Functional Anatomy of the Respiratory System

Overview

1. The Airway Tree

Conducting Zone (Generations 0-16)

Figure: Weibel's Model A of the human airways. Trachea (Z=0) to terminal bronchioles (Z=16) form the conducting zone; respiratory bronchioles (Z=17-19), alveolar ducts (Z=20-22), and alveolar sacs (Z=23) form the transitional and respiratory zones. (Murray & Nadel, Ch. 10)

• Trachea (Z=0): ~11 cm long, ~1.8 cm diameter, supported by C-shaped cartilage rings
• Main bronchi (Z=1): Right bronchus is more vertical, shorter, and wider - explains why aspirated foreign bodies tend to enter the right lung
• Lobar and segmental bronchi (Z=2-4): Supply individual lobes and bronchopulmonary segments
• Bronchioles (Z=5-16): Lack cartilage; walls maintained by elastic tissue and smooth muscle. Terminal bronchioles are the last purely conducting airways

Transitional Zone (Generations 17-19) - Respiratory Bronchioles

Respiratory Zone (Generations 20-23) - The Acinus

• Respiratory bronchioles (Z=17-19): intermittent alveoli in walls
• Alveolar ducts (Z=20-22): walls are completely lined with alveoli
• Alveolar sacs (Z=23): blind-ending clusters of alveoli

Gas Transport: Convection to Diffusion

2. The Alveolar-Capillary Unit

• Type I pneumocytes (thin, flat; 95% of alveolar surface) - primary gas exchange cells
• Basement membrane
• Pulmonary capillary endothelium

3. The Ventilatory Pump

Neural Control Architecture

Figure: Neuromuscular respiratory system flowchart showing pathways from cortex to muscles and the feedback control loop. (Murray & Nadel, Ch. 130)

• Pneumotaxic center (pons): fine-tunes respiratory rate and limits tidal volume
• Apneustic center (pons): promotes sustained inspiration
• Pre-Bötzinger complex (medulla): the primary rhythm generator

Respiratory Muscles

• Sternocleidomastoid, scalenes - elevate upper rib cage on inspiration
• Abdominal muscles - the primary expiratory muscles; active in forced expiration, coughing, and sneezing

Feedback Receptors

4. Lung Volumes and Functional Compartments

5. Distribution of Ventilation

• Intrapleural pressure is less negative at the lung base (due to lung weight)
• Basal alveoli are more compressed at FRC but have greater compliance slope - so they receive more ventilation per breath
• Apical alveoli are more expanded at FRC and sit on a flatter part of the pressure-volume curve

Summary of Functional Zones

UPPER RESPIRATORY TRACT          LOWER RESPIRATORY TRACT
Nose → Pharynx → Larynx    |    Trachea → Bronchi → Bronchioles
                            |          ↓
                      CONDUCTING ZONE (Z 0-16): Dead space, gas conditioning
                            ↓
                    TRANSITIONAL ZONE (Z 17-19): Respiratory bronchioles
                            ↓
                     RESPIRATORY ZONE (Z 20-23): Alveolar ducts & sacs
                             ← Gas exchange occurs here →

Recent Research on Surfactant Dysfunction in ARDS

Background: Surfactant Composition and Normal Function

How ARDS Disrupts Surfactant

1. Reduced Surfactant Production

2. Surfactant Inactivation in the Alveolar Space

3. Altered Surfactant Composition

• SP-B and SP-C levels decrease significantly
• Oxidized phospholipid species accumulate (dysfunctional)
• The phosphatidylcholine/phosphatidylglycerol ratio is altered
• Large aggregate (functional) surfactant is converted to small aggregate (non-functional) forms by inflammatory phospholipases

4. Functional Consequences

Recent Literature (2022-2026)

[Review · 2026] Hyperoxia and Surfactant Dysfunction in Critical Illness

• Hyperoxia directly disrupts surfactant lipids and proteins (particularly SP-A and SP-D), impairs surfactant synthesis and metabolism, and promotes atelectasis
• Redox imbalances and phospholipase A2 activation degrade surfactant phospholipids under oxidative stress
• Altered macrophage clearance of surfactant debris is proposed as a novel mechanism
• Hyperoxia-induced injury interacts synergistically with mechanical ventilation injury (VILI) and infection-driven ARDS
• The review calls for novel surfactant preparations resistant to functional inhibition and better human models to distinguish oxygen dose/duration effects

Key clinical implication: even when systemic SpO2 is maintained, alveolar hyperoxia from supplemental O2 may worsen surfactant function - supporting restrictive oxygen strategies in ARDS management.

[Review · 2026] Clinical, Molecular, and Therapeutic Approaches to ARDS

• Describes ARDS as characterized by "epithelial and endothelial injury, dysregulated inflammation, surfactant dysfunction, and impaired alveolar-capillary barrier integrity"
• Emphasizes biological heterogeneity across ARDS phenotypes - surfactant dysfunction is more prominent in the hyperinflammatory phenotype ("hot ARDS")
• Advocates for biomarker-based ARDS phenotyping to identify patients most likely to benefit from surfactant-directed therapies
• Notes precision-based approaches as the way forward, away from syndromic management

[Review · 2022] Surfactant Dysfunction in COVID-19/ARDS

• SARS-CoV-2 directly infects AT2 cells (which express ACE2), causing direct surfactant deficiency - distinct from secondary inactivation seen in classic ARDS
• Concurrent activation of the coagulation cascade and fibroblast proliferation creates fibrin plugs that further inactivate surfactant in the alveolar space
• Pulmonary fibrosis with myofibroblast proliferation and collagen deposition further impairs alveolar mechanics
• Offers insight into why COVID-ARDS had a distinct phenotype from classical ARDS, with deeper surfactant deficiency and worse response to conventional ventilatory strategies

Surfactant Therapy Trials: Why Adult ARDS Remains Challenging

Emerging Therapeutic Directions

1. Novel surfactant formulations resistant to protein inactivation - synthetic or semi-synthetic preparations that maintain function in protein-rich environments
2. SP-B/SP-C mimetics - peptide analogues of the hydrophobic surfactant proteins with improved inactivation resistance
3. Biomarker-guided patient selection - using SP-D, SP-B levels, or phenotypic clustering (hyper- vs. hypo-inflammatory) to identify ARDS patients with the greatest surfactant deficiency who may respond to replacement
4. Oxygen-sparing strategies - targeting SpO2 94-96% rather than >98% to reduce alveolar hyperoxia-mediated surfactant degradation (supported by the 2026 Watson et al. review)
5. Phospholipase A2 inhibitors - targeting the enzyme responsible for surfactant phospholipid degradation under oxidative stress

Summary Table: Mechanisms of Surfactant Dysfunction in ARDS