Thorax

The thorax, commonly known as the chest, is the upper part of the torso that houses and protects vital organs such as the heart and lungs. It is a bony framework formed by the thoracic vertebrae, ribs, costal cartilages, and sternum, enclosing the thoracic cavity. The thorax serves as a structural and protective unit while providing attachment for muscles of respiration and the upper limbs.

Location

The thorax is located between the neck (superiorly) and the abdomen (inferiorly). It extends from the base of the neck (the thoracic inlet) to the diaphragm (a muscular boundary) and is bounded posteriorly by the thoracic vertebrae, anteriorly by the sternum, and laterally by the rib cage.

Anatomy

The thorax, commonly known as the chest, is a vital region of the human body that provides protection to critical organs, including the heart and lungs, while also playing a role in respiration and structural support. It is a bony, muscular, and vascular framework that encloses the thoracic cavity and facilitates the movement of air during breathing.

The thorax is divided into bony structures, muscles, vessels, nerves, and the thoracic cavity. Below is a detailed breakdown of its anatomy:

Boundaries of the Thorax

The thorax has defined anatomical boundaries:

• Superior Boundary: Thoracic inlet (or superior thoracic aperture) at the base of the neck.
• Inferior Boundary: Diaphragm, separating the thoracic cavity from the abdominal cavity.
• Anterior Boundary: Sternum and costal cartilages.
• Posterior Boundary: Thoracic vertebrae and intervertebral discs.
• Lateral Boundaries: Rib cage (ribs and intercostal muscles).

Skeletal Framework of the Thorax

The bony framework of the thorax forms the rib cage, which protects the thoracic cavity and supports respiration.

Thoracic Vertebrae

• There are 12 thoracic vertebrae (T1–T12), forming the posterior wall of the thorax.
• Each vertebra articulates with ribs via costovertebral joints and costotransverse joints.
• The vertebral column provides stability, support, and attachment for muscles.

Ribs

The thorax contains 12 pairs of ribs, which are curved, flat bones:

• True Ribs (1–7): Directly connect to the sternum via costal cartilages.
• False Ribs (8–10): Attach indirectly to the sternum by connecting to the cartilage of the 7th rib.
• Floating Ribs (11–12): Do not attach to the sternum; they terminate in the posterior abdominal wall.

Each rib has key parts:

• Head: Articulates with the vertebral bodies.
• Neck: Connects the head to the tubercle.
• Tubercle: Articulates with the transverse process of the corresponding vertebra.
• Shaft (Body): The curved part that forms the rib cage.

Sternum

The sternum is a flat, elongated bone located in the midline of the anterior thorax. It consists of three parts:

• Manubrium: The uppermost part; articulates with the clavicles and first pair of ribs.
• Body: The central, elongated part; articulates with the costal cartilages of ribs 2–7.
• Xiphoid Process: A small, cartilaginous structure at the inferior end, ossifying with age.

The sternum provides a protective covering for the heart and serves as a key site for rib and clavicle attachment.

Muscles of the Thorax

The thoracic muscles can be categorized into respiratory muscles and supportive muscles:

Muscles of Respiration

Diaphragm:

• A dome-shaped muscle that forms the inferior boundary of the thoracic cavity.
• Separates the thoracic and abdominal cavities.

Intercostal Muscles:

Found between adjacent ribs; they include:

• External Intercostal Muscles: Elevate the ribs during inhalation.
• Internal Intercostal Muscles: Depress the ribs during forced exhalation.
• Innermost Intercostal Muscles: Work alongside internal intercostals.

Accessory Muscles:

Assist in forced respiration. Examples include:

• Sternocleidomastoid
• Scalene Muscles
• Pectoralis Minor

Supportive Muscles

• Pectoralis Major: Covers the anterior thorax and moves the upper limb.
• Serratus Anterior: A fan-shaped muscle on the lateral thoracic wall.
• Trapezius and Latissimus Dorsi: Large back muscles contributing to thoracic support and movement.

Thoracic Cavity

The thoracic cavity is enclosed by the rib cage and diaphragm, and it houses vital organs. It is divided into three main compartments:

Pleural Cavities (Right and Left)

Each cavity houses one lung, surrounded by a thin membrane called the pleura:

• Parietal Pleura: Lines the thoracic wall.
• Visceral Pleura: Covers the surface of the lungs.
• The space between the two layers, the pleural cavity, contains a small amount of serous fluid to reduce friction during breathing.

Mediastinum

• The central compartment between the pleural cavities.
• It contains the:
• Heart (within the pericardium)
• Great vessels (aorta, superior vena cava, pulmonary vessels)
• Trachea and esophagus
• Thymus gland
• Lymph nodes and nerves

Pericardial Cavity

• Located within the mediastinum.
• Houses the heart, enclosed in a fibrous, fluid-filled sac called the pericardium.

Blood Vessels and Nerves of the Thorax

Major Arteries

• Aorta: The largest artery, originating from the heart and arching posteriorly.
• Intercostal Arteries: Branches that supply blood to the intercostal muscles and ribs.
• Subclavian Arteries: Supply blood to the upper limbs and thoracic wall.

Major Veins

• Superior Vena Cava: Drains blood from the upper body into the heart.
• Azygos System: Includes the azygos vein, which drains blood from the thoracic wall.

Nerves

• Intercostal Nerves: Arise from the thoracic spinal nerves and supply the intercostal muscles.
• Vagus Nerve (CN X): Regulates thoracic and abdominal viscera.
• Sympathetic Trunk: Part of the autonomic nervous system, running parallel to the spine.

Lymphatic System

The thorax contains an extensive lymphatic network, including:

• Thoracic Duct: The largest lymphatic vessel, draining lymph into the venous system.
• Lymph Nodes: Mediastinal, intercostal, and diaphragmatic lymph nodes filter lymph from the thoracic organs and wall.

Surface Landmarks of the Thorax

Key external landmarks include:

• Suprasternal Notch: Depression at the superior border of the manubrium.
• Sternal Angle (Angle of Louis): Junction between the manubrium and body of the sternum, marking the 2nd rib.
• Costal Margin: Lower edge of the rib cage.
• Clavicles: Form the superior boundary of the thorax.

Function

The thorax, commonly known as the chest, serves as a crucial anatomical region that performs multiple life-sustaining functions. Its bony framework, muscles, and organs work together to support respiration, protection of vital organs, circulation, and movement. The thorax is essential for maintaining bodily functions, including oxygen exchange, blood circulation, and structural support.

Here is a detailed explanation of the functions of the thorax:

Protection of Vital Organs

The thorax provides a protective framework for the vital organs housed within the thoracic cavity:

• Heart: The rib cage, sternum, and thoracic vertebrae shield the heart from trauma and external forces.
• Lungs: The ribs and intercostal muscles protect the lungs, preventing direct injury.
• Great Vessels: Major blood vessels such as the aorta, superior vena cava, pulmonary arteries, and veins are safeguarded within the thorax.
• Esophagus and Trachea: These critical pathways for food, liquids, and air are protected within the thoracic cavity.

The rib cage acts as a rigid yet flexible shield that absorbs impacts and minimizes damage to these organs.

Respiration (Breathing)

The thorax is the primary region for the mechanics of breathing, facilitating inhalation and exhalation to support oxygen exchange in the lungs:

Expansion and Contraction

• The intercostal muscles and diaphragm work together to expand and contract the thoracic cavity, creating changes in pressure that drive airflow.
• During inhalation, the diaphragm contracts and flattens, increasing thoracic volume and drawing air into the lungs.
• During exhalation, the diaphragm relaxes, and the thoracic volume decreases, pushing air out of the lungs.

Pleural Cavities

The lungs are surrounded by the pleura (visceral and parietal layers), which reduces friction and allows smooth lung expansion and contraction during respiration.

Accessory Muscles

• Muscles such as the sternocleidomastoid, scalenes, and pectoralis minor assist in forced inspiration during physical activity or respiratory distress.
• The thorax creates the physical space and pressure gradient needed for efficient oxygen intake and carbon dioxide removal, ensuring survival.

Circulation and Blood Flow

The thorax plays a key role in circulation by housing the heart and major blood vessels:

Heart Function

The heart, located within the mediastinum and protected by the pericardium, pumps oxygenated blood to the body and deoxygenated blood to the lungs.

Major Blood Vessels

• The aorta distributes oxygenated blood from the heart to all body parts.
• The superior vena cava and inferior vena cava return deoxygenated blood to the heart.
• The pulmonary arteries and veins facilitate blood flow between the heart and lungs for oxygenation.

The thoracic cavity ensures the heart and vessels function optimally to supply nutrients and oxygen while removing metabolic waste.

Movement and Support

The thorax provides structural support and mobility for the upper body:

• Attachment of Upper Limbs: The thorax connects the clavicles and scapulae (shoulder girdle) to the sternum and rib cage, facilitating the movement of the arms and shoulders.
• Support for the Head and Spine: The thoracic vertebrae (T1–T12) support the upper body and provide attachment points for muscles that stabilize the head, neck, and torso.
• Core Stability: The muscles of the thorax, including the intercostals, pectoralis major, trapezius, and latissimus dorsi, stabilize the torso during movement, balance, and weight-bearing activities.

The thorax plays a role in maintaining posture and enabling activities such as lifting, bending, and twisting.

Passage for Vital Structures

The thorax serves as a conduit for critical structures that pass between the head, neck, and abdomen:

• Esophagus: A muscular tube that carries food and liquids from the pharynx to the stomach, passing through the esophageal hiatus of the diaphragm.
• Trachea: The airway for transporting air from the larynx to the lungs.
• Blood Vessels: The thorax houses and protects the aorta, vena cavae, and pulmonary vessels.
• Nerves:
  • The vagus nerve (CN X) regulates thoracic and abdominal organs.
  • The phrenic nerve innervates the diaphragm, enabling respiration.
  • The sympathetic trunk transmits autonomic nervous system signals to the thoracic and abdominal regions.

This passage of structures ensures communication, blood supply, and airflow between the upper and lower parts of the body.

Generation of Intra-Abdominal Pressure

The thorax assists in creating intra-abdominal pressure for various bodily functions:

• During activities like defecation, urination, and childbirth, the diaphragm contracts and increases intra-abdominal pressure.
• This pressure aids in expelling waste from the digestive system, urine from the bladder, and facilitating the delivery of a baby during labor.

Role in Immune Defense

The thorax houses lymphatic structures that are critical for the body’s immune defense:

• Thoracic Duct: The largest lymphatic vessel, located in the posterior thorax, drains lymph from most of the body into the venous system.
• Lymph Nodes:
  • Mediastinal lymph nodes filter lymph from the lungs, heart, and chest wall.
  • These nodes help trap pathogens and cancer cells, preventing their spread.

The thorax contributes to the body’s immune function by supporting the lymphatic network.

Thermoregulation

The thorax aids in maintaining body temperature:

• Muscular Activity: Movements of the respiratory muscles, such as the diaphragm and intercostals, generate heat.
• Vasodilation and Vasoconstriction: Blood vessels within the thorax regulate heat distribution throughout the body.

Clinical Significance

The thorax is of critical clinical importance as it houses vital organs like the heart, lungs, and major blood vessels. Disorders or injuries in this region can have life-threatening consequences.

Respiratory Conditions

• Diseases such as pneumonia, pleural effusion, pulmonary embolism, chronic obstructive pulmonary disease (COPD), and lung cancer affect lung function.
• Trauma can lead to pneumothorax (collapsed lung) or hemothorax (blood in the pleural cavity).

Cardiac Disorders

Conditions like coronary artery disease, pericarditis, heart failure, and myocardial infarction affect the heart housed within the thorax.

Thoracic Trauma

Rib fractures, flail chest, and injuries to the sternum can compromise breathing and protection of internal organs.

Esophageal Disorders

Esophageal cancer, gastroesophageal reflux disease (GERD), and esophageal rupture impact the digestive pathway through the thorax.

Musculoskeletal Issues

Conditions like scoliosis or kyphosis deform the thoracic vertebrae, affecting posture and respiratory capacity.

Thoracic Outlet Syndrome

Compression of nerves or blood vessels at the thoracic inlet causes pain, numbness, or swelling in the arms and chest.