11  Basics

11.1 Introduction

Pediatric pulmonology is a vital subspecialty of pediatrics that focuses on the structure and function of the lungs and respiratory tract in infants, children, and adolescents. To understand pediatric respiratory diseases and their clinical manifestations, a solid grasp of the anatomical, physiological, embryological, biochemical, and pathophysiological underpinnings of the pediatric respiratory system is essential.

This foundational knowledge enables clinicians to recognize what is normal, anticipate how and why diseases develop, and determine appropriate investigations and interventions. This write-up provides a focused introduction to these aspects, tailored to the context of medical education in Ghana.

11.2 Anatomy of the Pediatric Respiratory System

The pediatric respiratory system consists of the upper airway, lower airway, and lungs, with supporting structures including the thoracic cage and diaphragm.

11.2.1 Upper Airway:

Includes:

• Nasal cavity – filters, humidifies, and warms inspired air
• Nasopharynx, oropharynx, and laryngopharynx – direct airflow toward the larynx
• Larynx – houses the vocal cords; functions in phonation and protection during swallowing

Clinical relevance: Infants are obligate nose breathers. Even mild nasal congestion can lead to significant respiratory distress.

11.2.2 Lower Airway:

Includes:

• Trachea – extends from the cricoid cartilage to the carina
• Bronchi – right main bronchus is shorter and more vertical
• Bronchioles – terminal and respiratory
• Alveolar ducts and alveoli – site of gas exchange

Age-related note: The airway diameter in neonates is narrow, which increases resistance and the risk of obstruction.

11.2.3 Lungs:

• Right lung has three lobes, left lung has two lobes
• Lungs are surrounded by a pleural membrane
• Richly supplied with blood vessels and lymphatics

11.2.4 Thoracic Cage and Diaphragm:

• Ribs are more horizontal in infants
• Diaphragm is the main muscle of respiration; intercostal muscles assist with increasing age

11.3 Physiology of the Pediatric Respiratory System

Respiratory physiology involves ventilation, perfusion, and gas exchange, as well as control of breathing and defense mechanisms.

11.3.1 Ventilation:

The process of moving air into and out of the lungs.

• Tidal volume (VT): Volume of air moved in and out per breath (~6–8 mL/kg in children)
• Minute ventilation: VT × respiratory rate
• Compliance: Children have high chest wall compliance, meaning it deforms easily, but low lung compliance, especially in neonates

Clinical note: High compliance of the chest wall predisposes neonates to respiratory fatigue.

11.4 Gas Exchange:

Occurs at the alveolar-capillary interface:

• Oxygen (O₂) diffuses from alveoli to blood
• Carbon dioxide (CO₂) diffuses from blood to alveoli

Dependent on:

• Surface area of alveoli
• Thickness of the alveolar-capillary membrane
• Adequate ventilation-perfusion (V/Q) matching

11.5 Control of Breathing:

Controlled by centers in the medulla and pons, modulated by:

• Chemoreceptors (central: respond to CO₂; peripheral: respond to O₂)
• Stretch receptors in the lungs
• Voluntary control is limited in neonates

Age-specific physiology:

• Infants have periodic breathing and are prone to apneas
• Immature respiratory drive increases risk of hypoventilation

11.5.1 Defense Mechanisms:

• Nasal hairs and mucosa trap particles
• Mucociliary clearance moves mucus upward toward the oropharynx
• Cough reflex clears lower airways
• Immune defense: IgA in secretions, macrophages in alveoli

11.6 Embryology of the Respiratory System

11.6.1 Development Timeline:

• Week 4: Respiratory diverticulum (lung bud) arises from foregut endoderm
• Week 5–7: Formation of primary, secondary, and tertiary bronchi
• Week 16: Terminal bronchioles formed
• Week 24: Respiratory bronchioles begin to develop
• Week 28–36: Alveolar ducts and primitive alveoli form
• Birth to 8 years: Postnatal alveolar multiplication (from ~20 million at birth to ~300 million)

11.6.2 Embryological Germ Layers:

• Endoderm: Forms the epithelium of the airways and alveoli
• Mesoderm: Forms connective tissue, cartilage, smooth muscle, and blood vessels

11.6.3 Lung Maturation Stages:

1. Pseudoglandular (weeks 5–17): Branching of airways; no gas exchange possible
2. Canalicular (weeks 16–25): Formation of airspaces; capillary network appears
3. Saccular (weeks 24–36): Terminal sacs form; beginning of surfactant production
4. Alveolar (week 36 to 8 years): Alveoli mature and multiply

11.6.4 Surfactant:

Produced by type II pneumocytes from ~week 24, with sufficient amounts by ~week 34.

Function: Reduces surface tension in alveoli, preventing collapse during expiration

Clinical relevance: Premature infants often lack surfactant, a substance that can lead to respiratory distress.

11.7 Biochemistry of the Respiratory System

11.7.1 Gas Transport:

• Oxygen Transport:
  • 98% carried by hemoglobin
  • Oxyhemoglobin dissociation curve describes the relation between PaO₂ and SaO₂
  • Fetal hemoglobin (HbF) has a higher affinity for oxygen than adult hemoglobin
• Carbon Dioxide Transport:
  • Dissolved in plasma (~10%)
  • Bound to hemoglobin as carbaminohemoglobin (~20%)
  • As bicarbonate ions (~70%) via carbonic anhydrase reaction:

11.8 Acid-Base Balance:

• Lungs regulate pH by excreting CO₂
• Respiratory acidosis: from hypoventilation (↑CO₂)
• Respiratory alkalosis: from hyperventilation (↓CO₂)

Maintaining proper ventilation is crucial to acid-base homeostasis in children.

11.9 Surfactant Biochemistry:

• Composed mainly of phospholipids (especially dipalmitoylphosphatidylcholine - DPPC)
• Also contains surfactant proteins (SP-A, SP-B, SP-C, SP-D) that help spread and regulate surfactant

Synthesis is cortisol-dependent, which is why maternal corticosteroids are given antenatally in preterm labor.

11.10 Pathophysiology of the Pediatric Respiratory System

Pathophysiology describes the functional changes that occur in response to disease or injury. Understanding these responses helps to explain signs such as wheezing, cough, hypoxia, and tachypnea.

11.10.1 Airway Obstruction:

• Can occur extrathoracically (e.g., larynx) or intrathoracically (e.g., bronchioles)
• Narrow pediatric airways mean even minor swelling or secretions cause significant resistance
• Leads to increased work of breathing, wheezing, or stridor

11.10.2 Ventilation-Perfusion (V/Q) Mismatch:

• Ideal: ventilation matches perfusion
• In disease (e.g., mucus plugging, consolidation), mismatch occurs
  • Low V/Q: alveoli are perfused but not ventilated → hypoxemia
  • High V/Q: alveoli are ventilated but not perfused → wasted ventilation

11.10.3 Hypoventilation:

• Due to fatigue, CNS depression, or neuromuscular disease
• Leads to hypercapnia and respiratory acidosis

11.10.4 Surfactant Deficiency:

• Causes alveolar collapse (atelectasis)
• Reduces lung compliance
• Seen in premature infants or inactivation by infection/inflammation

11.10.5 Immature Immune System:

• Neonates have limited production of IgA, poor neutrophil function
• Makes them vulnerable to respiratory infections

11.11 Conclusion

The pediatric respiratory system is uniquely structured and regulated, necessitating a comprehensive understanding of its anatomy, development, biochemistry, physiology, and response to disease. Medical students should appreciate how these fundamental sciences interact in the context of health and disease. This understanding lays the groundwork for clinical reasoning, diagnosis, and management of respiratory illnesses in children.

In Ghana, where pediatric respiratory conditions are prevalent, this foundational knowledge becomes particularly crucial. As a future clinician, you are encouraged to integrate basic science with clinical practice to improve child health outcomes.