The anatomy of the respiratory system can be divided into 2 major parts, airway anatomy and lung anatomy.
Airway anatomy can be further subdivided into the following 2 segments:
The extrathoracic (superior) airway, which includes the supraglottic, glottic, and infraglottic regions
The intrathoracic (inferior) airway, which includes the trachea, the mainstem bronchi, and multiple bronchial generations (which have as their main function the conduction of air to the alveolar surface)
Lung anatomy includes the lung parenchyma, which carries part of the conduction system but is mainly involved in the gas exchange at the alveolar level. The lung parenchyma is further subdivided into lobes and segments.
The purpose of this chapter is to provide a better understanding of the anatomy of the airways and lungs, which will help the health provider to recognize and manage different respiratory abnormalities.
The trachea is a cartilaginous and fibromuscular tube that extends from the inferior aspect of the cricoid cartilage (sixth cervical vertebra level) to the main carina (fifth thoracic vertebra level). Its length is 3 cm at birth and 10-12 cm in adults (of which 2-4 cm is extrathoracic and 6-9 cm intrathoracic). Tracheal diameters vary widely, ranging from 13 to 25 mm (coronal plane) in men. In women, the variability is still noted, with a range of 10-21 mm (coronal plane). The shape of the intrathoracic trachea changes during expiration as a result of invagination of the posterior wall, causing as much as a 30% reduction of the anteroposterior diameter as seen on dynamic CT scanning (see the images below).
The trachea has multiple layers (see the image below). The mucosa is composed of a ciliated pseudostratified columnar epithelium and numerous mucus-secreting goblet cells that rest on a basement membrane with a thin lamina propria (mainly collagenous). The submucosa contains seromucous glands. The adventitia contains cartilaginous rings interconnected by connective tissue. The hyaline cartilage rings have the form of the letter C and are opened posteriorly. The open ends are connected by fibroelastic tissue and a band of smooth muscle (the trachealis).
As a human being develops from a fetus to a fully developed adult, several changes take place. Some of these changes follow regular patterns, and others either compensate for certain conditions or occur for unknown reasons.
Congenital anatomic variants of the lungs are present in the following forms:
Agenesis – A congenital complete absence of one or both lungs, the latter being incompatible with life (The condition is associated with other congenital abnormalities and is rare.)
Aplasia or hypoplasia – The presence of a rudimentary bronchus that ends in a blind pouch with no evidence of pulmonary vasculature or lung parenchyma
Accessory lobes, and fusion of lobes – Variations over the lung lobes that are mainly caused by the incomplete obliteration of the visceral pleural folds, result from the presence of abnormal vessels (creating extra lobes), or occur secondary to (completely or partially) fused lobes from obliteration of the normal lung fissures (See the images below.)
Pathologic variants are related to changes in the structure of the airways, the lung parenchyma, or adjacent structures that lead to disruption of the normal anatomy of the respiratory system. The most common such variants are presented below.
Emphysema/chronic obstructive pulmonary disease
Emphysema and chronic obstructive pulmonary disease are caused by an accumulation of inflammatory mucus that gives rise to a loss of elastic recoil resulting from lung tissue destruction or to an increase in the resistance of the conducting airways, leading to an abnormal permanent enlargement of air spaces distal to the terminal bronchioles (see the images below).