The anatomical, physiological, and immunological features of the respiratory system in children are closely related to their susceptibility to respiratory diseases during childhood.
Embryonic Development of the Respiratory System
The morphological development of the respiratory system occurs in five distinct stages:
Embryonic Period
The development of the respiratory system begins from the endoderm and mesoderm on gestational days 26–28. The formation of the primitive airway begins in the endoderm of the foregut, rapidly giving rise to the left and right main bronchi, referred to as the "lung buds." By the fifth to sixth week of gestation, the segmental bronchi are established.
Pseudoglandular Period
During this period, sections of the lung tissue appear glandular under the microscope, hence the name. By the end of this phase, the primitive airways begin to form lumens. During this stage, the trachea separates from the foregut. Incomplete separation results in a tracheoesophageal fistula, which is a significant congenital anomaly.
Canalicular Period
The bronchial branches continue to elongate, forming respiratory pathways. The growth of capillaries and the respiratory portions of the lungs are key features of this phase.
Saccular Period
Terminal airways widen and form columnar structures referred to as alveolar sacs.
Alveolar Period
Fully developed alveoli with intact capillary structures appear during this period. The expansion of the alveolar surface area provides the anatomical foundation for gas exchange.
Anatomical Characteristics
The respiratory system can be divided into the upper and lower airways demarcated by the lower edge of the cricoid cartilage. The upper airways include the nose, paranasal sinuses, pharynx, eustachian tube, epiglottis, and larynx, whereas the lower airways include the trachea, bronchi, bronchioles, respiratory bronchioles, alveolar ducts, and alveoli.
Upper Airways
Nose
In infants and young children, the nasal cavity is relatively short and narrow compared to adults. The absence of nasal hair and the delicate nature of the nasal mucosa, which is rich in blood vessels, make the nose prone to infection. Swelling of the mucosa during infection can easily lead to obstruction, resulting in breathing difficulties or mouth breathing.
Paranasal Sinuses
The development of the paranasal sinuses varies among different types. At birth, the maxillary and ethmoid sinuses are extremely small but enlarge rapidly after the age of 2 and reach full development by age 12. The frontal sinuses begin to appear between ages 2 and 3 and fully develop by ages 12–13, while the sphenoid sinuses appear at age 3, connect with the nasal cavity, and enlarge rapidly by age 6. The mucosa of the sinuses is continuous with that of the nasal cavity, and the sinus openings are relatively large. As a result, acute rhinitis can easily lead to sinusitis, which is not uncommon in preschool children.
Nasolacrimal Duct and Eustachian Tube
The nasolacrimal duct in infants is short, with an opening near the inner canthus, and the valve is underdeveloped. Consequently, nasal infections often invade the conjunctiva, causing inflammation. The eustachian tube in infants is wider, shorter, straighter, and more horizontally positioned, which makes middle ear infections more likely during nasopharyngitis.
Pharynx
The pharynx is relatively narrow and vertical. The tonsils include the palatine tonsils and the pharyngeal tonsils. The palatine tonsils begin to enlarge at around 1 year of age, develop rapidly between ages 4 and 10, and gradually regress by ages 14–15. As a result, tonsillitis is more common in older children but rare in infants. The pharyngeal tonsils, also known as adenoids, develop by 6 months of age and are located at the junction of the nasopharyngeal roof and posterior wall. Severe adenoid hypertrophy is an important cause of obstructive sleep apnea syndrome in children.
Larynx
The lower edge of the cricoid cartilage marks the boundary of the larynx. The larynx has a funnel shape, with a narrow lumen, small glottis, soft cartilage, delicate mucosa, and abundant blood vessels and lymphoid tissue. Mild inflammation can cause laryngeal narrowing, leading to inspiratory dyspnea.
Lower Airways
Trachea and Bronchi
In infants and young children, the trachea and bronchi are shorter and narrower compared to adults. The mucosa is delicate and vascular, and the cartilage is soft. The lack of elastic tissue reduces support, while insufficient mucus gland secretion results in dryness of the airways. In addition, the relatively poor ciliary function limits the clearance capacity. These factors contribute to a higher susceptibility to respiratory tract infections in infants, with congestion, swelling, and airway obstruction occurring more easily upon infection.
The left main bronchus is long and originates laterally from the trachea, while the right main bronchus is shorter, wider, and almost a direct continuation of the trachea. Foreign objects are, therefore, more likely to enter the right main bronchus. The smooth muscles of the bronchioles are thin and underdeveloped in infants under 5 months of age, with significant development only after 3 years. In young infants, airway obstruction is mainly caused by mucosal swelling and secretions.
Lungs
Infants have fewer and smaller alveoli, with poorly developed elastic tissue. The interstitial tissue contains abundant blood vessels, and its development is vigorous. As a result, the pulmonary blood content is high, while the air content is relatively low, increasing the susceptibility to infections. During infections, mucus blockage can lead to interstitial inflammation, pulmonary emphysema, or atelectasis.
Thorax
The thorax of infants and young children is relatively short, with a relatively long anteroposterior diameter, giving it a barrel-shaped appearance. The ribs are positioned horizontally, the diaphragm is relatively high, and the thoracic cavity is small while the lungs are relatively large. Respiratory muscles are underdeveloped. These factors limit lung expansion during breathing, particularly in areas adjacent to the spine and the lower posterior portions of the lungs, preventing full expansion and optimal ventilation and gas exchange. As a result, when lung pathology occurs, respiratory distress is likely, leading to hypoxia and carbon dioxide retention. The mediastinum in children is relatively large and surrounded by loose tissue, making it prone to displacement in cases of pleural effusion or pneumothorax.
Physiological Characteristics
Respiratory Rate and Rhythm
Children have a faster respiratory rate, with younger children displaying higher rates. Newborns have a rate of 40–44 breaths/min, while rates for 29 days to 12 months are 30 breaths/min, for 1–3 years are 24 breaths/min, for 4–7 years are 22 breaths/min, for 8–14 years are 20 breaths/min, and for 15–18 years are 16–18 breaths/min. Infants and young children have an immature respiratory center with limited regulatory capability, often resulting in irregular respiratory rhythms, pauses, and intermittent breathing. These phenomena are especially pronounced in preterm infants and newborns.
Type of Respiration
The range of thoracic movement is limited in infants and young children due to underdeveloped respiratory muscles. Muscle fibers are relatively thin, with more interstitial tissue and a lower proportion of fatigue-resistant fibers within muscle tissue. Consequently, the respiratory muscles in children are weaker, more easily fatigued, and prone to respiratory failure. The diaphragm is relatively more developed than the intercostal muscles, and the horizontal positioning of the ribs with narrow intercostal spaces results in abdominal respiration in infants and young children. As children grow, the diaphragm and abdominal organs descend, the ribs shift from a horizontal to an oblique position, and the thoracic volume increases, progressively transitioning to thoracoabdominal respiration. By age 7, the respiratory pattern increasingly resembles that of adults.
Characteristics of Respiratory Function
Vital Capacity (VC)
The vital capacity of children is approximately 50–70 ml/kg. Under resting conditions, older children use only about 12.5% of their vital capacity for respiration, whereas infants and young children use approximately 30%, indicating a smaller respiratory reserve in younger children. During respiratory distress, the maximum compensatory respiratory volume in children does not exceed 2.5 times the normal level, whereas adults can achieve up to 10 times the normal level. This makes children more susceptible to respiratory failure.
Tidal Volume (TV)
The tidal volume in children is approximately 6–10 ml/kg, with smaller tidal volumes in younger children. The ratio of dead space to tidal volume is larger compared to adults.
Minute Ventilation and Gas Diffusion Capability
Minute ventilation, when calculated per unit body surface area, is similar between children and adults. Gas diffusion capacity, when calculated per unit lung volume, is also comparable to adults.
Airway Resistance
The airway resistance in children is higher than in adults due to narrower airway diameters. As the airway diameter increases with age, resistance gradually decreases.
Immunological Characteristics
Both nonspecific and specific immune functions of the respiratory tract are weaker in children. The cough reflex and ciliary function are underdeveloped, making it difficult to effectively clear inhaled dust and foreign particles. Alveolar macrophage functions are insufficient, and helper T-cell function is temporarily reduced in infants and young children. Levels of immune factors such as IgG, secretory IgA (SIgA), and particularly IgG subclasses, are low. In addition, the quantity and activity of other protective factors, including lactoferrin, lysozyme, interferons, and complement proteins, are insufficient. These deficiencies make children more vulnerable to respiratory tract infections.