The Lungs: Two Organs That Are Not Twins
Almost everything paired in the body comes in matching halves — two kidneys, two eyes, two hands that are mirror images of each other. The lungs look as though they belong to that family, and they do not. Open a chest and you find one lung with three lobes and two fissures, and another with two lobes, one fissure, a bite taken out of its front edge and a strange tongue of tissue hanging below that bite. The reason is not deep or mysterious. The heart sits slightly to the left, and it takes its room from the left lung. Every asymmetry between the two — the number of lobes, the shape of the borders, even the order of the vessels at the hilum — is the downstream consequence of that single fact, and once you see it, the lungs stop being two objects to memorize and become one story you can reason out.
It is three in the morning on a respiratory ward and an elderly man has been lying flat on his back since his stroke. He coughs weakly; a mouthful of gastric content went the wrong way some hours ago and nobody saw it happen. By dawn he has a fever, and the chest film shows a dense patch in one specific place — high at the back of the right lung, in the superior segment of the lower lobe. His neighbour in the next bed, who aspirated while sitting upright after surgery, has his shadow somewhere else entirely: low down, in the basal segments. Neither pattern is a coincidence and neither is guesswork. Fluid obeys gravity, and inside the lungs gravity runs along a fixed map of branching tubes and pyramidal territories that was laid down before either man was born. A physiotherapist arriving at eight o'clock will tilt each patient in a different direction to drain a different segment — and she will choose the angle from anatomy alone.
The general form: apex, base, surfaces, borders
Each lung is a half-cone, and every part of that cone is named — because every part has a neighbour that matters. The APEX is the blunt top, and it is the surprise of the organ: it does not stop at the thoracic inlet but rises about two to three centimetres above the medial third of the clavicle, climbing past the first rib into the root of the neck. That means the top of your lung is not in your chest at all — it is in your neck, sitting immediately behind the subclavian vessels and immediately below the lower trunk of the brachial plexus and the cervical sympathetic chain. A tumour growing there does not first cause a cough; it presses on the nerves overhead, which is why an apical (Pancoast) tumour announces itself as pain down the inner arm and a drooping eyelid rather than as a chest complaint. The BASE is the opposite pole: concave, moulded onto the dome of the diaphragm, and therefore separated from the liver on the right and from the stomach and spleen on the left by nothing but a sheet of muscle.
Between apex and base run two surfaces and three borders. The COSTAL surface is large, smooth and convex, pressed against the ribs and costal cartilages of the thoracic cage; the MEDIASTINAL surface faces inwards, is concave, and carries both the hilum and a set of impressions moulded by the organs of the mediastinum — on a hardened cadaveric lung these impressions are so faithful that you can read the great vessels off them like a fossil. The ANTERIOR border is thin and sharp and slides forward into the costomediastinal recess of the pleura in deep inspiration; the POSTERIOR border is thick and rounded and lies in the deep groove beside the vertebral column; the INFERIOR border is a thin edge that separates the base from the costal surface and sweeps up and down with every breath. In life the lung is spongy, light enough to float in water, and elastic: cut into a chest and it recoils away from you, because it is permanently stretched by the negative pressure described in the pleura and the pleural cavity.
The right lung: three lobes and two fissures
The side the heart left alone kept the fuller design. The right lung has THREE lobes — superior, middle and inferior — carved out by TWO fissures. The OBLIQUE (major) fissure runs downwards and forwards from about the level of the T4 spine, following the line of the sixth rib to reach the sixth costochondral junction, and it separates the inferior lobe behind and below from everything in front and above. The HORIZONTAL (minor) fissure is unique to the right lung: it runs forward from the oblique fissure at about the mid-axillary line, horizontally along the fourth costal cartilage, and it splits off the middle lobe from the superior lobe. Because the horizontal fissure is a thin sheet lying almost in the plane of an X-ray beam, it is often visible on a plain PA chest film as a fine line at the level of the anterior fourth rib — the only fissure you can routinely see without a lateral view, and a quiet landmark that separates a right middle lobe collapse from a right lower lobe one. The right lung is also the slightly larger and heavier of the two, though the shorter: the liver pushes its dome of the diaphragm higher, so the right lung is broader but not as tall. Its mediastinal surface is a gallery of impressions, and each names a neighbour. A long vertical groove in front of the hilum takes the SUPERIOR VENA CAVA, continuing above as the groove for the right brachiocephalic vein and below as the shallow bed of the right atrium. Arching over the hilum from behind forwards, to join the cava, is the groove for the AZYGOS VEIN — the vein whose arch is the classic reason the right upper lobe bronchus behaves as it does. Behind that lies a vertical groove for the OESOPHAGUS, and above the hilum a small flattening for the trachea and the right subclavian artery. Read the right mediastinal surface and you have effectively read the right side of the mediastinum.
The left lung: two lobes, a notch and a tongue
Everything the left lung lacks, the heart took. The left lung has TWO lobes, superior and inferior, and only the OBLIQUE fissure between them; there is no horizontal fissure because there is no middle lobe to cut off. It is narrower, longer and lighter than the right. On its anterior border sits the CARDIAC NOTCH — a deep concave bite where the lung has simply given way to the heart and the pericardium, and it is the reason the beating heart lies directly against the chest wall over a small triangle of bare pericardium, so that a needle can be passed into the pericardial sac from below the xiphoid without traversing lung. Hanging beneath the notch is the LINGULA, a tongue-shaped projection of the superior lobe. Developmentally and functionally the lingula is the left lung's answer to a middle lobe: it carries its own two segments, superior and inferior lingular, and it collapses, consolidates and gets resected in exactly the way a middle lobe does. The left mediastinal surface is dominated by a different set of neighbours: a broad curved groove arches over the hilum for the ARCH OF THE AORTA and continues down behind it as the deeper, longer groove of the DESCENDING THORACIC AORTA — an impression far more pronounced than anything on the right. In front of the aortic groove and above the hilum runs a narrower groove for the LEFT SUBCLAVIAN ARTERY, and in front of that a faint one for the left brachiocephalic vein; below and behind, a shallow oesophageal impression appears only near the base, because on the left the aorta has claimed the space the oesophagus occupies on the right. And the great concavity over the whole anterior part of the surface — the cardiac impression — is deeper on the left than on the right for the obvious reason.
Picture two neighbours who moved into identical flats on the same landing, and then one of them agreed to let a grand piano live in his sitting room. Ten years later nothing about the two flats matches. The piano owner has knocked out an alcove for it (the cardiac notch), lost a whole room (the middle lobe), rearranged the hallway so that the pipes now run in a different order (the hilum), and has permanent grooves worn into his walls where the instrument leans (the aortic impressions). His neighbour, undisturbed, still has all three rooms and a plain hallway. If you were shown the two flats and asked why they differ, you would not memorise a list of forty differences — you would say "one of them has a piano", and derive the rest. The heart is the piano. Everything else in this article is derived.
The hilum and the root: RALS, and the order of things
The hilum is the doorway in the mediastinal surface; the root is the bundle that passes through it. Through the ROOT of each lung pass: the main bronchus, the pulmonary artery, two pulmonary veins (superior and inferior), the bronchial arteries and veins, lymphatic vessels with the bronchopulmonary lymph nodes, and the nerve fibres of the pulmonary plexus — the whole bundle sleeved in a cuff of pleura that continues downwards as the pulmonary ligament, a slack fold that lets the root move up and down with breathing. The arrangement inside that bundle is examined constantly, so learn it in two axes rather than as a list. FRONT TO BACK, on both sides, the order is identical: the pulmonary VEINS are the most ANTERIOR structures, the pulmonary ARTERY lies between, and the BRONCHUS is the most POSTERIOR. And the inferior pulmonary vein is always the lowest structure in the root, on either side.
TOP TO BOTTOM, the two sides finally part company, and this is where RALS earns its keep — Right Anterior, Left Superior, describing where the pulmonary artery sits relative to the main bronchus. On the RIGHT, the superior lobar bronchus leaves the main bronchus unusually early and unusually high, above the level of the pulmonary artery, which is why it is called the EPARTERIAL bronchus ("above the artery") — it is the highest structure in the right root, with the pulmonary artery lying ANTERIOR to it, and the remaining stem continuing down as the hyparterial bronchus. On the LEFT there is no eparterial bronchus at all: the pulmonary ARTERY is the highest structure, arching over the main bronchus before descending behind it. Relations complete the picture and are just as testable: the PHRENIC nerve, with the pericardiacophrenic vessels, passes ANTERIOR to the root of the lung, while the VAGUS passes POSTERIOR to it — both covered in the nerves of the thorax. On the left, the vagus gives off its recurrent laryngeal branch as it crosses the aortic arch, which is why left hilar disease can steal a voice.
Two mnemonics, one sentence each, will carry you through any hilum question. RALS: Right Anterior, Left Superior — on the right the pulmonary artery is in FRONT of the bronchus, on the left it is ABOVE it. And VAB from front to back on both sides: Vein, Artery, Bronchus — veins anterior and inferior, bronchus posterior. Then add the relations, which never swap sides: PHRENIC in FRONT of the root, VAGUS BEHIND it. There is even a way to remember that one — the phrenic nerve is heading down to the diaphragm and the front is the short way there; the vagus is heading for the oesophagus and the stomach, which lie behind. Every nerve in the thorax takes the shortest route to its target, and their relations to the lung root are just that geometry frozen in place.
Bronchopulmonary segments: the lung's independent republics
Below the lobe is a unit the surgeon cares about far more: the segment. A BRONCHOPULMONARY SEGMENT is a pyramid of lung tissue with its apex at the hilum and its base at the pleural surface, ventilated by one segmental (tertiary) bronchus and supplied by one segmental branch of the pulmonary artery — the two travelling together in the centre of the pyramid. Each segment is wrapped in a thin connective tissue septum, which makes it structurally and functionally independent of its neighbours. The RIGHT lung has TEN. In the superior lobe: apical, posterior and anterior. In the middle lobe: lateral and medial. In the inferior lobe: the superior (apical) segment plus four basal segments — medial, anterior, lateral and posterior. The LEFT lung has EIGHT to TEN, depending on how you count, because adjacent segments fuse: the apical and posterior merge into an APICOPOSTERIOR segment, and the medial and anterior basal merge into an ANTEROMEDIAL basal segment. The left superior lobe also carries the two lingular segments, superior and inferior, standing in for the missing middle lobe. The airways that deliver all this are traced in the trachea and the bronchial tree.
Now the detail that makes the whole scheme surgically usable: the VEINS do not obey the segments. While each segment has its own central bronchus and artery, its venous drainage is by INTERSEGMENTAL veins that run in the connective tissue septa BETWEEN segments, each vein collecting from the two territories on either side of it. A surgeon can therefore find a bloodless plane at the boundary: clamp and divide one segmental bronchus and one segmental artery, follow the intersegmental veins as a guide, and lift out a single pyramid of lung while leaving its neighbours ventilated and perfused. That is a SEGMENTECTOMY, and it is the reason a small peripheral tumour in an elderly patient with poor lung function need not cost a whole lobe. A LOBECTOMY removes a lobe along a natural fissure; a pneumonectomy removes the lung; a segmentectomy removes a republic and leaves the federation standing.
A double blood supply — and where the lymph goes
The lung is the only organ that receives blood twice, for two entirely different reasons. The first supply is the PULMONARY circulation: the right ventricle sends deoxygenated blood down the pulmonary trunk and the pulmonary arteries to the alveolar capillaries, and the four pulmonary veins return oxygenated blood to the left atrium. This blood is the lung's cargo, not its food — it passes through to be worked on, exactly as described in pulmonary circulation and breathing mechanics. The second supply is the BRONCHIAL circulation, and it is the lung's own nourishment: small BRONCHIAL ARTERIES carrying fully oxygenated systemic blood, usually one on the right (from the third posterior intercostal artery or the right superior intercostal) and two on the left (directly from the descending thoracic aorta), running on the back of the bronchi and feeding the bronchial walls, the connective tissue, the pleura and the lymph nodes down as far as the respiratory bronchioles. Bronchial veins drain partly to the azygos and hemiazygos system, and partly into the pulmonary veins — a small, permanent physiological shunt of deoxygenated blood into the systemic circulation, and one reason arterial oxygen saturation is never quite 100 per cent even in a perfectly healthy person.
The lymphatic drainage runs in two plexuses that meet only at the hilum. A SUPERFICIAL (subpleural) plexus lies beneath the visceral pleura and drains the peripheral lung tissue; a DEEP plexus follows the bronchi and pulmonary vessels through the substance of the lung. Both converge on the BRONCHOPULMONARY (hilar) nodes, and from there lymph passes to the TRACHEOBRONCHIAL nodes at the carina and around the bifurcation, then upwards to the PARATRACHEAL nodes, and finally into the right and left BRONCHOMEDIASTINAL trunks — the right joining the right lymphatic duct, the left usually joining the thoracic duct. This chain is why the staging of lung cancer is a chain: a tumour is a different disease depending on whether it has reached the hilar nodes, the ipsilateral mediastinal nodes or crossed to the other side, and the whole logic of assessment in lung cancer follows that anatomy node by node. Sensory and autonomic supply, meanwhile, arrives via the pulmonary plexus: sympathetic fibres from the sympathetic trunks (bronchodilator, vasoconstrictor) and parasympathetic fibres from the vagus (bronchoconstrictor, secretomotor, and carrying the cough reflex from the bronchial mucosa).
Because the right main bronchus is wider, shorter and more vertical, inhaled material preferentially enters the RIGHT lung — but which segment it reaches depends entirely on the posture at the moment of aspiration. Lying flat on the back (the unconscious patient, the anaesthetised patient, the drunk asleep), the most dependent territories are the POSTERIOR segment of the right upper lobe and the SUPERIOR segment of the right lower lobe: that is where the abscess or the aspiration pneumonia appears. Sitting or standing upright, gravity carries material to the BASAL segments of the right lower lobe instead. Lying on one side, it goes to whichever lateral segment is downmost. Physiotherapists use the same map in reverse — POSTURAL DRAINAGE positions each patient so that the segmental bronchus of the diseased territory points downhill, so secretions run out of the pyramid into the larger airways and can be coughed clear. Head-down tilt for the basal segments; sitting upright and leaning forward for the apical; lying on the good side for a lateral segment. It is one of the purest examples in medicine of anatomy performed as therapy.
Listening, tapping and reading the lungs
Where you put the stethoscope decides which lobe you are hearing. Trace the oblique fissure on the surface and the reason becomes obvious. It begins at the level of the T3–T4 spine posteriorly, runs downwards and forwards around the chest along the line of the sixth rib, and ends at the sixth costochondral junction anteriorly. Everything ABOVE and IN FRONT of that line is upper lobe (and, on the right, middle lobe below the fourth costal cartilage); everything BELOW and BEHIND it is lower lobe. So the upper lobes are examined mainly on the FRONT of the chest and in the upper axilla, and the lower lobes mainly on the BACK — a student who listens only to the front of the chest has effectively not examined the lower lobes at all, and will miss a basal pneumonia entirely. The right middle lobe has its own small window: the anterior chest to the right of the sternum between the fourth and sixth cartilages, and the right axilla. The inferior border of the lung itself crosses the sixth rib in the midclavicular line, the eighth in the midaxillary line and the tenth beside the vertebrae, while the pleura reaches two ribs lower at each of those points — the difference being the space the lung expands into on inspiration — and the reason a chest drain aimed for pleural fluid is placed low while a needle aimed for lung is not. In CONSOLIDATION the alveoli of a segment or lobe fill with inflammatory exudate while the airways stay open: the lung keeps its size, the opacity is bounded sharply by a fissure, and air in the patent bronchi shows as an air bronchogram. In COLLAPSE (atelectasis) a bronchus is obstructed — by a tumour, a mucus plug, an inhaled peanut — the trapped air is absorbed and the segment or lobe shrinks: the opacity comes with volume loss, so the fissures, the trachea, the mediastinum and the hemidiaphragm are all pulled TOWARDS the lesion. Same white shadow, opposite mechanics, and the difference is read from the movement of the structures around it rather than from the shadow itself.
A man of sixty-two, a lifelong smoker, sees his doctor about six months of aching pain down the inner side of his right arm and into the little finger — he has been treated for a trapped nerve in the neck. Over that time the small muscles of his hand have begun to waste. On examination his right upper eyelid droops, his right pupil is smaller than the left, and that side of his face does not sweat. The chest X-ray shows an apical shadow. This is a PANCOAST (superior sulcus) tumour, and every element of the presentation is anatomy: because the apex of the lung rises into the root of the neck, a tumour there invades the lower trunk of the brachial plexus (C8, T1), giving pain and wasting in a C8/T1 distribution; and it invades the cervical sympathetic chain and the stellate ganglion, giving HORNER'S SYNDROME — ptosis, miosis and anhidrosis. If it also reaches the recurrent laryngeal nerve the voice goes hoarse; if it reaches the phrenic nerve the hemidiaphragm rises and stops moving. A lung cancer that presented for half a year without a single respiratory symptom, because of where the lung's apex chooses to live.
- Each lung is a half-cone: an APEX rising 2–3 cm above the medial third of the clavicle into the neck, a concave BASE on the diaphragm, costal and mediastinal SURFACES, and anterior, posterior and inferior BORDERS.
- RIGHT lung: three lobes (superior, middle, inferior), OBLIQUE + HORIZONTAL fissures, slightly larger and heavier but shorter, with impressions for the superior vena cava, the azygos vein and the oesophagus.
- LEFT lung: two lobes, only the OBLIQUE fissure, a CARDIAC NOTCH on the anterior border and the LINGULA below it (its answer to a middle lobe), plus deep grooves for the aortic arch and descending aorta.
- ROOT contents: main bronchus, pulmonary artery, two pulmonary veins, bronchial vessels, lymph nodes and the pulmonary plexus, sleeved in pleura continuing as the pulmonary ligament.
- Hilar order — front to back on BOTH sides: veins ANTERIOR (and the inferior vein lowest), artery between, bronchus POSTERIOR. Top to bottom: RIGHT eparterial bronchus highest with the artery in front of it; LEFT pulmonary artery highest — RALS.
- The PHRENIC nerve passes ANTERIOR to the root of the lung; the VAGUS passes POSTERIOR to it — an exam favourite that never changes side.
- RIGHT segments (10): upper — apical, posterior, anterior; middle — lateral, medial; lower — superior (apical) + four basal (medial, anterior, lateral, posterior).
- LEFT segments (8–10): apicoposterior and anteromedial basal fusions, plus superior and inferior LINGULAR segments in the upper lobe and superior + basal segments in the lower.
- Each segment is a PYRAMID with its own segmental bronchus and artery centrally, but is drained by INTERSEGMENTAL veins running in the septa between segments — which is what makes segmentectomy possible.
- DOUBLE blood supply: pulmonary arteries carry deoxygenated blood for GAS EXCHANGE; bronchial arteries (from the aorta / posterior intercostal) carry oxygenated blood to NOURISH the lung tissue itself.
- Lymph: superficial (subpleural) and deep plexuses → bronchopulmonary (hilar) nodes → tracheobronchial → paratracheal → bronchomediastinal trunks — the exact chain lung-cancer staging follows.
- Surface marking: the oblique fissure runs from the T3–T4 spine along the sixth rib to the sixth costochondral junction — so the UPPER lobe is heard mostly in FRONT and the LOWER lobe mostly at the BACK.
- Calling the lingula "the left middle lobe". It is not a lobe at all — it is a tongue-like part of the left SUPERIOR lobe, carrying two segments, with no fissure separating it.
- Assuming the veins follow the segments like the bronchi and arteries do. The segmental bronchus and artery run in the CENTRE of the pyramid; the veins are INTERSEGMENTAL, running in the planes between segments — that is precisely why they mark the surgical boundary.
- Thinking the pulmonary arteries feed the lung. They carry the blood that is to be oxygenated — the lung's cargo. The lung tissue itself lives on the BRONCHIAL arteries from the systemic circulation.
During a right thoracotomy the surgeon exposes the root of the right lung and identifies the highest structure entering the hilum, then notes the vessel lying immediately in front of it. Which of the following correctly describes the right lung root?
- The lungs are not mirror images: the heart takes its room from the left, so the RIGHT lung has three lobes with oblique and horizontal fissures and is larger and heavier, while the LEFT has two lobes, only the oblique fissure, a cardiac notch and a lingula.
- At the hilum the pulmonary veins are anterior and inferior and the bronchus posterior on both sides; the right eparterial bronchus is highest with the artery in front (RALS = Right Anterior, Left Superior), the phrenic nerve passes in front of the root and the vagus behind.
- Ten bronchopulmonary segments on the right and eight to ten on the left, each a pyramid with its own segmental bronchus and artery but drained by intersegmental veins — the anatomical basis of segmentectomy, postural drainage and the posture-dependent sites of aspiration.
- The lung has a double blood supply — pulmonary arteries for gas exchange and bronchial arteries for its own nourishment — lymph draining hilar → tracheobronchial → paratracheal → bronchomediastinal; the apex reaches into the neck, which is why a Pancoast tumour causes arm pain and Horner's syndrome before any cough.
- Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students — Thorax: the lungs, pleural cavities and bronchopulmonary segments.
- Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy — The lungs: lobes, fissures, roots and lymphatic drainage.
- Netter FH. Atlas of Human Anatomy — Lungs: mediastinal surfaces, hila and bronchopulmonary segments.
- Last RJ. Last's Anatomy: Regional and Applied — The lungs and the roots of the lungs.
- Snell RS. Clinical Anatomy by Regions — Thorax: surface markings of the lungs, aspiration and postural drainage.
- TeachMeAnatomy — The Lungs; Bronchopulmonary Segments; The Hilum of the Lung.
