Introduction to the Respiratory System

Boundaries of the Thoracic Cavity

• The thoracic cavity is the space occupied by the lungs and their covering of visceral pleura.
• The lungs almost completely fill this space, leaving a tiny gap between the visceral pleura and the parietal pleura.
• The enclosed space between the pleural membranes is called the pleural cavity.
• The thoracic cavity is bounded posteriorly by the thoracic vertebral column, laterally by the ribs, and anteriorly by the sternum.

Thoracic Apertures

• The thoracic inlet, or superior thoracic aperture, is a bony-cartilaginous ring defined by the 1st thoracic vertebra, 1st rib, 1st costal cartilage, and upper margin of the manubrium.
• The suprapleural membrane attaches to this ring and rises upwards, extending 2.5 cm above the medial end of the clavicle.
• The thoracic outlet, or inferior thoracic aperture, is the space bounded by the costal margin in front, the tips of the 11th and 12th ribs, and the inferior margin of the 12th thoracic vertebra.
• The diaphragm closes off the thoracic outlet and is a bi-domed muscular sheet inserting into a central tendon.

Structures Passing Through the Thoracic Inlet

• Structures that pass through the thoracic inlet, such as the trachea, oesophagus, carotid artery, and internal jugular vein, take a sleeve of the fascial membrane as they do so.
• The trachea and oesophagus take pre-tracheal fascia, the vessels of the head and neck take the carotid sheath, and the vessels of the upper limb take the axillary sheath.

Diaphragm

• The diaphragm is the principal muscle of breathing.
• Each side of the diaphragm is referred to as a hemi-diaphragm.
• The diaphragm descends to enable inspiration and relaxes to allow expiration.
• The thoracic outlet transmits thoracic structures destined for the abdomen and permits passage of venous blood from the lower half of the body back into the heart.