The branching nature of the bronchi contributes to the lobar structure of the lungs. This structure allows for an exponential increase in the amount of surface area for gas exchange as the bronchi continue to branch until they reach the terminal bronchioles and finally the alveoli. This branching nature also permits lung segments and lobes to function relatively independently. Pathological processes in one lobe, such as pneumonia, will not necessarily affect other regions of the lung. It should be pointed out that considerable anatomical variation might exist between individuals. The bronchial anatomy described herein is illustrative of a typical bronchial pattern. The reader should be aware that oftentimes, two or three bronchi might arise from a common trunk rather than have separate and discrete origins. This is frequently the case for the apical and posterior segments of the upper lobe of the left lung (therefore typically combined into apicoposterior) and the anterior and medial basal segments of the lower lobe of the left lung (therefore often combined into anteromedial basal). This explains why some anatomy texts list 10 segments for the left lung and others only 8.
Conducting Airways and Lobes of the Lungs
After the bifurcation of the trachea into the main bronchi, the conducting airways pass through the hilum (the root of the lung) and continue to bifurcate. Throughout the generation of these bifurcations, the airway diameter decreases and contains less cartilage and smooth muscle (Fig. 4-4).
In the normal adult, the left main bronchus (LMB) measures approximately 4.5 cm in length and the right (RMB) measures approximately 2.5 cm in length. The shortness of the right main bronchus is due to the more proximal origin of the right upper lobe bronchus. Taken together, the two main bronchi, volumetrically, have 40% more cross-sectional area than the trachea. The left main bronchus leaves the trachea at a 135-degree angle, whereas the more superiorly located right main bronchus tends to be more vertically oriented, having a 155-degree angle of origin.
Aspiration of a foreign body (eg, button, food) almost always lodges in the right lung, because of the orientation of the right main bronchus (RMB).
Soon after its origin, the RMB gives rise to the right upper lobe bronchus, which typically is directed superiorly and slightly laterally, having an almost 90-degree angle from the RMB (Fig. 4-4). The upper lobe bronchial trunk measures approximately 1 cm in length and approximately 1 cm in diameter. The trunk then gives rise to the segmental bronchi of the RUL. A segmental bronchus supplies the apical segment of the right upper lobe and has a diameter ranging from 4 to 7 mm. Another bronchus, supplying the posterior segment, has a more horizontal course.
Finally, a third segmental bronchus supplies the anterior segment and, like the posterior segmental bronchus, has a generally horizontal course but proceeds somewhat inferiorly from its origin. The right main bronchus extends no farther inferiorly than the origin of the right upper lobe bronchus. The airway distal to the upper lobe bronchus is referred to as the bronchus intermedius (BI). The bronchus intermedius generally averages 2 cm in length and terminates at the point of origin of the right middle lobe bronchus.
Right Middle Lobe Bronchi
The middle lobe bronchial trunk measures approximately 12 mm in length and 8 mm in diameter. The origin of the middle lobe bronchus marks the point of origin of the right lower lobe bronchus. From its origin, off the anterior aspect of the BI, the right middle lobe bronchial trunk continues slightly inferiorly for a short distance before giving rise to the lateral and medial segmental bronchi. The medial segmental bronchus has a slightly more oblique course than the lateral segmental bronchus.
The right superior segmental bronchus may arise at, or above, the level of the origin of the right middle lobe bronchus but more frequently arises slightly more distally. Regardless, the superior segmental bronchus is the first branch off the lower lobe bronchus and has a predominantly horizontal course. The airway distal to the superior segmental bronchus is referred to as the basilar trunk. The basilar segmental bronchi have a predominantly vertical orientation. The anterior, posterior, and lateral basilar segmental bronchi typically arise from a common trunk. The medial basal bronchus, oriented medially, has its origin inferior to the superior segmental bronchus.
The origin of the left upper lobe bronchus occurs at a level lower than the origin of the right upper lobe bronchus. The left upper lobe bronchial trunk gives rise to the upper lobe and lingular segmental bronchi. Measuring 9 mm in length and approximately 12 mm in diameter, the left upper lobe bronchial trunk characteristically appears short but has a large diameter. The left upper lobe bronchial trunk divides into the ascending upper division (eventually giving rise to the apico-posterior and anterior segments) and the descending lower division (which then gives rise to the lingular segmental bronchi). Note that in the left upper lobe bronchus, the apical and posterior segments are combined and as such are supplied by one bronchus. The courses of apical and posterior segmental bronchi have vertically and horizontally oriented components as bronchial rami divide to supply the apicoposterior segment. The anterior segmental bronchus will have a more horizontal course similar to that seen on the right side. The lingular segmental bronchi have an oblique course. The superior lingular segmental bronchus has a more horizontal course and supplies the superior lingular segment. The superior lingular segmental bronchus is superior to the more vertically oriented inferior lingular segmental bronchus.
The left superior segmental bronchus is similar to that on the right side, having a typically horizontal course and supplying the superior segment. There are only four segments in the left lower lobe, compared to five on the right. The bronchial segment that supplies the medial basal segment on the right side is not a separate entity on the left. As a result, the anterior and medial segments are combined and supplied by an anteromedial bronchus. As is the case on the right side, the basilar segmental lower lobe bronchi course predominantly vertically. Like their contralateral counterparts, lateral and posterior basal segmental bronchi may arise from a common trunk.
The normal orientation of segmental airways that feed the bronchopulmonary segments of each lobe dictate the bronchial drainage positions that patients adopt in order to mobilize secretions from the distal airways to the mainstem bronchi and trachea, where the secretions may be expelled with forceful coughing. For example, the bronchial drainage position that brings the segmental airway supplying the anterior segment of the right upper lobe to its full upright (vertical) position is supine with the bed flat. This position maximizes the effect of gravity in moving secretions out of the anterior segment and into the more proximal airways.
The upper airways include the nose, pharynx, nasopharynx, oropharynx, larynx, and the trachea. These structures allow communication between the environment and the lungs. Knowledge of their structure is important for physical therapists in certain examination procedures as well as in airway clearance techniques.
The nose is supported by bone and cartilage and is covered by skin. Periosteal and perichondral membranes blend to connect the bones and cartilage to each other. The nasal cavity is a wedge-shaped passage divided vertically by a septum into right and left halves and into compartments by the nasal conchae. The nasal cavity begins with the nares and passes posteriorly to the nasopharynx. The lateral walls of each cavity contain prominent folds called conchae that project medially and inferiorly into the cavity and serve to increase the respiratory surface of the nasal mucous membrane to help warm and moisten air. The mucous membrane is formed from nasal epithelial cells; additional functions include protecting the airway from foreign substances by trapping particles in mucus and allowing for their removal by sneezing. The conchae occupy a large portion of the available space in the nasal cavity, and a small amount of inflammation can obstruct the nasal passage. The floor of the nasal cavity is formed by the palatine process of the maxilla and the horizontal part of the palatine bone. The paired nasal cavities open through the narrowed posterior apertures into the pharynx (Fig. 4-5).
Sagittal view of the upper airways. (Reproduced with permission from Tintinalli JE, Kelen GD, Stapczynski JS. Emergency Medicine: A Comprehensive Study Guide. 6th ed. New York: McGraw-Hill; 2004:102.)
The nasal conchae narrow the nasal passageways, and the mucous membrane is composed of fragile cells, making suction-catheter trauma likely in the case of blind nasotracheal suctioning. This is why low platelet counts may be a contraindication to this form of suctioning or, at the very least, require the insertion of a nasopharyngeal airway (nasal trumpet).
The pharynx is a shared structural throughway that allows the digestive system (from the mouth) and the respiratory system (from the nose) passage to their respective destinations—the esophagus and the larynx. The posterior and lateral walls of the pharynx are muscular, whereas the anterior wall consists of the opening to the nasal cavities, the soft palate, the opening to the mouth and the tongue, and finally to the posterior wall of the opening to the larynx.1 The pharynx is surrounded by the superior, middle, and inferior constrictor muscles that run horizontally; the stylopharyngeus, which is oriented longitudinally, disappears between the superior and middle constrictors. The inferior constrictor maintains a tonic contraction until swallowing, serving as a sphincter between the esophagus and the pharynx. The pharynx normally undergoes small changes in size during normal breathing; however, structural abnormalities may impede airflow through the pharynx, particularly during sleep.
The pharynx is divided by the soft palate into the nasopharynx and oropharynx. Muscles that form the soft palate, which assist with ventilation, include the levator and tensor veli palatini, the musculus uvulae, the palatopharyngeus, and the palatoglossus. Their coordinated action regulates the route of airflow between nasal and oral pathways to meet ventilatory demands (Fig. 4-5).10
The roof of the nasopharynx is called the fornix and consists of a mucous membrane in close proximity to the basal portions of the sphenoid and occipital bones. The ostium of the auditory tube is located in the lateral wall of the pharynx and provides a structural connection to the middle ear. The soft palate forms a mobile floor of the anterior portion of the nasopharynx. The pharyngeal isthmus is posterior to the soft palate and forms the opening to the oropharynx. The isthmus can be closed by the levator veli palatini muscle pulling the soft palate backward and upward.10 The soft palate will approximate the posterior wall to allow, for example, proper phonation of consonants, drinking under pressure, and expiration of air through the mouth and not the nose (ie, pursed-lip breathing).
The oropharyx is bordered anteriorly by the base of the tongue and extends downward posteriorly to the upward projection of the epiglottis. The epiglottis is united to the tongue by a midline and two lateral folds—the median and the lateral glossoepiglottic folds. The laryngeal part of the pharynx is continuous with the oropharynx at the level of the upper border of the epiglottis and is wide superiorly and narrows as it travels posteriorly. Distal to the cricoid cartilage of the larynx, the pharynx becomes continuous with the esophagus. At this point, the anterior wall of the pharynx is the opening to the larynx.
The larynx (composed of nine cartilages) forms a protective connection between the pharynx and the trachea. As part of the respiratory system, the larynx protects the trachea from food and foreign bodies by acting as a valve. The larynx is also equipped with a phonating mechanism designed for voice production. Laryngeal muscles, in addition to phonation, produce large changes in the size and therefore resistance of the laryngeal opening through the vocal cords. The larynx is approximately 5 cm in length in adult males.
The nine laryngeal cartilages form joints to allow normal functioning of the laryngeal structures. The cricoarytenoid joints and the cricothyroid joints both allow movement, which approximates, tenses, relaxes, tightens, or slackens the vocal cords.
The larynx is divided into three compartments by the projecting folds of the mucous membranes of the lateral walls. The vestibule lies between the inlet and the superior folds; the ventricle, between the superior folds and the vocal cords; and the infraglottic cavity, between the vocal cords and the cricoid cartilage, where it is continuous with the trachea. Contraction of the transverse and oblique arytenoid muscles and the aryepiglottic muscles has a sphincter action and closes the laryngeal inlet as a protective mechanism during swallowing.
The trachea begins at the level of the cricoid cartilage of the larynx, which generally is at the level of the sixth cervical vertebra. In adults, the trachea ranges from 9 to 15 cm in length and terminates as the carina, a ridge at the bifurcation of the trachea into the left and right main bronchi (Fig. 4-4). The trachea has a maximum transverse diameter of 16 mm, whereas sagittally the trachea is narrower, having a maximal diameter of 14 mm. The posterior wall of the trachea tends to appear slightly flattened due to posteriorly directed horse-shoe–shaped cartilages. The carina is a cartilaginous wedge at the bifurcation of the trachea into the right and left main stem bronchi. It resides approximately at the level of the fifth thoracic vertebral body and can be localized approximately at the same level as the sternal notch.
Tracheal suctioning requires the insertion of a catheter into the upper airway, where it is passed down the trachea to the level of the carina. The carina is richly innervated by the vagus nerve. When the tip of the suction catheter comes in contact with the carina, it can provoke a strong parasympathetic response, which in turn can trigger a sudden decrease in the heart rate and produce cardiac arrhythmias. Therapists should monitor their patients carefully for the appearance of such events and provide supplemental oxygen during the procedure.