2 - Normal anatomy, histology, and cytology

Summary

The lungs

The lungs, occupying the majority of the thorax, are covered by visceral pleura, which is in continuity with the parietal pleura that runs along the inner chest wall on each side (Fig. 2.1). The visceral and parietal pleura are normally in close approximation to each other, forming a potential space in the thorax; under normal conditions, there is only a small amount of serous fluid present to reduce friction during respiration. These paired organs, occupying the majority of the thoracic cavity, are divided into lobes by fissures, which are invested by pleura. The right lung has three lobes (superior, middle, and inferior), while the left lung has two (superior and inferior). While the right lung is larger, it is also wider and shorter than the left, due to the elevation of the right diaphragm. The superior lobe of the left lung has a cardiac notch at its anterioinferior aspect, creating the lingula. The lung segments usually are not separated by pleura, but are supplied by segmental bronchi. The lungs have a specialized dual vascular system. Oxygenated blood from the systemic circulation enters via the bronchial arteries and returns by bronchial veins, while deoxygenated blood from the right ventricle enters via the pulmonary arteries and returns to the left atrium after it has been oxygenated via the pulmonary veins. Under normal circumstances, the pulmonary circulation is a low pressure system at approximately 10 mmHg, while the bronchial vasculature maintains systemic pressures. Because of the dual blood supply, small pulmonary emboli do not tend to cause infarcts, but lead to hemorrhage instead.

The right main bronchus is wider and shorter than the left, and has a more vertical course, making this the more likely airway for foreign objects to lodge. After the main bronchi enter the hila of the lungs, they branch into lobar bronchi, each of which supplies a lung lobe. These in turn branch into segmental bronchi, which supply the pyramid-shaped bronchopulmonary segments that are surgically resectable. The airways continue to branch (approximately 25 times) to generate the terminal bronchioles. Further subdivisions include the respiratory bronchioles, alveolar ducts, and finally the alveolar sacs.