Respiratory System Structures

Why This Matters

The respiratory system isn't just a series of tubes—it's an elegantly designed pathway that conditions, conducts, and exchanges gases with remarkable efficiency. You're being tested on how each structure contributes to three core functions: air conditioning (warming, humidifying, filtering), conduction (moving air to and from gas exchange surfaces), and respiration (the actual $O_2$/$CO_2$ exchange at the cellular level). Understanding which structures perform which functions is essential for exam success.

When you study these structures, think about the progressive changes that occur as air moves deeper into the system: cartilage disappears, smooth muscle increases, epithelium thins, and surface area expands. These aren't random facts—they reflect functional adaptations. Don't just memorize anatomy; know what principle each structure demonstrates and how dysfunction in one area affects the entire system.

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Air Conditioning Zone: Preparing Air for the Lungs

Before air reaches delicate alveolar tissue, it must be warmed to body temperature, humidified to nearly 100% saturation, and filtered of particulates. These conditioning functions protect the fragile gas exchange surfaces from damage and optimize conditions for diffusion.

Nasal Cavity

• Warms, moistens, and filters incoming air through its highly vascularized mucosa and turbinate bones that create turbulent airflow
• Nasal septum divides the cavity into left and right passages, while conchae increase surface area for air conditioning
• Olfactory receptors in the superior region provide the sense of smell—a secondary but clinically relevant function

Cilia

• Microscopic hair-like projections line the respiratory epithelium and beat in coordinated waves toward the pharynx
• Mucociliary escalator—the combination of mucus-producing goblet cells and ciliary action that traps and removes debris
• First line of defense against inhaled pathogens; dysfunction (as in cystic fibrosis or smoking) leads to chronic infections

Epiglottis

• Elastic cartilage flap at the tongue base that reflexively covers the laryngeal inlet during swallowing
• Prevents aspiration by directing food and liquid toward the esophagus rather than the trachea
• Protective structure—not involved in gas exchange but critical for airway safety

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Conducting Zone: The Airway Pathway

The conducting zone moves air but performs no gas exchange—it's anatomical dead space. These structures progressively branch and narrow, transitioning from rigid cartilage-supported tubes to flexible muscular passages that can regulate airflow.

Pharynx

• Three-region passage (nasopharynx, oropharynx, laryngopharynx) connecting nasal cavity and mouth to the larynx
• Shared pathway for both respiratory and digestive systems—a key anatomical concept
• Muscular walls assist in swallowing and speech; the pharyngeal tonsils (adenoids) provide immune surveillance

Larynx

• Voice box containing the vocal cords; located between the pharynx and trachea
• Cartilage framework (thyroid, cricoid, arytenoid cartilages) maintains airway patency and protects vocal structures
• Dual function—sound production and airway protection via the cough reflex and laryngospasm

Vocal Cords

• True vocal folds vibrate as air passes through the rima glottidis, producing sound waves
• Intrinsic laryngeal muscles adjust tension and length to control pitch; airflow volume controls loudness
• Abduct during breathing to maximize airway opening; adduct during phonation and protective reflexes

Trachea

• Windpipe extending from the larynx to the carina, where it bifurcates into main bronchi
• C-shaped hyaline cartilage rings—the open posterior portion allows esophageal expansion during swallowing
• Pseudostratified ciliated columnar epithelium with goblet cells continues the mucociliary escalator function

Bronchi

• Primary bronchi branch at the carina; the right bronchus is wider, shorter, and more vertical—aspirated objects lodge here more often
• Progressive branching creates secondary (lobar) and tertiary (segmental) bronchi, each generation with less cartilage
• Transition zone—cartilage plates replace rings, and smooth muscle becomes more prominent for airflow regulation

Bronchioles

• Small airways less than 1 mm diameter that lack cartilage entirely—walls are smooth muscle and elastic fibers
• Terminal bronchioles are the last purely conducting structures; respiratory bronchioles begin gas exchange
• Bronchoconstriction and bronchodilation regulate airflow and resistance—the target of asthma medications

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Respiratory Zone: Gas Exchange Surfaces

The respiratory zone is where ventilation meets perfusion. Structural adaptations maximize surface area while minimizing diffusion distance—the blood-air barrier is only 0.5 micrometers thick.

Alveoli

• 300 million tiny air sacs provide approximately $70–100 \text{ m}^2$ of surface area for gas exchange
• Type I pneumocytes (simple squamous) form the thin diffusion barrier; Type II pneumocytes secrete surfactant
• Surrounded by pulmonary capillaries—gases diffuse according to partial pressure gradients ($P_{O_2}$ and $P_{CO_2}$)

Pulmonary Capillaries

• Dense capillary network wrapping each alveolus creates the respiratory membrane for gas exchange
• Diffusion distance of only 0.5 μm allows rapid equilibration—blood spends ~0.75 seconds in contact with alveoli
• Low-pressure system (mean ~15 mmHg) prevents fluid leakage into alveoli while maintaining perfusion

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Structural Support: The Thoracic Container

The lungs cannot expand themselves—they depend on the thoracic cage and associated structures to create the pressure gradients that drive ventilation. Boyle's Law governs this relationship: as thoracic volume increases, intrapulmonary pressure decreases, and air flows in.

Lungs

• Paired organs in the thoracic cavity; right lung has three lobes, left lung has two lobes (cardiac notch accommodates the heart)
• Elastic tissue throughout allows passive recoil during expiration—compliance measures how easily lungs expand
• Hilum is the medial entry point for bronchi, pulmonary vessels, lymphatics, and nerves

Pleura

• Double-layered serous membrane—visceral pleura adheres to lung surface, parietal pleura lines thoracic wall
• Pleural cavity contains serous fluid that creates surface tension, coupling lung movement to chest wall movement
• Negative intrapleural pressure (about $-4 \text{ cmH}_2\text{O}$) prevents lung collapse; pneumothorax occurs when this seal breaks

Diaphragm

• Primary inspiratory muscle—dome-shaped skeletal muscle separating thoracic and abdominal cavities
• Contraction flattens the dome, increasing thoracic volume by ~500 mL during quiet breathing
• Innervated by phrenic nerve (C3-C5)—"C3, 4, 5 keeps the diaphragm alive" is a critical clinical correlation

Intercostal Muscles

• External intercostals elevate ribs during inspiration (bucket-handle and pump-handle movements)
• Internal intercostals depress ribs during forced expiration—quiet expiration is passive elastic recoil
• Innervated by intercostal nerves (T1-T11); paralysis impairs deep breathing and coughing

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Quick Reference Table

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Self-Check Questions

1. Which two structures work together to form the mucociliary escalator, and what happens to respiratory function when this system fails?

2. Compare the structural composition of bronchi versus bronchioles—why does this difference explain the pathophysiology of asthma?

3. If a patient has a right-sided pneumothorax, which structure's function has been compromised, and how does this affect the pressure relationships needed for ventilation?

4. Trace the path of an inhaled oxygen molecule from the nasal cavity to a pulmonary capillary, identifying which structures condition the air, which conduct it, and which perform gas exchange.

5. A patient aspirates a peanut—which main bronchus is it most likely to enter, and what anatomical features explain this clinical pattern?