Lung: anatomy, location and function

Lung (pulmo) is a paired organ of the respiratory system that exchanges oxygen and carbon dioxide between air and blood. The lungs lie in the chest, on either side of the central compartment containing the heart, and rest on the diaphragm, the muscular partition between the chest and abdomen.

Where are the lungs located?

The right lung and left lung occupy the right and left sides of the thorax, or chest. Between them lies the mediastinum, the central compartment containing the heart, major blood vessels, trachea and oesophagus. The trachea is the main airway, while the oesophagus carries swallowed food towards the stomach.

Each lung is covered by visceral pleura, a thin membrane attached to its surface. Parietal pleura lines the inner chest wall, diaphragm and sides of the mediastinum. Between these membranes is the pleural cavity, a potential space containing a thin film of fluid that allows the surfaces to slide during breathing.

The apex, or upper tip, extends above the first rib into the root of the neck. The base is the concave lower surface resting on the diaphragm. The costal surface faces the ribs, while the medial surface faces the mediastinum and vertebral column.

What are the lobes and fissures of the lungs?

A lobe is a major subdivision of a lung. Fissures are deep clefts between lobes, lined by visceral pleura, and may be incomplete. The right lung has superior, middle and inferior lobes; the left lung has superior and inferior lobes.

The superior lobe of right lung lies above the horizontal fissure anteriorly and above the oblique fissure posteriorly. The middle lobe lies between the horizontal and oblique fissures, while the inferior lobe of right lung lies mainly behind and below the oblique fissure.

The superior lobe of left lung and inferior lobe of left lung are separated by an oblique fissure. The superior lobe includes the lingula, a tongue-shaped projection below the cardiac notch, the indentation in the anterior border that accommodates the heart.

The right lung is shorter partly because the liver raises the right side of the diaphragm. Both inferior lobes extend prominently onto the posterior surface, so they are not confined to the bottom of the chest.

How do airways enter the lung and support gas exchange?

The hilum is the area on the medial surface where airways, vessels and nerves enter or leave. These structures together form the lung root, the connection between the lung and mediastinum. The root contains a main bronchus, pulmonary vessels, bronchial vessels, lymphatic vessels and nerves.

A bronchus is an airway supported by cartilage. Each main bronchus divides into lobar bronchi supplying the lobes, then into segmental bronchi supplying bronchopulmonary segments. A bronchopulmonary segment is a subdivision of a lobe supplied by its own segmental bronchus and branch of the pulmonary artery.

Smaller branches become bronchioles, narrow airways without cartilage in their walls. These lead towards alveoli, microscopic air spaces surrounded by capillaries, the smallest blood vessels. Across the thin barrier between alveolar air and capillary blood, oxygen enters the blood and carbon dioxide passes into the air for exhalation.

What is the blood supply of the lung?

The lungs receive blood through pulmonary and bronchial circulations, which serve different purposes. A pulmonary artery carries oxygen-poor blood from the right side of the heart towards the lungs. Its branches accompany the airways and supply the capillaries around the alveoli.

Oxygen-rich blood returns towards the left side of the heart through a pulmonary vein. Within the lungs, pulmonary veins generally run between bronchopulmonary segments rather than accompanying the segmental bronchi centrally.

Bronchial arteries, arising from the main systemic arterial circulation, supply the airway walls and supporting tissues. Venous return from these tissues passes partly into bronchial veins and partly into pulmonary veins. Pulmonary vessels therefore support gas exchange, while bronchial vessels nourish lung tissues.

What nerves supply the lungs?

The pulmonary plexuses are networks of nerves around the lung roots. They contain fibres from the vagus nerves and the sympathetic trunks, chains of nerve connections beside the vertebral column.

Parasympathetic fibres, the autonomic fibres carried here by the vagus nerves, narrow the airways and increase glandular secretion. Sympathetic fibres, another division of the autonomic nervous system that regulates involuntary activity, promote airway widening. Sensory fibres carry information from the airways and lungs, including signals involved in breathing reflexes.

Where does lymph from the lungs drain?

Lymph is fluid collected from tissue spaces and carried through lymphatic vessels. The lungs have a superficial lymphatic network beneath the visceral pleura and a deep network accompanying the bronchi and pulmonary vessels.

The superficial network drains to bronchopulmonary nodes at the hila. The deep network drains through pulmonary nodes within the lungs and then to bronchopulmonary nodes. Lymph nodes are small organs that filter lymph. Drainage continues through tracheobronchial nodes near the division of the trachea and paratracheal nodes alongside the trachea, then towards the bronchomediastinal trunks, larger collecting lymphatic vessels.

How can you locate the lungs in the 3D model?

In the 3D model beside this article, rotate and zoom to change the viewing angle and scale. Select a lung and isolate it, or hide neighbouring structures to adjust the view; switch layers on and off, or fade selected structures to change their visibility.

You can also cut a section along the sagittal, coronal or transverse plane, or pull parts apart to change their arrangement in the view. When reviewing lung anatomy, remember that an oblique fissure separates regions lying partly in front of and behind one another, not simply upper and lower halves.