Organs of the Thorax

Overview
The thoracic cavity houses several essential organs that contribute to respiration, immune function, and endocrine activity. This section introduces these organs and describes their roles, along with their spatial relationships within the thorax.

Key Structures
This section covers the thymus, mammary glands, lungs, tracheobronchial tree, and pleural membranes. Each topic explains the structure and function of these organs while highlighting important anatomical features.

Clinical Relevance and Learning Focus
A thorough understanding of thoracic organ anatomy is vital when evaluating respiratory disorders, thoracic malignancies, breast disease, and pleural pathology. This section supports sound clinical reasoning and accurate interpretation of imaging findings.

The Thymus Gland

Contents

The thymus is a soft, pink, lobulated lymphoid organ found in the thoracic cavity and extending into the neck. During childhood and adolescence, it plays a key role in immune system development. After puberty, the gland gradually reduces in size and is progressively replaced by adipose tissue.

From an embryological perspective, the thymus develops from the third pharyngeal pouch.

This article reviews the anatomy of the thymus gland, including its structure, anatomical position, and blood supply.

Anatomical Structure and Position

The thymus has a flattened, asymmetrical shape with a distinctly lobular architecture. Each lobule is composed of multiple follicles, which contain two main regions:

• Cortex: Located at the periphery of each follicle, this region is densely packed with lymphocytes supported by epithelial reticular cells.
• Medulla: Found centrally within each follicle, the medulla contains fewer lymphocytes and a higher proportion of epithelial cells. It also contains Hassall’s corpuscles, which are concentric structures formed by epithelial reticular cells; their exact function remains uncertain.

The thymus is primarily situated within the superior mediastinum, lying posterior to the manubrium of the sternum. In some individuals, it may extend superiorly into the neck, reaching as high as the thyroid gland, or inferiorly into the anterior mediastinum, where it lies anterior to the fibrous pericardium.

Fig 1.0 – Anatomical position of the thymus within the superior mediastinum.

Vasculature

Arterial blood supply to the thymus is provided by anterior intercostal arteries and small branches arising from the internal thoracic arteries. Venous drainage occurs via the left brachiocephalic vein and the internal thoracic veins.

Clinical Relevance

DiGeorge Syndrome

DiGeorge syndrome is a genetic condition caused by deletion of a segment of chromosome 22. Its clinical presentation varies widely, but the most characteristic features can be remembered using the mnemonic CATCH:

• Congenital heart defects
• Abnormal facial features
• Thymic aplasia
• Cleft palate
• Hypoparathyroidism

Individuals with absent or underdeveloped thymic tissue are particularly prone to recurrent infections due to impaired immune system development.

The Breasts

Contents

The breasts are paired structures situated on the anterior chest wall within the pectoral region. They are present in both sexes, but undergo marked development in females after puberty.

In females, the breasts contain the mammary glands, which are accessory organs of the female reproductive system and are responsible for milk production during lactation.

This article explores the anatomy of the breasts, including their structure, nerve supply, blood supply, and clinical significance.

Note: The anatomical description in this article refers specifically to the female breast.

Surface Anatomy

The breast overlies the anterior thoracic wall. Horizontally, it extends from the lateral edge of the sternum to the mid-axillary line. Vertically, it typically spans from the second to the sixth costal cartilages. The breast lies superficial to the pectoralis major and serratus anterior muscles.

The breast can be divided into two main regions:

• Body of the breast: The larger, rounded, and more prominent portion
• Axillary tail: A smaller extension that passes superolaterally along the lower border of the pectoralis major toward the axilla

At the centre of the breast is the nipple, which is composed largely of smooth muscle fibres. Surrounding it is the areola, an area of pigmented skin containing numerous sebaceous glands. These glands enlarge during pregnancy and secrete an oily substance that helps protect and lubricate the nipple.

Fig 1 – Surface anatomy of the breast. Note that the axillary tail is not labelled.

Anatomical Structure

The breast consists of mammary glands embedded within a connective tissue framework.

Mammary Glands

The mammary glands are specialised sweat glands composed of secretory lobules and ducts. Typically, there are 15–20 lobules.

Each lobule is made up of multiple alveoli, which drain into a single lactiferous duct. These ducts converge toward the nipple in a radial pattern.

Connective Tissue Stroma

The connective tissue stroma surrounds and supports the mammary glands and contains both fibrous and fatty components.

The fibrous tissue condenses to form the suspensory ligaments (of Cooper). These ligaments:

• Anchor the breast to the dermis and the underlying pectoral fascia
• Separate and support the glandular lobules

Pectoral Fascia

The base of the breast rests on the pectoral fascia, a sheet of connective tissue associated with the pectoralis major muscle. This fascia provides attachment for the suspensory ligaments.

Between the breast and the pectoral fascia lies a layer of loose connective tissue known as the retromammary space. This potential space allows some movement of the breast and is commonly utilised in reconstructive surgery.

Fig 2 – Internal structure of the breast.

Vasculature

The medial portion of the breast receives arterial blood from the internal thoracic (internal mammary) artery, a branch of the subclavian artery.

The lateral breast is supplied by several vessels:

• Lateral thoracic and thoracoacromial branches from the axillary artery
• Lateral mammary branches from the posterior intercostal arteries (arising from the aorta), supplying the breast at the second to fourth intercostal spaces
• Mammary branches from the anterior intercostal arteries

Venous drainage mirrors the arterial supply, with blood returning via the axillary and internal thoracic veins.

Lymphatic Drainage

Lymphatic drainage of the breast is clinically significant due to its role in the spread of breast cancer.

Breast tissue drains primarily to three groups of lymph nodes:

• Axillary lymph nodes: ~75%
• Parasternal lymph nodes: ~20%
• Posterior intercostal lymph nodes: ~5%

The skin of the breast has additional lymphatic pathways:

• Skin: Drains to axillary, infraclavicular, and inferior deep cervical lymph nodes
• Nipple and areola: Drain into the subareolar lymphatic plexus

Fig 3 – Groups of axillary lymph nodes and their drainage into the apical nodes.

Nerve Supply

The breast is supplied by the anterior and lateral cutaneous branches of the fourth to sixth intercostal nerves. These nerves carry sensory fibres as well as autonomic fibres that regulate smooth muscle activity and vascular tone.

Milk production and secretion are not controlled by these nerves. Instead, lactation is hormonally regulated by prolactin and oxytocin, which are released from the pituitary gland.

Clinical Relevance

Breast Cancer

Breast cancer is the most frequently diagnosed cancer in the UK and has the second highest cancer-related mortality rate after lung cancer. It occurs far more commonly in women than in men.

Many clinical signs of breast cancer arise from obstruction of lymphatic drainage. Accumulation of lymph in the subcutaneous tissues can lead to features such as nipple deviation or retraction, and skin thickening with a dimpled appearance known as peau d’orange. Larger skin dimples are often caused by tumour invasion and fibrosis, which shorten the suspensory ligaments.

Spread of breast cancer most commonly occurs via lymphatic channels, with the axillary lymph nodes frequently affected. These nodes may become firm, fixed, and enlarged. From there, metastasis may occur to distant organs including the lungs, liver, bones, and ovaries.

Assessment of suspected breast cancer involves triple assessment, consisting of:

• Clinical examination
• Imaging (mammography and ultrasound)
• Tissue biopsy

Breast cancer is staged using either the I–IV system or the TNM (Tumour, Node, Metastasis) classification.

Standard treatment involves surgical excision of the affected tissue with adjuvant radiotherapy. Breast-conserving surgery is preferred where possible; mastectomy may be required if local excision is not suitable. Adjuvant chemotherapy has also been shown to improve survival outcomes.

Fig 4 – Mammogram showing a normal breast (left) and a lesion suspicious for malignancy (right).

The Heart (CLICK HERE FOR MORE)
Overview
The heart is the primary organ of the cardiovascular system, responsible for driving blood flow through both the pulmonary and systemic circuits. This section provides an introduction to its overall structure and the key elements that enable its function.

Key Structures
This section covers the cardiac chambers, heart valves, conduction pathways, coronary arteries and veins, as well as the pericardial support structures. Each topic explains the relevant anatomy and essential functional relationships.

Clinical Relevance and Learning Focus
A solid understanding of cardiac anatomy is essential for identifying arrhythmias, valvular disorders, myocardial ischaemia, and heart failure. This section aids interpretation of ECGs, echocardiographic studies, cross-sectional imaging, and findings from clinical examination.

The Lungs

Contents

• Anatomical Position and Relations
• Lung Structure
• Vasculature
• Nerve Supply
• Lymphatic Drainage
• Pulmonary Embolism

The lungs are the primary organs of respiration and are situated within the thorax on either side of the mediastinum.

Their main function is to oxygenate the blood. This is achieved by bringing inhaled air into close contact with deoxygenated blood within the pulmonary capillary network.

This article examines the anatomy of the lungs, including their anatomical relationships, blood and nerve supply, and key clinical correlations.

Note: The anatomy of the trachea, bronchi, and bronchioles is discussed in a separate article.

Anatomical Position and Relations

The lungs occupy the thoracic cavity on either side of the mediastinum. Each lung is enclosed within a pleural cavity formed by the visceral and parietal pleura.

They are connected to the mediastinum by the lung root, which contains structures entering and leaving the lungs. The medial (mediastinal) surfaces of the lungs lie adjacent to several important mediastinal structures:

Left Lung

• Heart
• Arch of the aorta
• Thoracic aorta
• Oesophagus

Right Lung

• Oesophagus
• Heart
• Inferior vena cava
• Superior vena cava
• Azygos vein

Lung Structure

The lungs are approximately conical in shape and each has an apex, a base, three surfaces, and three borders. The left lung is slightly smaller than the right due to the position of the heart.

Each lung consists of:

• Apex: The blunt superior end, projecting above the first rib into the root of the neck
• Base: The inferior surface resting on the diaphragm
• Lobes: Two or three sections separated by fissures
• Surfaces: Costal, mediastinal, and diaphragmatic
• Borders: Anterior, inferior, and posterior

Lobes

The two lungs differ in their lobular arrangement.

The right lung has three lobes—superior, middle, and inferior—separated by two fissures:

• Oblique fissure: Extends from the inferior border upward and posteriorly to meet the posterior border
• Horizontal fissure: Runs horizontally from the sternum at the level of the fourth rib to meet the oblique fissure

The left lung has two lobes—superior and inferior—separated by an oblique fissure similar to that of the right lung.

Surfaces

Each lung has three surfaces that correspond to adjacent thoracic structures:

• Mediastinal surface: Faces the mediastinum and contains the lung hilum, where structures enter and leave the lung
• Diaphragmatic surface: Forms the base of the lung and rests on the diaphragm; it is concave, with a deeper concavity on the right due to the liver
• Costal surface: Smooth and convex, facing the inner surface of the thoracic wall and separated from the ribs by the costal pleura

Borders

• Anterior border: Formed by the junction of the costal and mediastinal surfaces; the left lung features the cardiac notch, created by the heart
• Inferior border: Separates the base from the costal and mediastinal surfaces
• Posterior border: Rounded and smooth, formed where the costal and mediastinal surfaces meet posteriorly

Root and Hilum

The lung root anchors the lung to the mediastinum and contains:

• A main bronchus
• Pulmonary artery
• Two pulmonary veins
• Bronchial vessels
• Pulmonary nerve plexus
• Lymphatic vessels

These structures pass through the hilum, a wedge-shaped region on the mediastinal surface of each lung.

Bronchial Tree

The bronchial tree is a branching system of airways that delivers air to the alveoli.

It begins with the trachea, which divides into right and left main bronchi. The right bronchus is wider and more vertical, making it more prone to foreign body aspiration.

Each bronchus enters the lung through the hilum and divides into lobar bronchi, one for each lobe. These further divide into segmental bronchi, each supplying a bronchopulmonary segment—the functional units of the lungs.

Segmental bronchi branch into conducting bronchioles, which continue into terminal bronchioles. Terminal bronchioles give rise to respiratory bronchioles, which contain alveoli—thin-walled structures where gas exchange occurs.

Vasculature

Deoxygenated blood is delivered to the lungs by the paired pulmonary arteries. After oxygenation, blood returns to the heart via four pulmonary veins, two from each lung.

Additional nutritive blood supply to the bronchi, lung roots, visceral pleura, and supporting tissues is provided by the bronchial arteries, which arise from the descending aorta.

Venous drainage occurs through the bronchial veins. The right bronchial vein drains into the azygos vein, while the left drains into the accessory hemiazygos vein.

Nerve Supply

The lungs receive innervation from the pulmonary plexuses, which contain:

• Parasympathetic fibres (vagus nerve): Stimulate bronchial gland secretion, bronchoconstriction, and pulmonary vasodilation
• Sympathetic fibres: Cause bronchodilation and pulmonary vasoconstriction
• Visceral afferent fibres: Transmit pain sensations to the sensory ganglia of the vagus nerve

Lymphatic Drainage

Lymphatic drainage of the lungs is via two interconnected plexuses:

• Superficial (subpleural) plexus: Drains lung parenchyma
• Deep plexus: Drains structures within the lung root

Both systems drain into the tracheobronchial lymph nodes near the tracheal bifurcation. Lymph then passes into the right and left bronchomediastinal trunks.

Clinical Relevance

Pulmonary Embolism

A pulmonary embolism occurs when a pulmonary artery is obstructed by material originating elsewhere in the body. Common emboli include:

• Thrombus: The most frequent cause, usually arising from a peripheral vein
• Fat: Often following long bone fractures or orthopaedic surgery
• Air: May occur after cannulation of neck veins

Pulmonary embolism reduces lung perfusion, leading to impaired oxygenation and increased strain on the right ventricle. Typical symptoms include shortness of breath, chest pain, cough, haemoptysis, and rapid breathing. Clinically, the Wells’ score is used to estimate the likelihood of pulmonary embolism.

Definitive management involves anticoagulation and thrombolytic therapy to dissolve the embolus and prevent further clot formation.

The Tracheobronchial Tree

Contents

• The Trachea
• Bronchi
• Bronchioles
• Clinical Correlations: Asthma

The trachea, bronchi, and bronchioles together form the tracheobronchial tree—a branching network of airways that conducts air into the lungs, where gas exchange takes place. These structures are found within the neck and thoracic cavity.

This article reviews the anatomical location, structure, and neurovascular supply of the airways, and outlines their clinical significance.

The Trachea

Anatomical Position

The trachea marks the start of the tracheobronchial tree. It begins at the lower border of the cricoid cartilage in the neck as a continuation of the larynx.

It descends into the superior mediastinum and divides at the level of the sternal angle to form the right and left main bronchi. Throughout its course, the trachea lies anterior to the oesophagus and deviates slightly to the right as it descends.

Structure

Like other large airways, the trachea is kept patent by C-shaped rings of cartilage. The open posterior ends of these rings are bridged by the trachealis muscle, allowing some flexibility.

The trachea and bronchi are lined with ciliated pseudostratified columnar epithelium containing mucus-secreting goblet cells. Together, the mucus and coordinated ciliary movement form the mucociliary escalator, which traps inhaled particles and pathogens and moves them upward to be swallowed and destroyed.

At the point where the trachea divides, a cartilage ridge known as the carina projects between the two bronchi. This region is highly sensitive and plays an important role in initiating the cough reflex; it is clearly visible during bronchoscopy.

Neurovascular Supply

Sensory innervation of the trachea is provided by the recurrent laryngeal nerve.

Its arterial supply comes from tracheal branches of the inferior thyroid artery, while venous blood drains into the brachiocephalic, azygos, and accessory hemiazygos veins.

Bronchi

At the level of the sternal angle, the trachea divides into the right and left main bronchi. These airways branch further to form secondary (lobar) bronchi, each supplying a lobe of the lung, and then into tertiary (segmental) bronchi.

Together with branches of the pulmonary arteries and veins, the main bronchi contribute to the formation of the lung roots.

Structure

• Right main bronchus: Wider, shorter, and more vertically oriented than the left. Because of this anatomy, inhaled foreign bodies are more likely to enter the right lung. The superior lobar bronchus branches off before the right main bronchus enters the lung hilum.
• Left main bronchus: Travels inferior to the arch of the aorta and anterior to the thoracic aorta and oesophagus before reaching the hilum of the left lung.

Within the lungs, the main bronchi divide into lobar bronchi (three on the right, two on the left), which then branch into segmental bronchi. Each segmental bronchus supplies a bronchopulmonary segment, the functional subdivision of lung tissue.

Structurally, the bronchi resemble the trachea, though the cartilage arrangement differs. In the main bronchi, cartilage forms complete rings, whereas in smaller bronchi it appears as incomplete, crescent-shaped plates.

Neurovascular Supply

The bronchi are innervated by pulmonary branches of the vagus nerve (CN X). Their blood supply is derived from the bronchial arteries, and venous drainage occurs via the bronchial veins.

Bronchioles

Segmental bronchi continue to divide into progressively smaller airways known as bronchioles.

Structure

Bronchioles lack cartilage and mucus-producing goblet cells. Instead, they contain club cells, which secrete a surfactant lipoprotein that helps prevent airway collapse during expiration.

Initially, air passes through conducting bronchioles, which are involved only in air transport. These eventually become terminal bronchioles, which then branch into respiratory bronchioles. Respiratory bronchioles are distinguished by the presence of alveoli opening directly from their walls.

Alveoli are tiny, thin-walled air sacs lined by simple squamous epithelium and are the primary sites of gas exchange. Adult lungs contain approximately 300 million alveoli, providing an extensive surface area for efficient oxygen and carbon dioxide exchange.

Clinical Correlations: Asthma

Asthma is a chronic inflammatory condition of the airways characterised by hypersensitivity, reversible airflow obstruction, and bronchospasm.

Structural changes occur in the small airways, including thickening of smooth muscle, epithelial damage, and an enlarged basement membrane.

An asthma attack represents an acute worsening of symptoms triggered by factors such as allergens or exercise. These triggers provoke inflammation and contraction of bronchial smooth muscle, narrowing the airways and leading to breathlessness and wheezing—hallmark features of the condition.

The Pleurae

Contents

• Structure of the Pleurae
• Pleural Recesses
• Neurovascular Supply
• Pneumothorax

The pleurae are serous membranes that line both the lungs and the thoracic cavity, enabling smooth and efficient breathing. This article describes their structure and function, and explores their clinical significance.

Structure of the Pleurae

Each lung is associated with a pleura, giving a total of two pleurae in the body. Each pleura consists of a serous membrane made up of simple squamous epithelial cells supported by connective tissue. This epithelial layer is known as the mesothelium.

Each pleura has two continuous components:

• Visceral pleura – directly covers the surface of the lungs
• Parietal pleura – lines the internal surface of the thoracic cavity

These layers are continuous at the lung hilum. Between them lies a potential space called the pleural cavity.

Parietal Pleura

The parietal pleura lines the inner surface of the thoracic cavity. It is thicker than the visceral pleura and is subdivided based on the structures it contacts:

• Mediastinal pleura: Covers the lateral surfaces of the mediastinum
• Cervical pleura: Lines the extension of the pleural cavity into the neck
• Costal pleura: Covers the inner surfaces of the ribs, costal cartilages, and intercostal muscles
• Diaphragmatic pleura: Covers the superior (thoracic) surface of the diaphragm

Fig 1 – Subdivisions of the parietal pleura.

Visceral Pleura

The visceral pleura adheres closely to the outer surface of the lungs and extends into the interlobar fissures. It becomes continuous with the parietal pleura at the lung hilum, where structures enter and leave the lungs.

Pleural Cavity

The pleural cavity is the narrow, potential space between the parietal and visceral pleurae. It contains a small amount of serous fluid, which serves two key purposes:

• Lubrication: Allows the pleural surfaces to glide smoothly over one another during respiration
• Surface tension: Keeps the lungs closely apposed to the thoracic wall so that lung expansion follows thoracic expansion

If air enters the pleural cavity, this surface tension is lost, resulting in a pneumothorax.

Fig 2 – Relationship between the parietal and visceral pleurae and the pleural cavity.

Pleural Recesses

In certain regions, the lungs do not fully occupy the pleural cavities, creating pleural recesses where opposing layers of parietal pleura come into contact.

Each pleural cavity contains two main recesses:

• Costodiaphragmatic recess: Between the costal and diaphragmatic pleurae
• Costomediastinal recess: Between the costal and mediastinal pleurae, behind the sternum

These spaces are clinically important as they can collect fluid, such as in a pleural effusion.

Neurovascular Supply

Parietal Pleura

The parietal pleura is sensitive to pain, pressure, and temperature, producing well-localised pain when irritated. It is innervated by the intercostal nerves and the phrenic nerve.

Its blood supply comes from branches of the intercostal arteries.

Visceral Pleura

The visceral pleura is insensitive to pain, touch, and temperature, responding only to stretch. It receives autonomic innervation from the pulmonary plexus, which contains fibres from both the vagus nerve and the sympathetic trunk.

Arterial supply is provided by the bronchial arteries, which also supply lung tissue.

Clinical Relevance

Pneumothorax

A pneumothorax occurs when air or gas enters the pleural cavity, disrupting the normal surface tension and impairing lung expansion.

Common clinical features include chest pain, shortness of breath, and unequal chest movement. On percussion, the affected side may sound hyper-resonant due to the presence of excess air.

Pneumothoraces are broadly classified into two types:

• Spontaneous pneumothorax: Occurs without an obvious cause; subdivided into primary (no underlying lung disease) and secondary (associated with existing lung pathology)
• Traumatic pneumothorax: Results from blunt or penetrating chest injury, such as a rib fracture

Management depends on the cause and severity. Primary pneumothoraces are often small and may resolve with minimal treatment, while secondary or traumatic pneumothoraces frequently require decompression using a chest drain to allow lung re-expansion.

Fig 3 – Chest radiograph demonstrating a left-sided pneumothorax.