16th February 2024

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.