Guide — Fundamentals
Respiratory System Anatomy & Physiology
How the respiratory system is built and how it moves air: the conducting and respiratory zones, the alveolar-capillary membrane and surfactant, the pleura, the muscles of ventilation, and the pressures that drive every breath. Master this map once and every later topic — ABGs, ventilation, lung disease — has somewhere to land.
11 min read · Fundamentals
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
The respiratory system has two jobs — move air (ventilation) and exchange gas (respiration, meaning diffusion of O₂ and CO₂). Structurally it divides into the conducting zone, which moves air but does no gas exchange (that volume is anatomic dead space, ~150 mL, or roughly 1 mL per lb of ideal body weight), and the respiratory zone, where gas exchange happens. The upper airway (nose → pharynx → larynx) warms, humidifies, and filters inspired gas; the lower airway begins at the trachea.
| Zone | Generations | Structures | Role |
|---|---|---|---|
| Conducting zone | 0 – 16 | Trachea → main bronchi → lobar → segmental bronchi → bronchioles → terminal bronchioles | Moves air; no gas exchange = anatomic dead space (~150 mL) |
| Respiratory zone | 17 – 23 | Respiratory bronchioles → alveolar ducts → alveolar sacs | Gas exchange; ~300 million alveoli, ~70 m² surface |
Key Concepts
- Conducting zone (generations 0–16). Trachea → main bronchi → lobar → segmental bronchi → bronchioles → terminal bronchioles. Cartilage supports the larger airways; smooth muscle dominates the bronchioles. The lining is pseudostratified ciliated columnar epithelium with goblet cells — the mucociliary escalator that clears secretions upward. No gas exchange occurs here, so this volume is anatomic dead space.
- Respiratory zone (generations 17–23). Respiratory bronchioles → alveolar ducts → alveolar sacs. About 300 million alveoli provide roughly 70 m² of gas-exchange surface.
- Alveolar-capillary membrane. Type I pneumocytes form the thin gas-exchange surface (~95% of alveolar area); Type II pneumocytes produce surfactant and can regenerate Type I cells; alveolar macrophages clear debris. The membrane is only about 0.5 µm thick.
- Surfactant.A phospholipid (chiefly dipalmitoylphosphatidylcholine) made by Type II cells. It lowers alveolar surface tension, which keeps alveoli open at low volumes (Laplace’s law, P = 2T ÷ r), raises compliance, and prevents atelectasis. Deficiency drives neonatal respiratory distress syndrome and contributes to ARDS.
- Pleura.The visceral pleura covers the lung, the parietal pleura lines the chest wall, and a thin fluid film couples them. Intrapleural pressure is normally negative (about −5 cmH₂O at rest), which holds the lung expanded against the chest wall. Losing that negative pressure (air entry) causes pneumothorax.
- Muscles of ventilation.The diaphragm is primary (~70% of tidal volume; innervated by the phrenic nerve, roots C3–C5). The external intercostals assist inspiration. Accessory inspiratory muscles (scalenes, sternocleidomastoid, trapezius) are recruited in distress. Expiration is normally passive (elastic recoil); active expiration recruits the abdominals and internal intercostals.
- Gross anatomy.The right lung has 3 lobes, the left has 2 (plus the lingula and a cardiac notch). The right mainstem bronchus is wider, shorter, and more vertical than the left — which is why aspiration and inadvertent right mainstem intubation favor the right side.
| Muscle | Phase | Notes |
|---|---|---|
| Diaphragm | Inspiration (primary) | ~70% of tidal volume; phrenic nerve, roots C3 – C5 |
| External intercostals | Inspiration (assist) | Elevate the ribs during quiet inspiration |
| Scalenes, SCM, trapezius | Inspiration (accessory) | Recruited in respiratory distress |
| Abdominals, internal intercostals | Active expiration | Quiet expiration is passive (elastic recoil) |
Assessment & Findings
The anatomy you just mapped shows up directly at the bedside — the structures predict the signs.
- High work of breathing. Accessory muscle use, tracheal tug, and paradoxical (abdominal) breathing signal a high work of breathing and impending diaphragmatic fatigue.
- Right mainstem intubation. Produces diminished or absent leftbreath sounds and asymmetric chest rise — a direct consequence of the right mainstem’s vertical takeoff.
- Anatomic dead space estimate. About 1 mL per lb of ideal body weight (roughly 2.2 mL/kg).
RT Priorities & Interventions
- Position to optimize the diaphragm. Upright or semi-Fowler positioning reduces abdominal pressure pushing up on the diaphragm and improves tidal volume.
- Protect the mucociliary escalator. An endotracheal tube bypasses the upper airway’s heat-and-moisture function, so always provide humidification to prevent thick secretions and mucus plugging.
- Recognize surfactant-related disease. Know RDS and ARDS, and the role of PEEP in keeping recruited alveoli open.
Common Pitfalls
- Confusing ventilation (air movement) with respiration (gas exchange) — a patient can ventilate yet fail to oxygenate, and vice versa.
- Forgetting that an artificial airway bypasses normal humidification, leading to mucus plugging.
- Assuming expiration is active — at rest it is passive; visible expiratory effort points to obstruction or distress.
Board Exam Pearls
- Phrenic nerve = C3, C4, C5 (“keeps the diaphragm alive”).
- Type II pneumocytes make surfactant and regenerate Type I cells.
- The right mainstem bronchus is wider, shorter, and more vertical (right mainstem intubation).
- Anatomic dead space is about 150 mL (~1 mL/lb ideal body weight).
- Conducting zone (generations 0–16) = dead space; respiratory zone (17–23) = gas exchange.
FAQ
What is the difference between ventilation and respiration?
Ventilation is the bulk movement of air into and out of the lungs; respiration is gas exchange across membranes (external respiration in the lung, internal respiration at the tissues). You can ventilate without adequately oxygenating, and oxygenate poorly even with adequate ventilation.
Why does an endotracheal tube require humidification?
It bypasses the nose and upper airway, which normally warm and fully saturate inspired gas. Dry gas thickens secretions, cripples the mucociliary escalator, and causes plugging, so heated humidification is added to artificial airways.
What does surfactant do?
Produced by Type II alveolar cells, surfactant lowers surface tension so alveoli stay open at low lung volumes (Laplace's law), which increases compliance and prevents atelectasis. Deficiency causes neonatal RDS and worsens ARDS.
Why is right mainstem intubation so common?
The right mainstem bronchus branches at a more vertical, less acute angle than the left, so an advanced tube tends to follow it - hypoventilating the left lung and producing diminished left breath sounds.
Put it to work
Estimate minute and alveolar ventilation, and see how dead space (the conducting-zone air you just mapped) subtracts from every breath.
Open the Minute Ventilation calculator →Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. The respiratory system: anatomy and physiology chapters.
- West JB, Luks AM. West's Respiratory Physiology: The Essentials. 11th ed. Wolters Kluwer; 2021.