12.3: Respiratory Anatomy

The Nose

The primary entry and exit points for the respiratory system are located in the nose. 

The external nose comprises surface and skeletal structures contributing to its various functions. Between the eyebrows lies the root of the nose, connecting to the rest of the nose through the bridge. The length of the nose, known as the dorsum nasi, extends to the apex or tip. The ala, a cartilaginous structure, forms the lateral side of each naris (nostril), while the philtrum creates a concave surface linking the nose's apex to the upper lip. While the root and bridge of the nose comprise bone structures, the prominent part of the nose is primarily cartilage (Figure 2). The alar cartilage encompasses the nose's apex and the naris, while the septal cartilage, a flexible hyaline cartilage, forms the dorsum nasi, connecting to the nasal bone. Nostrils or nares are the anterior nasal apertures opening into the right and left nasal cavities.

​Figure 2 | Nose |² External and skeletal features of the nose. This illustration provides an anatomical overview of the external nose and nasal cartilage. The top panel highlights surface landmarks, including the root, which is located between the eyebrows, and the bridge that connects the root to the rest of the nose. The dorsum nasi runs down the middle of the nose, ending at the apex, or tip of the nose. The ala forms the outer walls of the nostrils, and the philtrum is the vertical groove between the nose and upper lip. The lower panel focuses on the skeletal structure and cartilage of the nose. It shows the frontal bone, nasal bone, and maxillary bone, which support the upper portion of the nose. The septal cartilage separates the nostrils, while the major alar cartilage shapes the nostrils and tip of the nose.

The nasal cavity is divided into left and right sections by the nasal septum, formed anteriorly by the septal cartilage and posteriorly by the perpendicular plate of the ethmoid bone and the vomer bones (Figure 3). The nasal cavity is organized into three distinct parts: the vestibule (dilated space inside the nares), the respiratory zone (large central portion), and the olfactory region (small superior part housing olfactory receptors). Lateral walls of the nasal cavity house three bony projections termed nasal conchae. These conchae, named superior, middle, and inferior, increase the nasal cavity's surface area and disrupt airflow for filtering and warming. The conchae and meatuses between them retain moisture and prevent dehydration. The floor of the nasal cavity is composed of the palate, consisting of the hard palate in the anterior region and the soft palate in the posterior part (Figure 3). Posterior nasal aperture, choanae, connects nasal cavities posteriorly with the nasopharynx.³

Figure 3 | Sagittal view of the Nose and Pharynx | ⁴ This diagram provides a sagittal view of the head and neck, highlighting key structures of both the upper respiratory and digestive tracts. In the nasal cavity, the nasal vestibule, conchae, and the superior, middle, and inferior nasal meatuses are shown, illustrating the pathway for filtering and conditioning inhaled air. The oral cavity includes the hard and soft palate, tongue, and palatine tonsil, all of which are involved in both respiratory and digestive functions. The pharynx is divided into three regions: the nasopharynx, oropharynx, and laryngopharynx, serving as a shared passageway for air and food. In the larynx, structures such as the vestibular and vocal folds, along with the thyroid and cricoid cartilage, are essential for voice production and protecting the airway during swallowing. Additionally, the image includes the frontal and sphenoidal sinuses, olfactory epithelium for the sense of smell, and the opening of the auditory tube, which helps equalize pressure in the ear.       

The nerve supply to the nasal cavity is chiefly governed by two branches of the trigeminal nerve: the anterior ethmoidal nerve and the maxillary branch (Figure 4). The anterior ethmoidal nerve, a part of the ophthalmic branch of the trigeminal nerve, supplies sensation to the anterior third of the septum and the lateral wall of the nasal cavity. Meanwhile, the posterior two-thirds of the septum and the lateral wall of the nasal cavity receives sensory innervation from the maxillary branch of the trigeminal nerve. This extensive innervation helps to regulate sensory perception and responses in various regions of the nasal cavity.

Figure 4 | Innervation of the Nasal Cavity |​ ⁵  Anatomical depiction of the nasal nerve innervations, showing both medial and lateral branches. In the top image, the medial branches are highlighted, including the olfactory nerve, anterior ethmoidal nerve, external nasal nerve, posterior superior nasal nerve, and nasopalatine nerve. The bottom image focuses on the lateral branches, displaying the pterygopalatine ganglion, lateral posterior superior nasal nerve, external nasal nerve, internal nasal nerve, lesser palatine nerve, and greater palatine nerve. 

The nasal mucous membranes benefit from a complex vascular structure comprising the ophthalmic, maxillary, and facial arteries. The anterior and posterior ethmoidal arteries, stemming from the ophthalmic artery, provide circulation to the upper part of the nasal septum and the lateral nasal walls. Meanwhile, the sphenopalatine artery, a division of the maxillary artery, delivers blood flow to the rear portion of the nasal septum and the lateral nasal wall. Additionally, branches originating from the greater palatine artery, part of the maxillary artery, serve the lower rear section of the lateral nasal wall. Furthermore, the septal branch of the superior labial artery, which belongs to the facial artery, nourishes the front lower part of the septum. This extensive network ensures a robust and comprehensive blood supply throughout the nasal cavity.⁶

Figure 5 | Blood Supply to the Nasal Cavity |⁷ Anatomical diagram illustrating the arterial supply to the nasal cavity. The image highlights key arteries, including the posterior ethmoidal arteries, anterior ethmoidal arteries, branch of sphenopalatine artery, greater palatine artery, and septal branch of the superior labial artery. The Kisselbach Area (Little’s Area), marked in orange, is emphasized as a common site for anterior nosebleeds (epistaxis). The incisive canal is also labeled, indicating its anatomical relevance to arterial connections in the nasal septum. 

Paranasal Sinuses

The paranasal sinuses are air-filled cavities within skull bones that contribute to the structure of the nasal cavity's walls, playing a role in the warming and humidification of incoming air. Each sinus is named after the specific skull bone in which it resides: the frontal sinus, maxillary sinus, sphenoidal sinus, and ethmoidal sinus (Figure 6). The linings of these sinuses, along with the conchae meatuses, consist of pseudostratified ciliated columnar epithelium (Figure 7). This epithelial lining includes goblet cells that generate mucus. In addition to mucus, serous and mucus-producing cells secrete lysozyme and defensins, proteins possessing antibacterial properties. These components are involved in protecting the respiratory system against potential infections and maintaining its optimal condition.⁸

Figure 6 | Paranasal Sinuses | ⁹  The paranasal sinuses are air-filled spaces located within the bones surrounding the nasal cavity, including the frontal, ethmoid, sphenoidal, and maxillary bones. These sinuses are shown in the anatomical diagram and are lined with a mucous membrane that helps humidify and warm inhaled air, as well as lighten the weight of the skull. Each sinus is connected to the nasal cavity through small openings, allowing mucus drainage and air circulation. 

The Pharynx

The pharynx is a continuous cavity surrounded by skeletal muscles and lined with a mucous membrane that connects to the nasal cavities (see Figure 3). It extends from the base of the skull to the level of the C6 vertebra and consists of three main regions: the nasopharynx, oropharynx, and laryngopharynx.

The nasopharynx primarily functions as an airway. The pharyngeal tonsil, also known as the adenoid, lies at the upper portion of the nasopharynx. It tends to be more prominent in children but regresses with age. The uvula, a teardrop-shaped structure, is the continuation of the soft palate and moves during swallowing to close off the nasopharynx and prevent material from entering the nasal cavity. Additionally, the auditory (Eustachian or oropharyngeal) tubes connect the middle ear cavity to the nasopharynx.

The oropharynx, adjacent to the oral cavity, serves as a passage for both air and food. This region experiences a transition from pseudostratified ciliated columnar epithelium to stratified squamous epithelium. It houses two tonsil sets, the palatine and lingual tonsils, which trap and destroy pathogens from the oral or nasal cavities. The palatine tonsils are situated laterally at the fauces, while the lingual tonsils are located at the base of the tongue.

The laryngopharynx is located beneath the oropharynx and behind the larynx, acting as a shared passage for both air and food until it reaches its endpoint, where the digestive and respiratory systems separate. The stratified squamous epithelium of the oropharynx extends into the laryngopharynx. This region opens into the larynx anteriorly and connects to the esophagus posteriorly.³

The Larynx

The larynx, a complex anatomical structure critical for respiration, vocalization, and airway protection, extends from the anterior aspect of the C3 to C6 vertebrae. Its primary function includes protecting the lower airway by closure to prevent the ingress of foreign matter. Swallowing prompts the pharynx and larynx to elevate, expanding the pharynx while the epiglottis moves downward, effectively sealing the tracheal opening and avoiding food entry into the larynx. Although not a part of the larynx, the hyoid bone provides essential support for laryngeal motion during respiration and phonation, serving as a pivotal landmark for laryngoscopy evaluation. The vallecula, situated between the base of the tongue and the epiglottis, serves as a crucial reference point during medical procedures involving laryngoscope insertion. The pyriform fossa is a sinus in the orifice of the larynx where lodging of foreign bodies or a non-centered endotracheal tube is common.

The larynx is divided into three compartments: supraglottic, glottic, and subglottic. Supraglottic (vestibule) portion is above false vocal cords to the tip of the epiglottis. The glottis is the opening at the superior part of the larynx and is composed of vestibular folds, or a false vocal cord. Subglottic portion is below the true vocal cords and above the beginning of the trachea. The lining transitions from stratified squamous epithelium in the superior region to pseudostratified ciliated columnar epithelium, aiding mucus production to trap foreign elements entering the trachea. The specialized epithelium, similar to the nasal cavity and nasopharynx, helps clear debris and pathogens by moving mucus toward the laryngopharynx for swallowing.

The larynx is made of  three single cartilages and three small paired cartilages (Figure 8). The thyroid cartilage is the largest cartilage that makes up the larynx and is referred to as the Adam’s apple. The epiglottis is a fibroelastic cartilage that is attached to the thyroid cartilage through thyroepiglottic ligament above vocal cords and to the base of the tongue by glossoepiglottic fold and covers the opening of the trachea. When it is in the “closed” position, the unattached end of the epiglottis rests on the glottis. Cricoid cartilage forms a ring around the larynx with a wide posterior region and a thinner anterior region. It is palpable just below the thyroid gland. The position of the cricoid cartilage matches the beginning of the trachea and esophagus and is a landmark for laryngotracheal anesthesia (local anesthesia) and surgical airway for tracheostomy. Any incisions or needle punctures to the cricothyroid membrane should be made in its inferior third and be directed posteriorly because of the neurovascular bundle that runs along the superior margin of the cricoid cartilage. The three smaller, paired cartilages are the arytenoids, corniculates, and cuneiforms. The arytenoid cartilages articulate on the superior posterior aspect of the cricoid cartilage. They are attached to the posterior ends of the vocal cords and move them up and down and rotate. Corniculate cartilages are positioned more medially and cuneiform cartilages are situated more laterally and are embedded in the aryepiglottic folds.³

Figure 8 | Anterior and Lateral Views of the Larynx |​¹²  This diagram illustrates the key anatomical structures of the larynx including cartilages, ligaments, highlighting their arrangements and relationships, presented in both anterior and right lateral views.

The true vocal cord, a membranous fold connecting to the thyroid and arytenoid cartilages (Figure 9), allows for sound production by oscillation. Variations in the size of these folds among individuals contribute to different pitch ranges of voices. The narrowest portion of the adult airway is between the vocal cords. In children below 10 years of age, the narrowest part is just below the vocal cords at the cricoid cartilage. Folds in males tend to be larger compared to those in females, which creates a deeper voice. Anterior-posterior dimension is 23 mm in the male and 17 mm in the female. Mechanoreceptors play a role in regulating air pressure by influencing vocal cord abduction during inhalation and adduction during exhalation to prevent alveolar collapse.

Figure 9 | Vocal Cords | ¹³ The true vocal cords and vestibular folds of the larynx are viewed inferiorly from the laryngopharynx. 

Numerous intrinsic and extrinsic ligaments, along with skeletal muscles within the larynx, maintain the positioning of laryngeal cartilages. Intrinsic muscles of the larynx include aryepiglottic and oblique arytenoid muscles which help close the laryngeal inlet whereas thyroepiglottic muscle helps open the same. The glottic opening is through posterior cricoarytenoid muscles and its closing is by the transverse arytenoid and the lateral cricoarytenoid muscles. Cricothyroid muscles are important in lengthening the true vocal cords and thyroarytenoid muscles help reduce the length of these folds. The muscles within the larynx include aryepiglottic, thyroepiglottic, cricoarytenoid, interarytenoid, cricothyroid, and thyroarytenoid. Their roles are summarized in the table below (Table 1).  

The nerves responsible for laryngeal function include the superior and recurrent laryngeal nerves, branches of the vagus nerve. Superior laryngeal nerve branches as internal and external where the external branch supplies the inferior constrictor muscle of the pharynx and also to the cricothyroid muscles (Figure 10). The internal branch innervates the interarytenoid muscles which are important in phonation. It carries sensory signals from the epiglottis, base of the tongue, supraglottic mucosa, aryepiglottic folds, arytenoids, thyroepiglottic joint, and cricothyroid joint.

The recurrent laryngeal nerve gets its name due to its nature of forming a loop. The left branch loops around the aorta whereas the right branch loops around the brachiocephalic artery (Figure 10). The recurrent laryngeal nerve supplies sensation to the larynx below the level of the vocal cords and innervates all the muscles of the larynx except the cricothyroid and part of the interarytenoid muscles. Both superior and recurrent laryngeal nerves can be vulnerable to injury during certain procedures due to their proximity to the thyroid gland. Damage to these nerves could lead to issues such as hoarseness. Airway devices or anesthetic blocks can lead to damage to recurrent laryngeal nerves which in turn could cause unilateral or bilateral vocal cord paralysis, hoarseness and dyspnea.

Figure 10 | Innervation of the Larynx | ¹⁴ The diagram illustrates the pathway of the vagus nerve and its branches, specifically highlighting the superior and recurrent laryngeal nerves and their role in innervating the larynx. The looping of the left recurrent laryngeal nerve around the aortic arch is also clearly depicted. 

Figure 11 | Nerve Supply to the Larynx showing Branches of the Superior Laryngeal Nerve | ¹⁵  The diagram illustrates the role of the vagus nerve and its superior laryngeal branches in controlling laryngeal function. It highlights the distinct sensory and motor functions of the internal and external branches, respectively.

 
The larynx receives blood from the superior thyroid artery, a branch of the external carotid artery, and the cricothyroid artery, a branch of the superior thyroid artery, both of which run through the upper part of the cricothyroid membrane. Stretching these vessels can lead to hoarseness.¹⁶

Figure 12 | Blood Supply to the Larynx | ¹⁷ This diagram illustrates the arterial blood supply to the larynx and its associated structures.

The Trachea

The trachea, commonly known as the windpipe, extends from the inferior border of the cricoid cartilage, aligning with the level of the sixth cervical vertebra, and extends towards the lungs, slightly inclining to the right. Measuring approximately 12 centimeters in length with an internal diameter of 12 millimeters, it displays variations between genders, being narrower in females than in males. Its structural integrity is attributed to 16 to 20 C-shaped hyaline cartilages interconnected by connective tissue (Figure 13).

Crucial to the trachea's design is the fibroelastic membrane, constituted by the trachealis muscle and elastic connective tissue, effectively closing the trachea's posterior surface. This membrane allows the trachea to stretch and contract slightly during the breathing cycle. The C-shaped cartilages primarily offer structural reinforcement, preventing the trachea from collapsing while maintaining an open passageway for air. Additionally, the trachealis muscle can contract to propel air during exhalation.¹⁸

​​Figure 13 | Trachea and Primary Bronchi | ¹⁹ This diagram illustrates the structural features of the upper respiratory tract, including the cartilaginous rings of the trachea and the branching pattern of the primary and secondary bronchi.

The trachea, considered part of the lower respiratory tract, forms the base of the airway hierarchy. It is designated as the "zero generation," while the primary bronchi, branching from it, are classified as the first generation in the airway system. With subsequent branchings into bronchi and bronchioles, the terminal generation is represented by the alveolar sacs, forming the twenty-third generation (Figure 14). This hierarchical classification illustrates the progressive branching of the airway, highlighting the trachea's fundamental role as a conduit for the subsequent generations in the respiratory system.|²⁰ 

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Figure 14 | Schematic representation of the subdivisions of the conducting airways and respiratory Units |²¹  The diagram illustrates the hierarchical branching of the airways and the functional differences between the conducting, transitional, and respiratory zones. The generation numbers provide a quantitative representation of the airway branching pattern.

Bronchial Tree

Alveolar Ducts and Alveolar Sacs

Both alveolar ducts (generations 20 to 22) and alveolar sacs (generation 23), lined by alveolar epithelium, participate in gas exchange. The alveolar sacs are the last stage of division and terminate at the alveoli (Figure 15). On the other hand, the walls of the alveolar ducts are made of alveoli that communicate with the ducts and are separated by septa. These septa contain smooth muscle cells that form rings across the length of the alveolar ducts. These muscles, by their contractions, can affect the caliber of the ducts. About half the total number of alveoli arise from the alveolar ducts and half from the alveolar sacs; approximately 17 alveoli arise from each alveolar sac. The branching of bronchopulmonary segments is completed in utero, and any growth of conduction airways after birth is through an increase in size. However, the number of alveoli continues to proliferate in the first decade of life; after that, the alveoli do not increase in number but increase only in size.

Alveoli

Alveoli, the fundamental units of gas exchange in the lungs, present as numerous small, grape-like sacs interconnected to the alveolar ducts (Figures 15 and 16). These structures, forming clusters of individual alveoli, are primarily responsible for gas exchange. An alveolus, approximately 250 micrometers in diameter, features elastic walls that exhibit substantial stretching during inhalation, significantly augmenting the available surface area for gas exchange. The size of alveoli varies across different regions of the lung due to gravitational influences, with larger alveoli predominantly found in the upper lung segments. Alveoli interconnect through alveolar pores, facilitating equalization of air pressure throughout the lung and within the alveolar units. These pores play a pivotal role in maintaining uniform air pressure and promoting optimal gas exchange across the respiratory surfaces.

The lungs house an astounding 300 million alveoli, and each alveolus is encompassed by an intricate network of approximately 1,000 pulmonary capillaries. This arrangement provides an extensive surface area of nearly 100 square meters, optimizing the exchange of oxygen and carbon dioxide through diffusion. The remarkable efficiency of these microscopic structures in the alveoli enables the vital process of gas exchange fundamental to respiration.¹⁰

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Figure 16 | Structures of the Respiratory Zone |²³ The diagram emphasizes the close proximity of the alveolar air space to the capillary blood, highlighting the structural adaptations that facilitate efficient gas exchange across the respiratory membrane. The inset images provide both a macroscopic and microscopic perspective of the alveolar structure.

The alveolar wall, a pivotal component of the respiratory system, comprises three distinct cell types: Type I alveolar cells, Type II alveolar cells, and alveolar macrophages. Type I alveolar cells represent the predominant cell type in the alveoli, accounting for approximately 97% of the alveolar surface area. They are exceedingly thin, approximately 25 nm thick, and are constituted by squamous epithelial cells. Their ultra-thin structure renders them highly permeable to gases, enabling efficient gas exchange between the alveoli and pulmonary capillaries. Scattered amidst the Type I cells, Type II alveolar cells play a crucial role by secreting a substance called pulmonary surfactant. The surfactant, composed of phospholipids and proteins, serves two vital functions. Firstly, it significantly reduces the surface tension within the alveoli. Secondly, it enhances compliance, which decreases the force required for lung ventilation. This decrease in surface tension is instrumental in stabilizing the alveoli, preventing their collapse. The role of surfactant in the law of Laplace elucidates that, in the absence of surfactant, smaller alveoli with increased tension would collapse due to surface tension. Surfactant reduces surface tension, balancing pressure across alveoli of different sizes and preventing collapse. However, surfactant mitigates this scenario by diminishing surface tension, particularly in smaller alveoli, thereby preventing their collapse. It is crucial in conditions such as neonatal respiratory distress syndrome, where the deficiency of surfactant can lead to severe breathing difficulties in newborns.

Pleura

Each lung is enveloped within a cavity encased by a serous membrane known as the pleura. The right and left pleurae, which encompass the respective right and left lungs, are separated by the mediastinum. The pleurae consist of two layers (Figure 17). The Visceral Pleura is situated superficially and envelops the lungs, extending into and lining the lung fissures. Its closeness to the lung tissue aids in providing structural support. The Parietal Pleura is the outer layer of the pleura that attaches to the thoracic wall, the mediastinum, and the diaphragm, essentially lining the thoracic cavity. At the hilum, the visceral and parietal pleurae interconnect, forming the pleural cavity, which is the space between these two layers. The pleurae secrete a small amount of pleural fluid. This fluid lubricates the surfaces, significantly reducing friction between the layers during breathing which helps prevent any trauma caused by friction.The pleurae also act as a protective barrier, delineating different organs to prevent interference caused by organ movements. Additionally, they serve as a barrier against the spread of infection.²⁴

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 Figure 17 | Parietal and Visceral Pleura of the Lungs |²⁵ The diagram highlights the arrangement of the pleural membranes and the importance of the pleural cavity in allowing smooth lung movement during respiration. It also emphasizes the role of the intercostal muscles and diaphragm in the mechanics of breathing.

Pulmonary Blood Supply

Until the sixteenth generation, the blood supply of the bronchi is derived from the systemic vascular tree via the bronchial circulation. The main function of the respiratory passages at these levels is the conduction of air and humidification. Beyond this level, the pulmonary circulation nourishes the respiratory passages where gas exchange occurs.

The lungs, responsible for the critical process of gas exchange, receive a dual blood supply that contributes to their functional support and maintenance. The first source of blood is oxygenated blood derived from the bronchial arteries, representing branches of the descending aorta. While these arteries supply approximately two percent of the total cardiac output, their main function is providing blood to the non-gaseous exchange portions of the lungs. The principal blood supply necessary for gas exchange is sourced from the deoxygenated blood within the pulmonary arteries (Figure 18). The primary role of pulmonary circulation is to facilitate the exchange of gases, delivering deoxygenated blood to the lungs, where it becomes oxygenated for subsequent distribution to the body tissues by erythrocytes.

As the pulmonary artery traverses along the bronchi, it progressively branches into smaller vessels. Each arteriole, accompanied by a venule, supplies and drains a single pulmonary lobule. Upon nearing the alveoli, the pulmonary arterioles evolve into the pulmonary capillary network. These capillaries, with extremely thin walls, follow the branching of the bronchioles and the intricate structure of the alveoli, where the capillary wall converges with the alveolar wall, forming the respiratory membrane – the site where gas exchange takes place. Upon oxygenation, blood exits the alveoli via multiple pulmonary veins, which exit the lungs through the hilum. The venous blood from the bronchi's initial segment returns via bronchial veins, which then drain into vessels such as the azygos, hemiazygos, or posterior intercostal veins. The remaining deoxygenated blood is carried back via the pulmonary veins. This distinctive venous system not only carries oxygenated blood from the respiratory units but also transports deoxygenated blood from the visceral pleura and distal bronchi. This bronchopulmonary anastomosis is instrumental in contributing to the normal anatomic shunt.

Figure 18 | Pulmonary Circulation |²⁶ The image depicts the heart and lungs, highlighting the major blood vessels involved in pulmonary circulation. 

The pulmonary circulation is a unique low-pressure, low-resistance system, primarily due to the dense microcirculation and shorter vessels within the lungs. Unlike systemic circulation, which faces higher pressures and resistance, the pulmonary circulation accommodates the entire cardiac output, making it the only vascular bed that receives all of the blood pumped by the heart. The mean arterial pressure in the pulmonary circulation is about 15 mm Hg, and the mean pressure within the pulmonary capillaries is even lower, averaging around 10 mm Hg. The pulmonary blood volume is approximately 600 milliliters, with about 100 milliliters contained within the capillaries at any given time.

One of the critical factors influencing pulmonary blood flow is gravity, which affects the distribution of blood perfusion in the lungs. Since the apex of the lung is positioned roughly 15 centimeters higher than the pulmonary artery, and the base is a similar distance below, pressure in the capillaries at the apex is significantly lower, often resulting in these capillaries remaining closed due to insufficient pressure to keep them open. In contrast, the capillaries at the base of the lung are better perfused due to the gravitational effect increasing the pressure there.

Additionally, the pulmonary system has mechanisms to respond to changes in blood flow or pressure. When the pressure in the pulmonary artery increases, recruitment and distension of pulmonary capillaries occur, which opens previously closed vessels and allows for greater blood flow. However, an increase in lung volume can compress the alveolar vessels, leading to a rise in resistance. This dynamic relationship between vessel recruitment, distension, and resistance helps maintain an efficient system for gas exchange while accommodating changes in blood flow and lung volume during respiration.²⁷

Pulmonary Nerve Supply

The control of airway diameter, regulated by the parasympathetic and sympathetic nervous systems, is pivotal in modulating respiratory function. The parasympathetic nervous system, mainly through the vagus nerve, orchestrates bronchoconstriction. Primarily, the cholinergic stimulation activates muscarinic M3 ACh receptors, resulting in bronchoconstriction through G-protein-mediated activation of phospholipase C, which elevates IP3 levels and increases the release of calcium ions. receptors targeted by this system are concentrated in the larger airways, where their activation leads to the narrowing of the air passages. In contrast, the sympathetic nervous system acts on \(\beta2\) receptors, leading to bronchodilation via a G-protein-mediated increase in cAMP, which reduces the release of calcium ions.

The influence of the ANS on the lungs is critical for the precise regulation of oxygen and carbon dioxide levels. Sensory nerve fibers originate from the vagus nerve and the second to fifth thoracic ganglia. These nerves form the pulmonary plexus at the lung root, situated at the hilum. From this plexus, nerves follow the bronchial tree into the lungs, extensively branching to innervate muscle fibers, glands, and blood vessels within the lung tissue. 

Additionally, humoral factors can influence bronchial tone: histamine, leukotrienes, thromboxane A2, serotonin, \(\alpha\)-adrenergic agonists, vasoactive intestinal peptide, and endothelin contribute to bronchoconstriction, while nitric oxide, \(\beta2\)-agonists, and prostacyclin promote bronchodilation.²⁸