The Thoracic Cage Anatomy

The Thoracic Cage

• The thoracic cage serves as the protective structure around the heart, lungs, and other organs in the chest area.
• It is formed by the vertebral column at the back, the sternum and costal cartilages in the front, and the ribs and intercostal spaces on the sides.
• The cage has two openings, the Superior and Inferior Apertures.

Superior Aperture of the Thoracic Cage

• The superior aperture of the thoracic cage is a narrow, kidney-shaped opening located at the top of the chest.
• It is bordered by the manubrium sterni anteriorly, the body of T1 vertebrae posteriorly, and the first rib and its cartilage laterally.
• This opening allows passage of the trachea, esophagus, lung apices, remnants of the thymus, brachiocephalic artery, left common carotid artery, left subclavian artery, right and left brachiocephalic veins, right and left phrenic nerves, right and left vagus nerves, right and left sympathetic trunks, and the right and left 1st thoracic nerves.

Inferior Aperture of the Thoracic Cage

• The inferior aperture is the lower opening of the thoracic cage, and is bordered by the infrasternal angle anteriorly, the T12 vertebral body posteriorly, and the costal margin (7th to 12th ribs) laterally.
• The diaphragm forms the floor of this aperture.

The Sternum

• The sternum is composed of fused sternebrae.
• It consists of three separate bones: the manubrium, the body, and the xiphoid process.
• The manubrium has the jugular notch, the articulation points for the first and second ribs, and clavicular articular facets. The sternal angle is formed by the manubrium and the second rib.
• The body of the sternum articulates with ribs 2-7, and is joined to the xiphoid process via the xiphisternal joint.
• The xiphoid process starts as cartilage but calcifies with age. It serves as a partial attachment point for multiple muscles.

The Ribs

• There are typically 12 pairs of ribs.
• The first seven ribs are "true ribs" due to their direct attachment to the sternum.
• The remaining five are "false ribs" due to their indirect or no attachment to the sternum.
• Ribs 11 and 12 are "floating ribs" due to their lack of ventral attachment.
• Ribs 3-9 are considered "typical ribs" and have a head, neck, tubercle, angle, shaft, and sub-costal groove.
• Ribs 1, 2, 10, 11, and 12 are considered "atypical ribs" due to their unique features.

Thoracic Vertebrae

• There are 12 vertebrae in the thoracic region.
• They are characterized by the presence of transverse costal facets, costal facets on the vertebral body, long spinous processes that point inferiorly, superior articular facets facing dorsally/posteriorly, inferior articular facets facing ventrally/anteriorly, a circular vertebral foramen, and a heart-shaped body.
• The first and 9th-12th thoracic vertebrae are atypical compared to the typical vertebrae (2nd-8th).

The Diaphragm

• This dome-shaped, musculo-aponeurotic partition separates the thorax and the abdomen.
• Its convex superior surface faces the thorax.
• The central part of the diaphragm is tendinous, while the peripheral part is muscular.
• It has three main openings:
  • The vena caval opening, located at the level of the T8 vertebra in the central tendon, allows passage of the inferior vena cava and branches of the right phrenic nerve.
  • The esophageal opening, located at the level of the T10 vertebra, allows passage of the esophagus, the right and left vagus nerves, the esophageal branches of the left gastric vessels, and lymph vessels.
  • The aortic opening lies anterior to the body of the T12 vertebra between the crura, and allows passage of the aorta, thoracic duct, and azygos vein.
• It also has minor openings, such as the space of LARREY, which transmits superior epigastric vessels and a few lymphatics of the liver. When enlarged, this space is called the "foramen of Morgagni".

The Lungs

• The lungs are the key organs for respiration and lie in the thoracic cavity, one on each side of the mediastinum.
• Each lung has an apex, base, costal surface, and medial surface.
• The apex of the lung is situated above the border of the first rib and close to the blood vessels and nerves at the root of the neck.
• The base of the lung is related to inferior surface of the diaphragm.
• The costal surface of the lungs is convex and abuts the costal cartilages, ribs, and intercostal muscles.
• The medial surface of the lungs is concave and contains the hilum, where the structures forming the lung root enter and leave the lung.
• The lung root includes the primary bronchus, pulmonary artery, two pulmonary veins, bronchial arteries and veins, lymphatics, and nerves.

Mediastinum

• The mediastinum refers to the region between the lungs, containing the heart, great vessels, trachea, right and left bronchi, esophagus, lymph nodes, lymph vessels, and nerves.

Right and Left Lungs

• The right lung has three lobes, while the left lung has only two lobes.

Pleura and Pleural Cavity

• The parietal pleura lines the thoracic cavity walls, while the visceral pleura covers the lung surface.
• The pleural cavity is a potential space between these two pleura layers, containing a serous fluid.
• The two pleura layers are continuous at the hilum.

Hilum

• The hilum is a depression or slit-like opening in the medial surface of the lung through which the lung root passes.

Lower Border of the Lungs

• The level of the lower border of the right lung is typically indicated by a line starting at the sternal end of the 6th rib in the midclavicular line, extending to the 8th rib in the midaxillary line, and finally to the 10th rib at the vertebral column.
• The lower border of the left lung is represented by a line starting at the sternal end of the 7th rib in the midclavicular line, extending to the 8th rib in the midaxillary line, and finally to the 11th rib adjacent to the vertebral column.

Parietal Pleura

• The inferior margin of the parietal pleura takes an oblique path laterally, starting from the level of rib 8 in the midclavicular line to rib 10 in the midaxillary line and then continuing to the T12 vertebra at the vertebral column.

Interior of the Lung

• The interior of the lung consists of bronchi, smaller air passages, alveoli, connective tissue, blood vessels, lymph vessels, nerves, and elastic connective tissue.

Pulmonary Blood Supply

• The bronchial arteries supply blood to the lung tissue.
• The bronchial veins drain the lung tissue, with the right side draining into azygos vein and the left side draining into the superior intercostal vein.

Bronchi and Bronchioles

• The trachea divides at the level of the 5th thoracic vertebra, forming the right and left bronchi.
• The right bronchus is wider, shorter, and more vertical than the left bronchus, making it more prone to obstruction by inhaled foreign bodies.
• After entering the lung, the right bronchus divides into three lobar bronchi.
• The left bronchus is longer, narrower, and more oblique than the right.
• It divides into two branches after entering the lung.
• The wall of the conducting passages are thick-walled, while the cartilage becomes irregular towards the end, eventually disappearing at the level of the bronchiole.
• Smooth muscle replaces cartilage in the bronchiole wall, making it responsive to ANS stimulation and irritation.

Epithelial Change

• The epithelium changes from ciliated columnar to non-ciliated cuboidal and finally to squamous epithelium as the airway divides and the passages get smaller.

Nerve Supply

• The vagus nerve provides parasympathetic innervation, resulting in bronchoconstriction.
• The sympathetic nervous system innervates the lungs, leading to bronchodilation.

Respiratory Bronchioles and Alveoli

• The lung is further divided into lobules by fine sheets of connective tissue.
• As the airway splits, the wall becomes thinner and thinner with a gradual decrease in smooth muscle and connective tissue.
• Alveolar ducts and alveoli have a single layer of squamous epithelium and are supported by elastic fibers, which are surrounded by a dense network of capillaries, forming the respiratory membrane.
• Septal cells (type II pneumocytes) are located between the squamous cells and produce surfactant, a substance that reduces surface tension in the lungs. Surfactant production starts between 24th and 35th weeks of fetal life.

Function

• The lungs have protective cells, such as plasma cells, macrophages, and lymphocytes, that act in distal air passages to defend against microbes.