Pulmonary ventilation, more commonly referred to as breathing, is the movement of air into and out of the lungs. Inhalation is the act of drawing air into the lungs, and exhalation is the expulsion of that air out of the lungs. This action follows Boyle’s law, which explains that the pressure of a gas decreases as the volume increases if the temperature is held constant.[1] Increasing the volume of the thoracic cavity causes a decrease in the intrathoracic pressure and an influx of air. Meanwhile, decreasing the volume, typically through the relaxation of the muscles of inspiration, causes an increase in intrathoracic pressure and results in exhalation. The thoracic wall is essential to this process and is made up of the sternum, 12 pairs of ribs, 12 thoracic vertebrae, and the muscles, fascia, and skin that connect to this bony cage.

The structures of the thoracic wall protect the heart, lungs, and great vessels as well as some abdominal organs. Additionally, the bony structures provide attachment points for muscles and allow for the mechanical function of ventilation.

Ribs are curved, flat bones that attach posteriorly to vertebrae and anteriorly to the sternum. Ribs originate at each thoracic vertebra. The head of each rib has 1 or 2 facets for articulation with the costal facets on the bodies of thoracic vertebrae. Ribs 1, 10, 11, and 12 have a single facet on their head that only articulates with its respective vertebra. Ribs 2 through 9 all have 2 facets on their heads; the superior facet articulates with the vertebra above the rib, while the inferior facet articulates with the respectively named vertebra. In addition, ribs 1 through 10 have tubercles with facets that articulate with the costal facet of the transverse process of each respective vertebra.[2]. Ribs 11 and 12 have no tubercle and therefore do not articulate with the transverse process of their respective vertebra.

The sternum is composed of 3 parts: the manubrium, body, and xiphoid process.[3] The manubrium is the widest portion and contains the jugular, or sternal notch as well as the clavicular notch. The sternal angle, or angle of Louis, is where the manubrium joins the body of the sternum. The second rib attaches at this point. The xiphoid process is attached inferiorly to the body of the sternum and provides an attachment point for the diaphragm and rectus abdominis, but no ribs.

Ribs 1 through 7 are considered true, or vertebrocostal, ribs and attach directly to the sternum. Ribs 8 through 10 are considered false, or vertebrochondral, ribs in which the cartilage of each rib attaches to the cartilage of the rib above it which terminates where the seventh rib attaches to the sternum. Ribs 11 and 12 are considered floating, vertebral, or free, ribs because they terminate in the posterior abdominal musculature and do not attach to the sternum. The costal cartilage of ribs 1 through 10 facilitates the elastic recoil of the thoracic wall which contributes to passive exhalation.

The thoracic wall forms part of the axial skeleton and is comprised of bone, muscle, and connective tissue. It develops from the mesoderm where blocks become somites from which the sclerotome separates. The sclerotome is the origin of the vertebrae and transverse elements which begin elongating into ribs during the fifth week of gestation. Ossification begins during the fetal period but will not be complete until adulthood.[4] The myotome gives rise to the muscles of the thoracic wall. The sternum forms from sternal bars that meet in the midline and begin to fuse during the seventh week of gestation. Fusion starts superiorly and ends inferiorly by the tenth week.[5]

Each intercostal space receives its blood supply from 3 arteries, a posterior intercostal artery and a pair of anterior intercostal arteries. The posterior arteries of the first 2 intercostal spaces are fed from the superior (supreme) intercostal artery which comes off the subclavian artery.[6] The remaining pairs of posterior intercostal arteries and a pair of subcostal arteries emerge directly from the thoracic aorta.[7] These posterior arteries enter the costal groove near the angle of each rib where they travel between the intercostal vein and nerve. The posterior and anterior intercostal arteries anastomose laterally.

The internal thoracic artery is another branch of the subclavian artery that supplies the anterior intercostal arteries of spaces 1 through 6 before dividing into the superior epigastric and musculophrenic arteries.[8] The musculophrenic artery supplies the anterior intercostal arteries of spaces 7 through 9. The 2 most inferior intercostal spaces are supplied from the posterior intercostal arteries and their collateral branches and do not have anterior intercostal arteries.

The intercostal veins run most superiorly in the costal grooves. There are 11 pairs of posterior intercostal veins and one pair of subcostal veins. Like the posterior arteries, the posterior veins also anastomose with the anterior intercostal veins. Several of the superior posterior intercostal veins merge to form the left and right superior intercostal veins at the level of T3 or T4 which can then empty into the brachiocephalic vein or the superior vena cava (SVC).[9] The remaining posterior intercostal veins end at the azygous vein before emptying into the SVC. The internal thoracic veins accompany the internal thoracic arteries before emptying into the brachiocephalic vein.

The respiratory center in the medulla oblongata controls the rate and depth of ventilation. While the dorsal respiratory group controls the motor neurons for all inspiration, the ventral respiratory group only controls forced exhalation since normal exhalation is passive. The phrenic nerve is a branch of the cervical plexus. It originates mostly from C4, with some involvement of C3 and C5. The right and left sides travel through the neck and thorax before innervating the diaphragm.[10]. The anterior rami of spinal nerves T1 through T11 form the intercostal nerves while the anterior rami of spinal nerve T12 form the subcostal nerve.[11] A portion of the anterior ramus of T1 forms part of the brachial plexus. However, the remaining intercostal nerves do not form plexuses. While the afferent fibers of these nerves conduct sensory information to include that of the overlying skin, the efferent fibers relay motor information to the muscles of respiration.