Appendix 1: Anatomy and development of the lungs (Fig. 47.36)

During the fourth week of intrauterine growth the lung bud develops from the ventral surface of the primitive foregut. This becomes a bibbed structure, the ends of which ultimately become the lungs. The trachea and bronchi derive their blood supply from the bronchial arteries coming from the aorta. Although development is initially symmetrical in each lung bud, the typical lobar arrangement is defined early in foetal life. Congenital abnormalities may occur during development, the most important ones being agenesis (nonformation) or accessory lobes.

The primitive lungs drain into the cardinal veins which ultimately become the pulmonary veins draining into the left atrium. Abnormalities of venous drainage may occur at this stage of development (compare anomalous venous drainage).

Surgical anatomy of the lungs

There are three lobes to the right lung and two to the left, although the two sides are basically very similar (Fig. 47.37). Each lobe is composed of segments (see above) with its own discrete blood supply of pulmonary artery and pulmonary vein: an important point when resecting individual segments. No bronchi or arteries cross the intersegmental plains.

The right main bronchus is shorter, wider and nearly vertical compared with the left. As a consequence, inhaled foreign bodies are more likely to enter the right main bronchus than the left. The bronchi are supplied with blood from the bronchial arteries which arise directly from the aorta. They drain into both the systemic and pulmonary circulations (Figs 47.38and 47.39).

Lymphatic drainage tends to follow the bronchi and certain collections of lymph nodes are recognised around the hila and trachea (Fig. 47.40).

These are important as their presence or absence is useful in staging malignant disease (noninvasively by radiography or invasively by needle biopsy, mediastinotomy or mediastinoscopy). They are also important in determining prognosis following resection of a lung neoplasm.