16th February 2024

Thoracic cavity

Thoracic cage

The bony thoracic wall, aka thoracic cage, comprises the thoracic vertebrae, ribs, costal cartilage and sternum. There are a total of 12 ribs. The first seven ribs are true ribs, connected to the vertebral column on the back and directly with the sternum anteriorly. The next three are false ribs, as their front attachment with the sternum is indirectly via combined costal cartilage. 

The last two are the floating ribs, which do not have anterior attachment to the sternum. Inferior and just internal to each rib is the costal groove. The costal groove contains the intercostal vein, artery and nerve from superior to inferior.

Image Source: Wikimedia Commons (2014). Endothoracic fascia, Licensed under a creative commons licence. Available for free at: https://upload.wikimedia.org/wikipedia/commons/e/e2/Endothoracic_fascia.svg (Accessed: 9 January 2024).

Pleural lining

The pleura is a thin double membrane which lines the lungs on each side. The outer membrane is called the parietal pleural; the inner is known as the visceral pleura. The parietal pleural is closely attached to the thoracic wall and the visceral pleura to the lungs. A tiny physiological space contains fluid between the two layers, which helps maintain the negative pressure for the lungs to expand. 

Somatic nerves innervate the visceral layer; hence, it is sensitive to touch, pain, temperature and pressure. The visceral layer is only sensitive to stretch due to its visceral nerve innervation.

Lungs

The respiratory system is divided into the conducting zone and the respiratory zone. The conducting zone starts from the nose and ends at the bronchioles. The trachea is part of the conducting zone and starts in the neck from the larynx and ends at the carina, at the level of the sternal angle and the T4/T5 vertebral level, by dividing into the right and left main bronchus. The right main bronchus is shorter, wider and more vertical than the left. Each of these main bronchi enters its respective lungs at the hilum and further divides into secondary lobar, then tertiary segmental bronchioles, which supply bronchopulmonary segments. Each bronchopulmonary segment is functionally and structurally independent.

Image Source: Wikimedia Commons (2013). 2312 Gross Anatomy of the Lungs, Licensed under a creative commons licence. Available for free at: https://upload.wikimedia.org/wikipedia/commons/7/7e/2312_Gross_Anatomy_of_the_Lungs.jpg (Accessed: 9 January 2024).

The respiratory zone includes the respiratory bronchioles and alveoli and moves the respiratory gases, that is, oxygen and carbon dioxide, in and out of the blood. Oxygenation occurs in the respiratory zone, from where the oxygenated blood is taken by the alveolar capillaries in the connective tissue surrounding the bronchopulmonary segment and drains to the segmental and main pulmonary veins.

The right lung is divided into three lobes: the upper, middle and lower lobes. The horizontal and oblique fissures delineate these. The left lung has only two lobes, upper and lower, divided by a single oblique fissure. The parietal pericardium lies directly in contact with the left lower lobe.

The pulmonary plexus supplies the lung tissue, providing parasympathetic, sympathetic and visceral afferent innervation. Two pulmonary veins from each lung bring the oxygenated blood to the left atrium. Deoxygenated blood is supplied to the lungs by the pulmonary arteries, accounting for the majority of blood supplied to the lungs. Oxygen supplies for the tissues are delivered via the bronchial arteries.

Lymphatic drainage

The thoracic duct brings the lymph from the lower half and the left side of the upper half of the body. It starts from the cisterna chyli at the level of L1; it is initially to the right of the midline in the thorax but then crosses the midline to the left at T4-6 and drains at the origin of the left brachiocephalic vein. The lymph drainage from the right upper half of the body is via the right lymphatic duct, which drains at the origin of the right brachiocephalic vein.

Image Source: Wikimedia Commons (2013). 2203 Lymphatic Trunks and Ducts System, Licensed under a creative commons licence. Available for free at :https://upload.wikimedia.org/wikipedia/commons/4/43/2203_Lymphatic_Trunks_and_Ducts_System.jpg (Accessed: 9 January 2024).

References

Wineski, L. E., & Snell, R. S. (2019). Snell’s clinical anatomy by regions. 

Chaurasia, B. D. (2019). Human anatomy: Regional and applied, dissection and clinical. New Delhi, India: CBS Publishers & Distributors.

Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). Moore’s Clinically Orientated Anatomy (7th ed.). China: Lippincott.