertebrae dorsally, the ribs and costal cartilages laterally, and the sternum (consisting of the manubrium, xiphoid process and sternebrae) ventrally. It is separated from the abdomen by the diaphragm.

The thoracic skeleton consists of 13-18 thoracic vertebrae (species dependent), a corresponding number of ribs articulating with 2 successive vertebrae and the sternum.

Ribs

Costae are arranged in pairs and articulate with 2 successive vertebrae (costovertebral joint – ball and socket). More caudally, there may be a tubercle for articulation with the transverse processes of more caudal vertebra (costotransverse joint). Costae join ventrally on the midline at the sternum. The sternebrae is the main body joining ribs on the midline. True ribs articulate directly with the sternum, whilst false ribs articulate indirectly through a connection with the cartilage of the costae directly cranial. The last rib may not have contact with its neighbour, and is termed a floating rib.

Individual ribs have a dorsal bony part, a body of rib and a ventral cartilaginous part (costal cartilage).

Intercostal Muscles

External and internal intercostal muscles occupy the intercostal spaces.

External intercostal mm. lie superficially and their fibres run caudoventrally between the ribs (but not between costal cartilages). Their main action is inspiratory – when contracted, they move the ribcage up and outwards expanding the thoracic cavity.

Internal intercostal mm. lie deep to the external intercostal muscles, and their fibres run cranioventrally. They have an expiratory action (interosseus mm.), but are usually only active during exercise. Their second component, the interchondral mm., have an inspiratory action.

Pleura

The pleura is a serous membrane that lines the thoracic cavity. It consists of a single flat layer of epithelium with and underlying propria (connective tissue).

Each lung is invested by a pleural membrane called the visceral pleura, so there are 2 pleural membranes arranged as closed sacs. The space in between the left and right pleural sac is the mediastinum. The visceral pleura derives from the splanchnic mesoderm.

The visceral pleura is continuous with the mediastinal pleura, which in turn in continuous with diaphragmatic and costal pleura. The mediastinal, costal and diaphragmatic pleura are called the parietal pleura. The parietal pleura effectively lines the thoracic cavity, and is derived from the somatic mesoderm.

The narrow space between the parietal and visceral pleura is called the pleural cavity. It contains a small amount of serous fluid spread over the surface of the pleura, which creates adhesion. The pressure within the pleural cavity is sub-atmospheric. The pleura translates changes in thoracic volume into changes in lung volume.

The costodiaphragmatic recess in the space between costal and diaphragmatic pleura.

Mediastinum

The mediastinum is the partition between the left and right pleural sacs. It contains …

• Heart (pericardial sac)

• Thoracic trachea

• Thoracic oesophagus

• Lymph nodes

• Caudal vena cava

• Right phrenic nerve

And other structures depending on species.

Diaphragm

The diaphragm is a dome-shaped musculotendinous sheet that separates the thoracic and abdominal cavities. The muscular part is peripheral, and the tendinous part is central. The muscle attaches to the xiphoid process and the inside of the lower costal cartilages and ribs, and the vertebral column. The diaphragmatic crura are attached to the lumbar vertebrae.

It receives motor and sensory innervation from the phrenic nerve.

The diaphragm is the major muscle of inspiration. When it contracts, it flattens and moves downwards, increasing the volume of the thoracic cavity.

The diaphragm has 3 openings …

• Aortic haitus – the most dorsal opening. Contains the aorta, azygous vein and thoracic duct.

• Oesophageal hiatus – contains the oesophagus, dorsal and ventral vagal trunks.

• Caval foramen – lies within the central tendinous region of the diaphragm and contains the caudal vena cava. This opening does not allow movement, as the diaphragm is fused with the vessel wall.

Left and Right Thoracic Cavities

Both the left and right thoracic cavities contain …

• Sympathetic trunk

• Aorta

• Brachiocephalic trunk

• Oesophagus

• Vagal nerve

• Laryngeal recurrent nerve

• Trachea

• Vena cava

• Lymph nodes and thoracic duct

• Pericardium with heart

The left thoracic cavity only contains the thymus. The right thoracic cavity only contains the azygous vein.

Lungs

Each lung is contained within a pleural sac. They are attached by the roots only to the mediastinum. The right lung is always larger than the left due to the positioning of the heart. The cranial point of the lungs is termed the apex. Each lung has a concave base relating to the face of the diaphragm.

In most species, the lungs are divided into lobes by the bronchial tree. The left lung has a cranial and caudal lobe, whilst the right lung has a cranial, caudal, middle and accessory lobe (although this pattern varies a little between species).

The majority of lung tissue is provided by bronchi and bronchioles, pulmonary vessels, peribronchial and perivascular connective tissue. The terminal bronchioles have alveoli scattered along their length, and are continued by alveolar ducts, alveolar sacs and finally alveoli.

Nervous supply to the lung is via the pulmonary plexus.

Trachea, Bronchi and Bronchioles

The trachea leads from the pharynx through the visceral space of the neck and enters the mediastinum at the thoracic inlet. It bifurcates into 2 principle bronchi above the heart, as approximately the 4^th-6^th intercostal space.

The trachea is supported by C-shaped rings of hyaline cartilage (being dorsally incomplete). The ends of the incomplete rings are joined by the trachealis muscle, whilst the edges are joined by sheets of elastic connective tissue continuous with the perichondrium. The tracheal walls are composed of a number of layers. The innermost layer is a mucosal layer, followed by a fibrocartilagenous middle layer (the incomplete cartilage rings or strips), and then the adventitia (in the neck) or serosa (in the thorax).

The 2 principle bronchi divide into 2 lobar bronchi for the left lung and 4 lobar bronchi for the right lung. These divide further into smaller bronchi and bronchioles within the tissue.

The structure of the larger bronchi is the same as that of the trachea. On the smaller bronchi the C-shaped cartilage rings are gradually replaced by irregular plaques of cartilage. The lumen of the bronchus is lined by a pseudostratified epithelium, including goblet and serous cells. Beneath the epthielial layer is a connective tissue layer, then well-developed spiral bands of smooth muscle.

Bronchioles have no cartilage at all, and this defines the bronchobronchiolar division. The bronchioles have instead well-developed muscular walls to enclose the lumen. The epithelium is reduced to cuboidal, and doesn’t contain mucus secreting glands. There are many lymphoid nodules beneath the epithelial layer.

Alveoli

Alveolar ducts lead from the bronchioles to the alveoli. They have alveoli which open on all their sides, and so have no walls as such. Openings to individual alveoli are guarded by smooth muscle.

The alveolar sacs are rotunda-like areas on the end of each alveolar duct, and are usually present in clusters.

Alveoli are minute, polygonal chambers. Their walls are extremely thin, consisting of 2 irregular layers of epithelial sheets with a layer of capillaries in between. The alveolar interstitium is formed from connective tissue fibres and cells, which include collagen fibrils and elastin fibres.

The Heart

The heart is the central organ that pumps blood through the vascular system by rhythmic contraction. It is located in the thoracic cavity, with 60% of it lying to the left of the median plane. Size of the heart varies between species. It is cone-shaped...