What Is in the Thoracic Cavities?
The thoracic cavity is basically the chest, including everything between the neck and the diaphragm. It’s home to the thoracic organs and is protected by the thoracic cage. The heart and lungs are essential for survival and both are prone to certain diseases, so you need to be able to examine them.
The thoracic cavity has three compartments: the mediastinum and two pleural cavities. The mediastinum is home to the heart, trachea, great vessels, and some other structures. The pleural cavities are on either side of the mediastinum and contain the lungs and the pleural linings.
The mediastinum is the compartment that takes up the middle portion of the thoracic cavity. It’s lined by mediastinal pleura and extends from the superior thoracic aperture (where the thoracic cavity opens into the neck) down to the diaphragm (the main muscle for breathing). It has several different regions:
Superior mediastinum: This region covers the area from the superior thoracic aperture to a horizontal plane at the level of the intervertebral disc between the 4th and 5th thoracic vertebrae. It contains the superior vena cava, brachiocephalic veins, arch of the aorta, thoracic duct, trachea, thymus, and vagus and phrenic nerves.
Inferior mediastinum: This region of the mediastinum starts where the superior mediastinum leaves off and extends inferiorly (in other words, toward the feet) to the diaphragm. It has three sections:
The anterior mediastinum contains lymph nodes, fat, connective tissue, and remnants of the thymus (it shrinks after childhood). It’s close to the front of the body.
The middle mediastinum is home to the pericardium, heart, arch of the azygos vein, main bronchi, and roots of the great vessels.
The posterior mediastinum contains the esophagus, thoracic aorta, azygos and hemiazygos veins, vagus nerve, sympathetic trunks, and splanchnic nerves. It’s close to the rear of the thorax.
The pleural cavities
Two pleural cavities house the lungs, one on each side of the mediastinum. Each lung is covered by a pleural sac, which is made up of two layers of pleura:
Visceral pleura: This pleura adheres to the lungs.
Parietal pleura: Lining the cavities and attached to the thoracic wall, the mediastinum, and the diaphragm, this pleura has four parts:
The costal part covers the sternum, ribs and cartilage, intercostal muscles, and sides of the thoracic vertebrae.
The mediastinal part covers the sides of the mediastinum.
The diaphragmatic part covers the parts of the diaphragm on each side of the mediastinum.
The cervical pleura runs from the superior thoracic aperture into the root of the neck to form a dome over the lungs.
Each cavity contains recessed spaces called the costodiaphragmatic and costomediastinal recesses, which allow room for full expansion of the lungs during inspiration (breathing in).
Much of the focus when you explore the thoracic region is on the heart and lungs, but the following organs also live in the thoracic cavity:
The diaphragm: The diaphragm is a dome-shaped muscle that divides the thorax from the abdomen. Its function is to expand and contract the thoracic cavity during inhalation and exhalation along with the muscles of the thoracic wall. The central tendon of the diaphragm is attached to the pericardium.
The thymus: The pink-lobed thymus is located between the sternum (breastbone) and the pericardium in the anterior portion of the mediastinum. The thymus grows during childhood, but after puberty it shrinks (or involutes). The thymus is the site for the maturation of T cells. The thymus gets its blood supply from the inferior thyroid and internal thoracic arteries.
The esophagus: The esophagus is a tube about 10 inches long that joins the oropharynx to the stomach. It runs downward and to the left through the superior and posterior mediastinum.
The nerves of the thorax: A number of nerves can be found in the thoracic cavity:
The right vagus nerve enters the mediastinum anterior to the right subclavian artery, where it gives off the right recurrent laryngeal nerve. The left vagus nerve enters the mediastinum between the left common carotid and left subclavian artery. It passes anterior to the arch of the aorta where it gives off the left recurrent laryngeal nerve. The left and right vagus nerves join the esophageal plexus and continue into the abdomen.
On the right side, the recurrent laryngeal nerve loops around the subclavian artery. On the left, it passes under the arch of the aorta. Both nerves run up to the larynx, one on each side.
The right and left phrenic nerves enter the superior thoracic aperture and travel between the mediastinal pleura and the pericardium to the diaphragm.
The cardiac plexus receives branches from the vagus nerve and the sympathetic trunk and runs to the arch of the aorta and heart.
The pulmonary plexus also receives branches from the vagus nerve and the sympathetic trunk and runs to the bronchial subdivisions in the lungs.
The esophageal plexus receives fibers from the vagus nerve and sympathetic ganglia and form a plexus on the esophagus inferior to the bifurcation of the trachea.