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HLT-RESP80-02 · Lesson 1 · 30 clock hours · theory

Overview of the Respiratory System: Structures, Functions, and Clinical Relevance

Objective. Students will identify and label all major structures of the upper airway, lower airway, and thoracic cage on anatomical diagrams and explain the primary function of each structure.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed clinician in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it to make decisions about any patient or medication. Always follow your instructor, your supervising professional, and current official references and regulations.

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Overview of the Respiratory System

The respiratory system delivers oxygen to the blood and removes carbon dioxide from the body. As a respiratory therapist, you will assess, support, and manage this system daily. A precise knowledge of its structures and functions is the foundation for every clinical skill you will learn.

Upper Airway

The upper airway begins at the nose and mouth and ends at the larynx. Its primary jobs are to conduct air toward the lungs, filter out particles, warm inspired air to near body temperature, and humidify it to near full saturation before it reaches the delicate lower airway tissues.

Nose and nasal cavity. Air enters through the nares (nostrils) and passes over the nasal conchae (turbinates)—curved bony shelves covered with vascular mucosa. The conchae create turbulent airflow that increases contact between air and the mucosa, maximizing warming and humidifying. Coarse hairs (vibrissae) and mucus trap larger particles.

Paranasal sinuses. These air-filled spaces in the skull bones communicate with the nasal cavity and contribute to mucus production and resonance of the voice.

Pharynx. The pharynx is a muscular passage divided into three regions: the nasopharynx (behind the nasal cavity), the oropharynx (behind the mouth), and the laryngopharynx (hypopharynx), which leads to both the esophagus and the larynx. The eustachian tubes open into the nasopharynx. The tonsils and adenoids, located in the pharynx, are lymphoid tissue involved in immune defense.

Larynx. The larynx connects the pharynx to the trachea. It is formed by cartilages, the most prominent being the thyroid cartilage (Adam's apple) and the cricoid cartilage (the only complete ring in the airway). The epiglottis, a leaf-shaped flap, folds down over the laryngeal inlet during swallowing to prevent aspiration. The vocal cords (true vocal folds) within the larynx produce voice and can close reflexively to protect the airway. The glottis is the opening between the vocal cords.

Clinical note — upper airway obstruction: Swelling, foreign bodies, or loss of muscle tone can rapidly obstruct the upper airway. Recognizing the anatomical bottleneck at the glottis is essential for understanding why intubation or emergency cricothyrotomy may be required.

Lower Airway

The lower airway begins just below the larynx and includes all conducting and respiratory zones down to the alveoli.

Trachea. The trachea is a flexible tube, roughly 10–12 cm long in adults, reinforced by C-shaped cartilaginous rings. The open posterior wall is closed by the trachealis muscle, which allows the esophagus to expand during swallowing. The inner surface is lined with pseudostratified ciliated columnar epithelium and goblet cells (the mucociliary escalator), which traps and moves particles toward the pharynx.

Carina. At the carina, the trachea bifurcates into the right and left main-stem bronchi. The carina is a highly sensitive area—stimulation causes a powerful cough reflex. The right main-stem bronchus is shorter, wider, and more vertical than the left, making it the more common site for accidental intubation or foreign-body aspiration.

Bronchi and bronchioles. The bronchi branch repeatedly in a pattern called dichotomous branching. Main-stem bronchi → lobar bronchi (three on the right, two on the left, matching the lung lobes) → segmental bronchi → subsegmental bronchi → bronchioles. Bronchioles lack cartilage; they remain open by the elastic recoil of surrounding lung tissue. Terminal bronchioles are the last purely conducting airways.

Respiratory bronchioles and alveolar ducts. Beyond the terminal bronchioles, the respiratory bronchioles mark the beginning of the respiratory zone, where gas exchange starts to occur. They lead to alveolar ducts and alveolar sacs.

Alveoli. Alveoli are tiny, thin-walled air sacs where virtually all gas exchange takes place. The alveolar wall consists of type I pneumocytes (flat, covering most of the surface, ideal for gas diffusion) and type II pneumocytes (cuboidal cells that produce surfactant). Surfactant reduces surface tension and prevents alveolar collapse at end-expiration. Alveolar macrophages patrol the surface and remove particles that reach this level.

Thoracic Cage

The thoracic cage—formed by the sternum, ribs, costal cartilages, and thoracic vertebrae—protects the lungs and heart, and its movement drives ventilation. The diaphragm is the primary muscle of inspiration; contraction flattens it, increasing thoracic volume and drawing air in. External intercostal muscles assist during normal breathing; accessory muscles (scalenes, sternocleidomastoid) recruit during increased work of breathing.

The pleura are thin serous membranes: the visceral pleura covers the lung surface, and the parietal pleura lines the thoracic wall. The pleural space between them contains a small amount of fluid that allows frictionless movement and transmits the negative pressure that keeps the lungs expanded.

Why Anatomy Matters Clinically

Every intervention you perform—placing an oral airway, passing a suction catheter, setting ventilator parameters, or interpreting a chest X-ray—depends on knowing exactly what structure you are dealing with and where it sits. Misidentifying anatomy can lead to patient harm: an endotracheal tube in the right main-stem bronchus collapses the left lung; unrecognized epiglottitis closes the airway without warning. Accurate anatomical knowledge is therefore a direct patient-safety skill.

Key terms8

CarinaThe ridge at the inferior end of the trachea where it bifurcates into the right and left main-stem bronchi; highly sensitive to stimulation and triggers a strong cough reflex.
EpiglottisA leaf-shaped cartilaginous flap at the base of the tongue that covers the laryngeal inlet during swallowing, protecting the airway from aspiration.
GlottisThe opening between the true vocal cords; the narrowest part of the adult larynx and the anatomical landmark used to guide endotracheal tube placement.
Mucociliary escalatorThe combined action of mucus secreted by goblet cells and the rhythmic beating of cilia lining the airway, which traps inhaled particles and moves them toward the pharynx for removal.
SurfactantA phospholipid-rich substance produced by type II pneumocytes that reduces alveolar surface tension, preventing alveolar collapse at the end of expiration.
Terminal bronchioleThe last generation of purely conducting airways, located just proximal to the respiratory bronchioles; contains no cartilage and no alveoli.
Pleural spaceThe potential space between the visceral and parietal pleura that normally contains a thin film of fluid; maintains the negative pressure needed to keep the lungs expanded against the chest wall.
Type II pneumocyteA cuboidal alveolar epithelial cell responsible for producing and recycling pulmonary surfactant; also capable of differentiating into type I pneumocytes to repair the alveolar surface.
In class: Lecture with projected cross-sectional diagrams of the airway from nares to alveoli; students annotate printed diagrams in real time and compare with a partner. · Small-group card-sorting activity matching structure names, locations, and functions; groups defend their final arrangements to the class. · Short written response: students describe in plain language what happens to inspired air at each anatomical landmark from the nose to the respiratory bronchiole.
Materials: Tortora & Derrickson 'Principles of Anatomy and Physiology' (current edition); Scanlan, Egan's Fundamentals of Respiratory Care (current edition); full-color airway anatomy posters; printed blank airway diagrams; whiteboard.
Check: Labeled diagram quiz: student must correctly identify and name at least 90% of 30 labeled structures to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Patient Assessment & Diagnosis
1. Which structure is the narrowest part of the adult upper airway and serves as the primary landmark for endotracheal intubation?
Patient Assessment & Diagnosis
2. A patient aspirates a peanut. In which airway is the foreign body most likely to lodge, and why?
Respiratory Pathophysiology
3. Which cell type produces pulmonary surfactant, and what is the primary consequence of surfactant deficiency?
Respiratory Pathophysiology
4. The mucociliary escalator is the airway's primary particle-clearance mechanism. Which of the following would MOST directly impair its function?
Cardiopulmonary Anatomy & Physiology
5. The pleural space normally contains a small amount of fluid and is maintained at a pressure slightly below atmospheric pressure. What is the primary clinical significance of this negative pleural pressure?
Therapeutic Procedures
6. A respiratory therapist is preparing to suction a patient through an endotracheal tube. Understanding airway anatomy, the therapist knows the catheter should be advanced no farther than which landmark to avoid mucosal trauma and severe coughing?
Cardiopulmonary Anatomy & Physiology
7. Which of the following correctly describes the structural difference between bronchi and bronchioles?
Cardiopulmonary Anatomy & Physiology
8. During a lecture, a student asks where gas exchange actually begins in the airways. The correct answer is:
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