Key Respiratory Structures

Why This Matters

The respiratory system isn't just a series of tubes and sacs—it's an elegantly designed pathway that conditions air, protects your airway, and ultimately delivers oxygen to every cell in your body while removing carbon dioxide. In anatomy and physiology, you're being tested on how structure determines function at every level, from the cartilage rings that keep your trachea open to the thin-walled alveoli that maximize gas diffusion. Understanding these relationships is what separates memorization from true comprehension.

As you study these structures, pay attention to the underlying principles: mucociliary clearance, pressure gradients, surface area optimization, and protective reflexes. Exam questions often ask you to explain why a structure has a particular feature or what would happen if that feature were compromised. Don't just memorize that the trachea has C-shaped cartilage—know that this design allows the esophagus to expand during swallowing while maintaining airway patency.

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Air Conditioning and Filtration Structures

The upper respiratory tract does far more than simply allow air to pass through. These structures warm, humidify, and filter inspired air before it reaches the delicate gas exchange surfaces below.

Nasal Cavity

• Warms and humidifies air to nearly body temperature and 100% humidity—protecting the lower respiratory tract from cold, dry air damage
• Mucus and cilia trap particles as small as 10 micrometers, providing the first line of defense against pathogens and debris
• Olfactory epithelium in the superior region houses chemoreceptors for smell; the nasal septum divides the cavity into bilateral passages

Epiglottis

• Elastic cartilage flap that reflexively covers the laryngeal inlet during swallowing—preventing aspiration of food and liquids
• Attaches to the base of the tongue and tilts posteriorly when the larynx elevates during the swallowing reflex
• Failure of this protective mechanism results in aspiration, which can cause choking or aspiration pneumonia—a key clinical correlation

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Conducting Passageways

These structures form the respiratory tree's branching pathway, moving air from the external environment to the gas exchange surfaces. No gas exchange occurs here—this is anatomical dead space, but these passages are critical for air conduction and continued conditioning.

Pharynx

• Three distinct regions—nasopharynx (posterior to nasal cavity), oropharynx (posterior to oral cavity), and laryngopharynx (superior to larynx and esophagus)
• Shared pathway for both respiratory and digestive systems, making it a critical junction where protective reflexes must coordinate
• Lymphoid tissue (pharyngeal and palatine tonsils) provides immune surveillance at this entry point for pathogens

Larynx

• Cartilaginous framework including the thyroid cartilage ("Adam's apple"), cricoid cartilage, and paired arytenoids that support the vocal cords
• Houses the vocal folds (true vocal cords) which vibrate during phonation; the rima glottidis is the opening between them
• Protective function via the cough reflex and laryngospasm—the larynx can close forcefully to prevent foreign material from entering the lower airways

Trachea

• C-shaped hyaline cartilage rings (16–20 total) keep the airway patent while the open posterior end allows esophageal expansion during swallowing
• Pseudostratified ciliated columnar epithelium with goblet cells creates the mucociliary escalator—cilia beat upward to move trapped debris toward the pharynx
• Trachealis muscle spans the posterior gap; contraction narrows the airway during coughing to increase air velocity and expel mucus

Bronchi

• Primary bronchi branch from the trachea at the carina—the right main bronchus is wider, shorter, and more vertical, making it the more common site for aspirated objects
• Progressive branching into lobar (secondary) and segmental (tertiary) bronchi follows the lung's lobar organization
• Cartilage plates replace C-rings as bronchi branch; smooth muscle increases proportionally, allowing bronchoconstriction and bronchodilation

Bronchioles

• Lack cartilage entirely—walls consist of smooth muscle and elastic fibers, making them highly responsive to autonomic control
• Terminal bronchioles represent the end of the conducting zone; respiratory bronchioles mark the beginning of the respiratory zone where gas exchange starts
• Smooth muscle tone regulated by sympathetic (bronchodilation via $\beta_2$ receptors) and parasympathetic (bronchoconstriction) input—clinically relevant in asthma treatment

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Gas Exchange Structures

This is where respiration actually happens. The respiratory membrane must be extremely thin to allow rapid diffusion while maintaining an enormous surface area to meet metabolic demands.

Alveoli

• Primary site of gas exchange—approximately 300 million alveoli provide roughly 70 $m^2$ of surface area (about the size of a tennis court)
• Type I pneumocytes form the thin squamous epithelial lining (0.1–0.5 μm thick); Type II pneumocytes secrete surfactant to reduce surface tension and prevent alveolar collapse
• Respiratory membrane consists of alveolar epithelium, fused basement membranes, and capillary endothelium—gases diffuse according to Fick's Law

Pulmonary Capillaries

• Dense capillary network surrounds each alveolus, creating a "sheet" of blood for maximum gas exchange efficiency
• Blood-gas barrier is only 0.5 μm thick at its thinnest points, allowing oxygen and carbon dioxide to equilibrate within 0.25 seconds
• Low-pressure system (mean pulmonary arterial pressure ~15 mmHg) prevents fluid from being forced into alveoli while still perfusing the entire respiratory surface

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Structural Support and Containment

These structures create the protected environment necessary for the lungs to expand and contract without friction or damage.

Lungs

• Asymmetrical paired organs—right lung has three lobes (superior, middle, inferior) while left lung has two lobes plus the cardiac notch to accommodate the heart
• Spongy, elastic tissue composed of bronchial trees, alveoli, blood vessels, and connective tissue; elasticity is critical for passive recoil during expiration
• Hilum on the medial surface is where bronchi, pulmonary vessels, lymphatics, and nerves enter—the "root" of the lung

Pleura

• Double-layered serous membrane—visceral pleura adheres to lung surface; parietal pleura lines the thoracic wall and diaphragm
• Pleural cavity contains a thin layer of serous fluid that creates surface tension, coupling lung expansion to chest wall movement
• Intrapleural pressure is normally subatmospheric (approximately $-4$ cmH₂O); pneumothorax occurs when air enters this space and breaks the pressure gradient

Thoracic Cavity

• Bony framework of ribs, sternum, and thoracic vertebrae protects the lungs and heart while providing attachment points for respiratory muscles
• Contains three compartments—two pleural cavities housing the lungs and the central mediastinum containing the heart, great vessels, and esophagus
• Pressure changes within this cavity drive ventilation; Boyle's Law ($P_1V_1 = P_2V_2$) explains how volume changes create pressure gradients for airflow

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Ventilation Mechanics

These muscular structures create the pressure gradients that drive air into and out of the lungs. Ventilation is fundamentally about changing thoracic volume to change intrapulmonary pressure.

Diaphragm

• Primary muscle of inspiration—contraction flattens the dome shape, increasing vertical thoracic dimension and dropping intrapulmonary pressure below atmospheric
• Innervated by phrenic nerve (C3–C5: "C3, 4, 5 keeps the diaphragm alive")—spinal cord injuries above this level result in respiratory paralysis
• Accounts for 75% of air movement during quiet breathing; relaxation allows elastic recoil of lungs and abdominal contents to push it superiorly during expiration

Intercostal Muscles

• External intercostals run obliquely downward and forward; contraction elevates the ribs ("bucket handle" and "pump handle" movements) to increase thoracic dimensions during inspiration
• Internal intercostals run perpendicular to externals; contraction depresses the ribs during forced expiration (quiet expiration is passive)
• Intercostal nerves (T1–T11) provide motor innervation; accessory muscles (sternocleidomastoid, scalenes) assist during labored breathing

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Sound Production

While not directly involved in gas exchange, these structures enable the respiratory system's secondary function of phonation.

Vocal Cords (Vocal Folds)

• True vocal folds are mucosal folds containing the vocalis muscle and vocal ligament; vibration during expiration produces sound waves
• Pitch controlled by tension—cricothyroid muscle stretches the folds to increase pitch; thyroarytenoid muscle relaxes them to lower pitch
• Abduction and adduction controlled by intrinsic laryngeal muscles; the folds must adduct to phonate and abduct during breathing to minimize airway resistance

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Quick Reference Table

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Self-Check Questions

1. Which two structures rely on smooth muscle rather than cartilage to regulate airway diameter, and why does this distinction matter clinically in conditions like asthma?

2. Compare the protective functions of the nasal cavity and the epiglottis—what type of threat does each guard against, and when is each mechanism active?

3. If a patient suffers a spinal cord injury at C2, which respiratory muscle would be paralyzed and why? What would happen to the external intercostals in this scenario?

4. Trace the path of an inhaled dust particle from the nasal cavity to its eventual removal—which structures and mechanisms are involved in the mucociliary escalator?

5. A pneumothorax disrupts the normal pressure relationship in the thoracic cavity. Using your knowledge of the pleura and Boyle's Law, explain why the affected lung collapses when air enters the pleural space.