Lobes and fissures
Each lung has an apex, a base on the diaphragm, costal and mediastinal surfaces, and anterior, posterior and inferior borders, and is divided into lobes by fissures. The apex rises into the root of the neck above the medial third of the clavicle.
- Oblique fissure: on both lungs, from about the spine of T3 or T4 behind, round and down to the sixth costal cartilage in front.
- Horizontal fissure: on the right lung only, along the fourth costal cartilage from the sternum back to meet the oblique fissure near the midaxillary line.
| Feature | Right lung | Left lung |
|---|---|---|
| Lobes | Upper, middle, lower | Upper, lower |
| Fissures | Oblique and horizontal | Oblique only |
| Shape | Shorter and wider, raised by the liver | Longer and narrower |
| Anterior border | Straight | Cardiac notch, with the lingula below |
| Mediastinal impressions | Superior vena cava, azygos arch, oesophagus | Deep cardiac impression, aortic arch and descending aorta |
Hilum and root
The root of the lung is the bundle of structures joining the lung to the mediastinum; the hilum is the area on the mediastinal surface where they enter. The root contains the main bronchus, pulmonary artery, two pulmonary veins, bronchial vessels, the pulmonary nerve plexus and lymph nodes, wrapped in a sleeve of pleura. Below the root the sleeve hangs down as the pulmonary ligament.
The arrangement differs slightly between the sides:
- Right: the upper lobe bronchus (the eparterial bronchus) leaves the main bronchus above the pulmonary artery. The artery lies in front of the main bronchus, and the veins are the most anterior and inferior.
- Left: the pulmonary artery is the highest structure, arching over the bronchus. The bronchus lies behind and below, and the veins are anterior and inferior.
On both sides the veins are lowest and most anterior, and the bronchus is the most posterior. The phrenic nerve passes in front of the root and the vagus behind it. The azygos vein arches over the right root, and the aortic arch over the left.
Bronchopulmonary segments
A bronchopulmonary segment is the part of a lung supplied by one segmental (tertiary) bronchus and its accompanying branch of the pulmonary artery. Segments are pyramidal, with the apex towards the hilum. Tributaries of the pulmonary veins run between them, so a diseased segment can be removed on its own.
| Lobe | Right lung | Left lung |
|---|---|---|
| Upper | Apical, posterior, anterior | Apicoposterior, anterior, superior lingular, inferior lingular |
| Middle | Lateral, medial | |
| Lower | Superior, medial basal, anterior basal, lateral basal, posterior basal | Superior, anteromedial basal, lateral basal, posterior basal |
The right lung has ten segments. The left is usually described as having eight to ten, depending on whether the apicoposterior and anteromedial basal segments are counted as one or two.
Blood supply and innervation
The lungs have two arterial supplies: pulmonary arteries bring deoxygenated blood for gas exchange, and bronchial arteries bring oxygenated blood to nourish the airways and supporting tissue.
The pulmonary trunk divides under the aortic arch at about the level of the sternal angle. The right pulmonary artery is longer and passes behind the ascending aorta and superior vena cava. The left pulmonary artery is shorter, and it is attached to the underside of the arch by the ligamentum arteriosum. Each follows the bronchial tree. Two pulmonary veins on each side carry oxygenated blood to the left atrium.
Bronchial arteries usually arise from the descending aorta on the left and from a posterior intercostal or left bronchial artery on the right. Part of the bronchial venous blood returns to the azygos system and part to the pulmonary veins.
The pulmonary plexuses at the roots carry vagal fibres, which constrict the bronchi and increase secretion, and sympathetic fibres, which dilate them. The visceral pleura and lung tissue have no pain fibres.
Clinical relevance
The geometry of the bronchial tree decides where aspirated material goes and where signs are heard.
- Aspiration: when upright, material tends to enter the right lower lobe, because the right main bronchus is wider and more vertical. When supine, it enters the superior segment of the right lower lobe or the posterior segment of the upper lobe.
- Auscultation: upper lobes are heard mainly in front, lower lobes mainly behind, and the right middle lobe only in front on the right, between the fourth and sixth ribs.
- Apical (Pancoast) tumour: can invade the lower brachial plexus and the sympathetic chain, causing hand weakness and Horner's syndrome.
- Hoarseness: a left hilar tumour or nodes can compress the left recurrent laryngeal nerve where it hooks under the arch.
- Segmentectomy and lobectomy follow the segmental and lobar planes.
On the specimen
This station shows isolated lungs, so side each one before naming the lobes and hilar structures. Hold the lung with the apex up and the hilum facing you: the sharp anterior border is in front and the rounded posterior border faces the spine. Two fissures and three lobes mean a right lung; a cardiac notch and lingula mean a left lung.
- Bronchus: thick-walled, pale, with palpable cartilage plates; it holds its shape and is the most posterior structure at the hilum.
- Pulmonary artery: thin-walled, usually collapsed, above or in front of the bronchus.
- Pulmonary veins: the lowest and most anterior vessels, two on each side.
- Middle lobe: the wedge between the horizontal and oblique fissures on the right, visible from the front.
- Left main bronchus: longer and more horizontal than the right, as it passes under the aortic arch.
Grooves on the mediastinal surface confirm the side: an aortic groove on the left, azygos and superior vena cava grooves on the right.