Anatomy of the Thorax

The thorax is divided by a transverse plane at the angle of Louis (T4/5) into the superior and inferior mediastinum. The latter can be subdivided into the anterior, middle and posterior compartments.

Ribs

There are a total of 12 ribs:

– Ribs 1 to 7 attached to sternum by costal cartilage – “true ribs”

– Ribs 8 to 10 attached to the rib above by costal cartilage – “false ribs”

– Ribs 11 and 12 are so-called “floating ribs,” as do not articulate with the sternum anteriorly

Airways

The airway starts with the trachea which begins, beneath the cricoid cartilage (C6).

Trachea –> main bronchi –> lobar bronchi –> segmental bronchi  –> conducting bronchiole –> terminal bronchiole –> respiratory bronchiole 

Lungs:

The lungs sit in the thoracic cavity and are split into different lobes by fissures.

– The right lung is shorter, wider and more vertical – hence a foreign body is more likely to pass down this way.

The lungs have specific surface markings which are useful when auscultating the different parts.

– Apex –> 2.5cm above medial third of clavicle

– 2nd costal cartilage –> lungs from both sides meet

– 4th costal cartilage –> left lung diverges to accommodate the heart

– 6th costal cartilage –> lungs lie opposite mid-clavicular line

– 8th costal cartilage –> lungs cross mid-axillary line

– 10th rib –> lungs reach posteriorly on both sides.

Bronchopulmonary Segments

Each lung is split into 10 bronchopulmonary segments

– Upper lobe – S1-3                                 

– Middle lobe – S4-5                            

– Lower lobe – S6-10

– In a supine patient, a foreign body is more likely to enter right apical lower lobe segment

– In standing patient, a foreign body is more likely to enter a basal segment of lower lobe 

Pleura

Both lungs are covered by 2 pleural layers which have a closed intrapleural space, needed for breathing

– Visceral layer – this is the layer that lines the lungs

– Parietal layer – lines the thoracic wall, upper diaphragm and sides of the mediastinum.

 Neurovascular supply and lymphatics

 Blood supply

– Pulmonary arteries -> supply deoxygenated blood for gas exchange

– Bronchial arteries -> supply oxygenated blood to the bronchial tree

Nervous supply

– Vagus -> cause bronchoconstriction: ACh on M₃ receptor – Gq coupled (parasympathetic)

– Thoracic sympathetic chain -> cause bronchodilation – Na on B₂ receptor – Gs coupled (sympathetic)

Lymphatic supply

The lymph drainage of the lungs runs along the bronchi and under the pleurae.

– These converge on lymph nodes at the lung hila (bronchopulmonary nodes)

– These then drain to nodes around the carina (tracheobronchial nodes)

– This drains to mediastinal nodes –> terminate into the thoracic duct/right lymphatic duct

In the superior mediastinum, the key features are the major blood vessels and thoracic duct.

Aortic arch:

This gives off three major branches:

– On the right -> brachiocephalic trunk (divided into right subclavian and right common carotid artery)

– On the left -> left common carotid and subclavian arteries

SVC:

The subclavian and internal jugular vein unite forming the brachiocephalic vein at sternoclavicular joint

– The SVC is then formed by the union of the brachiocephalic veins behind the first costal cartilage

– This drains into the right atrium at level of 3rd costal cartilage

Thoracic duct:

This enters the junction of the left subclavian and left internal jugular vein