Thorax

Introduction

The thorax is the part of the body located between the neck and the abdomen. It houses vital organs such as the heart and lungs and plays a crucial role in respiration and circulation. Understanding the anatomy and contents of the thorax is essential for both clinical practice and anatomical studies.

Anatomy of the Thorax

Bony Thorax

The bony thorax forms the skeletal framework of the thoracic cavity and provides protection to the underlying organs.

• Sternum: Composed of the manubrium, body, and xiphoid process, the sternum forms the anterior midline of the thorax.
• Ribs: There are 12 pairs of ribs categorized as true ribs (1-7), false ribs (8-10), and floating ribs (11-12). They articulate with the thoracic vertebrae and sternum via costal cartilages.
• Thoracic vertebrae: Twelve vertebrae form the posterior boundary of the thorax and serve as attachment points for ribs and muscles.
• Costal cartilages: Cartilaginous extensions of the ribs that connect them to the sternum and provide flexibility to the thoracic cage.

Muscles of the Thorax

The muscles of the thorax facilitate respiration and movement of the thoracic cage.

• Intercostal muscles: These include external, internal, and innermost layers. They occupy the spaces between adjacent ribs and assist in inspiration and expiration.
• Diaphragm: A dome-shaped muscle forming the floor of the thoracic cavity. It is the primary muscle of respiration.
• Accessory muscles of respiration: Muscles such as the scalenes, pectoralis minor, and serratus anterior aid in forced inspiration.

Fascia and Connective Tissue

The thoracic fascia and connective tissue layers provide structural support and separate different compartments within the thorax.

• Endothoracic fascia: A thin layer of connective tissue that lines the inner surface of the thoracic cage and supports the pleura.
• Thoracic cavity layers: The thoracic cavity is lined by the parietal pleura on the walls and the visceral pleura covering the lungs. These layers reduce friction during respiratory movements.

Thoracic Cavities and Contents

Pleural Cavities

The pleural cavities are potential spaces surrounding each lung, lined by a serous membrane known as the pleura. These cavities allow smooth lung movements during respiration.

• Structure and layers of pleura: The parietal pleura lines the thoracic walls, diaphragm, and mediastinum, while the visceral pleura covers the lungs. The pleural fluid within the cavity reduces friction.
• Pleural recesses: Areas where the parietal pleura folds back on itself, including the costodiaphragmatic and costomediastinal recesses. They are clinically significant for fluid accumulation.
• Clinical relevance: Conditions such as pleural effusion and pneumothorax involve the accumulation of fluid or air within the pleural cavity, affecting respiration.

Mediastinum

The mediastinum is the central compartment of the thoracic cavity located between the two pleural sacs. It contains vital structures including the heart, great vessels, and parts of the respiratory and digestive systems.

• Superior mediastinum: Located above the pericardium, it contains the thymus, trachea, esophagus, aortic arch, and major veins and arteries.
• Inferior mediastinum: Divided into anterior, middle, and posterior compartments.
  • Anterior compartment: Contains connective tissue, lymph nodes, and remnants of the thymus.
  • Middle compartment: Contains the heart enclosed in the pericardium, roots of great vessels, and phrenic nerves.
  • Posterior compartment: Contains the descending aorta, esophagus, thoracic duct, azygos and hemiazygos veins, and sympathetic chains.
• Contents of each compartment: Structures are organized to optimize function and allow safe passage of vessels, nerves, and airways.

Pericardial Cavity

The pericardial cavity is a potential space between the layers of the pericardium surrounding the heart. It allows free movement of the heart within the thoracic cavity during cardiac cycles.

• Pericardium layers: Includes the fibrous pericardium, which provides protection and anchors the heart, and the serous pericardium, which has parietal and visceral layers separated by pericardial fluid.
• Heart position within the thorax: The heart is located in the middle mediastinum, slightly left of the midline, and rests on the diaphragm with its apex pointing anteriorly and to the left.

Vascular Structures

Arteries

The thoracic arteries supply oxygenated blood to the thoracic wall, lungs, and mediastinal organs.

• Aorta: The main arterial trunk with ascending, arch, and descending parts. The aortic arch gives rise to major branches supplying the head, neck, and upper limbs.
• Branches of the aortic arch: Includes the brachiocephalic artery, left common carotid artery, and left subclavian artery.
• Pulmonary arteries: Carry deoxygenated blood from the right ventricle to the lungs for oxygenation.

Veins

The thoracic veins drain deoxygenated blood from the thoracic organs and wall back to the heart.

• Superior and inferior vena cava: Major veins that return blood to the right atrium of the heart from the upper and lower parts of the body, respectively.
• Azygos system: Includes the azygos, hemiazygos, and accessory hemiazygos veins, which provide an alternative pathway for venous return from the thorax.
• Pulmonary veins: Carry oxygenated blood from the lungs to the left atrium of the heart.

Lymphatic Drainage

The thoracic lymphatic system plays a key role in immune defense and fluid balance.

• Thoracic duct: The largest lymphatic vessel in the body, draining lymph from most of the body into the venous system at the junction of the left subclavian and internal jugular veins.
• Right lymphatic duct: Drains lymph from the right upper quadrant of the body into the venous system.
• Thoracic lymph nodes: Located along the trachea, bronchi, and mediastinal vessels; they filter lymph and participate in immune responses.

Nervous Structures

The thoracic nerves control motor and sensory functions of the thoracic wall, as well as autonomic regulation of thoracic organs.

• Vagus nerve: Provides parasympathetic innervation to the heart, lungs, and digestive organs within the thorax.
• Phrenic nerve: Motor supply to the diaphragm and sensory fibers to the pericardium and mediastinal pleura.
• Sympathetic trunk and thoracic ganglia: Part of the autonomic nervous system controlling vascular tone, cardiac function, and bronchial smooth muscles.
• Intercostal nerves: Arise from thoracic spinal nerves and supply motor innervation to intercostal muscles and sensory innervation to the thoracic skin and parietal pleura.

Respiratory System within the Thorax

The thoracic cavity houses the major components of the respiratory system, facilitating the exchange of gases essential for life.

• Trachea and bronchi: The trachea extends from the larynx and divides into the right and left main bronchi, which further branch into secondary and tertiary bronchi supplying each lung.
• Lungs: Each lung is divided into lobes and further into bronchopulmonary segments. The right lung has three lobes, and the left lung has two. Surfaces include the costal, mediastinal, and diaphragmatic surfaces, with borders shaping the lung contours.
• Blood supply and innervation: The lungs receive blood from the pulmonary arteries for oxygenation and from the bronchial arteries for nutritional supply. Innervation includes sympathetic, parasympathetic, and sensory fibers regulating bronchial tone and reflexes.

Cardiovascular System within the Thorax

The thoracic cavity contains the heart and major vessels, which are essential for systemic and pulmonary circulation.

• Heart anatomy: The heart has four chambers: two atria and two ventricles. It contains valves, including the tricuspid, pulmonary, mitral, and aortic valves, ensuring unidirectional blood flow.
• Coronary circulation: The coronary arteries supply oxygenated blood to the myocardium, and the coronary veins drain deoxygenated blood into the coronary sinus and ultimately into the right atrium.
• Pericardial relationships: The heart is enclosed within the pericardium and lies in close relation to the diaphragm, lungs, and major vessels, allowing protection and facilitating cardiac movements within the thorax.

Thoracic Wall and Surface Anatomy

The thoracic wall provides protection to the thoracic organs and serves as a framework for respiratory movements. Knowledge of surface anatomy is important for clinical examination and procedures.

• Surface landmarks: Important landmarks include the sternal angle, nipple line, clavicles, and costal margins. These help in locating underlying structures such as the heart, lungs, and major vessels.
• Thoracic inlet and outlet: The thoracic inlet is bordered by the first thoracic vertebra, first ribs, and manubrium, allowing passage of vessels and airways. The thoracic outlet is formed by the diaphragm, ribs, and sternum, marking the inferior boundary of the thorax.
• Clinical palpation and auscultation points: Surface anatomy guides the placement of stethoscope for heart and lung auscultation and assists in procedures like thoracentesis and pericardiocentesis.

Development and Variations

Understanding the development and anatomical variations of the thorax is essential for recognizing congenital anomalies and planning surgical interventions.

• Embryology of thoracic structures: The thoracic skeleton develops from mesenchymal condensations, ribs form from costal processes, and the diaphragm develops from multiple embryonic components including the septum transversum.
• Congenital anomalies: Common variations include pectus excavatum, pectus carinatum, cervical ribs, and variations in rib number or fusion. These anomalies can affect respiratory function and may require surgical correction.

Clinical Considerations

The thorax is prone to a variety of injuries and diseases that can compromise respiratory and cardiovascular function. Understanding these conditions is essential for diagnosis and management.

• Thoracic injuries: Includes rib fractures, flail chest, and sternal fractures, which can impair breathing and damage underlying organs.
• Pleural diseases: Conditions such as pneumothorax, pleuritis, and pleural effusion affect the pleural space and may cause respiratory distress.
• Cardiopulmonary conditions: Includes pericardial effusion, cardiac tamponade, and thoracic aortic aneurysms, which can have life-threatening consequences if untreated.
• Surgical approaches: Thoracotomy, video-assisted thoracoscopic surgery, and minimally invasive procedures are performed for diagnostic and therapeutic interventions within the thoracic cavity.

References

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