Anatomy · Respiratory · Thorax

Lung Anatomy

Also known as: lung, pulmonary

The lungs are two spongy, cone-shaped organs filling most of the chest on either side of the heart. Air reaches them through a branching tree of airways ending in about 300 million alveoli, whose combined surface, roughly the size of a tennis court, is where oxygen enters the blood and carbon dioxide leaves it.

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Key facts

LobesThree on the right (upper, middle, lower), two on the left (upper, lower) with a lingula
AirwaysTrachea, main bronchi, lobar and segmental bronchi, bronchioles, alveolar ducts, alveoli
Segments10 bronchopulmonary segments per lung, each with its own bronchus and artery
Blood supplyPulmonary arteries (for gas exchange) and bronchial arteries (for the lung tissue itself)
Nerve supplyVagus nerve (constricts airways, mucus) and sympathetic fibres (dilate airways)
BreathingAbout 12-16 breaths a minute at rest, 500 mL each

Location and pleura

Each lung sits in its own pleural cavity, wrapped in visceral pleura that is continuous with the parietal pleura lining the chest wall, diaphragm and mediastinum. The thin film of fluid between the two layers lets the lung slide as the chest expands and holds it against the chest wall by surface tension, so that when the diaphragm descends the lung is pulled open. Air entering this space (a pneumothorax) breaks the seal and the lung collapses. The right lung is larger and shorter, pushed up by the liver; the left has a cardiac notch that makes room for the heart.

Lobes and segments

Fissures divide the right lung into upper, middle and lower lobes and the left into upper and lower lobes. Each lobe is built from bronchopulmonary segments, pyramids of lung tissue with their own segmental bronchus and artery and no connection to their neighbours, which is why surgeons can remove one segment and radiologists describe a shadow by segment.

Airways

The trachea divides at the carina into right and left main bronchi. The right is wider and more vertical, so inhaled objects usually go that way. Each main bronchus divides into lobar, then segmental bronchi, then some twenty further generations of ever-smaller bronchi and bronchioles. Cartilage rings stiffen the larger airways; smooth muscle in the bronchioles adjusts their calibre, tightening in asthma. Ciliated cells and mucus sweep dust and microbes upward to be swallowed. Beyond the terminal bronchioles, respiratory bronchioles and alveolar ducts open into clusters of alveoli.

Alveoli and gas exchange

An alveolus is a thin-walled sac about 0.2 mm across, lined by flat type I cells for gas exchange and type II cells that secrete surfactant, the detergent that stops the sacs collapsing at the end of each breath. Capillaries form a dense net around every alveolus, so the barrier between air and blood is under a micrometre thick. Oxygen diffuses into red cells and binds haemoglobin; carbon dioxide diffuses the other way and is breathed out.

Blood supply

The pulmonary trunk from the right ventricle divides into right and left pulmonary arteries, which follow the bronchi into every segment and carry deoxygenated blood to the alveolar capillaries. Four pulmonary veins return oxygenated blood to the left atrium. Separately, small bronchial arteries from the aorta nourish the bronchi and lung tissue. A clot lodging in a pulmonary artery (pulmonary embolism) blocks flow to the segment it serves.

Innervation

The pulmonary plexus around each lung root carries parasympathetic fibres from the vagus nerve, which constrict the airways and stimulate mucus, and sympathetic fibres, which relax airway muscle. Salbutamol and adrenaline relieve wheeze by acting on the same beta-2 receptors that the sympathetic nerves use. The lung itself has no pain fibres; chest pain from lung disease comes from the parietal pleura, supplied by the intercostal and phrenic nerves.

Function

Breathing in is active: the diaphragm flattens and the external intercostals lift the ribs, enlarging the chest so air flows in. Breathing out at rest is passive elastic recoil. The brainstem sets the rhythm, driven mainly by the level of carbon dioxide in the blood. Beyond gas exchange the lungs filter small clots, make angiotensin-converting enzyme, and defend against inhaled microbes.

Clinical relevance

Asthma and COPD narrow the airways; pneumonia fills alveoli with fluid and pus; fibrosis stiffens the lung; emphysema destroys alveolar walls; cancer most often arises from the bronchial lining. Spirometry measures how much and how fast air moves; a chest X-ray and CT show the lobes, segments and pleural spaces; a pulse oximeter reads how well the alveoli are loading oxygen onto haemoglobin.

Structures in the atlas 81

The modelled pieces that make up the lungs in the 3D atlas. Each opens in the explorer.

The respiratory in brief

Air enters through the nose, where cartilages support the nostrils and conchae warm and moisten it, passes the larynx, which guards the airway and produces the voice, and descends the trachea. The trachea divides into the main bronchi, which branch again and again into the segmental bronchial trees modelled here, one for each bronchopulmonary segment. Gas exchange takes place in the alveoli at the end of this tree; the lung tissue itself is not a separate piece in this dataset, so the bronchial trees stand in for the lungs.

All structures of the respiratory