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Respiratory Tract
and Alveoli

Explore the anatomy of the respiratory tract from the nasal cavity to the alveoli, and understand how gas exchange keeps every cell alive.

Anatomy & Physiology

What Is the Respiratory System?

The respiratory system is a network of organs and tissues that facilitates breathing: the intake of oxygen (\( O_2 \)) and the removal of carbon dioxide (\( CO_2 \)). It is divided into two functional zones:

Conducting Zone
Transports air but performs no gas exchange: nose, pharynx, larynx, trachea, bronchi, and bronchioles.
Respiratory Zone
Where gas exchange occurs: respiratory bronchioles, alveolar ducts, and alveoli.

The Upper Respiratory Tract

Nasal Cavity Pharynx Larynx Warms, filters, humidifies air Shared passage for air & food Voice production & epiglottis guard Pharynx Divisions Nasopharynx (superior) Oropharynx (middle) Laryngopharynx (inferior)

The Lower Respiratory Tract

Trachea Left Main Bronchus Right Main Bronchus Bronchioles Bronchioles Alveoli Alveoli

The Trachea: Your Windpipe

C-Shaped Cartilage
16 to 20 rings of hyaline cartilage keep the trachea open and prevent collapse during breathing.
Mucociliary Escalator
Goblet cells secrete mucus that traps debris; cilia sweep it upward toward the pharynx for removal.
Trachealis Muscle
Smooth muscle on the posterior wall allows the esophagus to expand during swallowing.

Bronchi and Bronchioles

As airways branch deeper into the lungs, their structure changes progressively:

1
Primary bronchi enter each lung at the hilum. The right bronchus is wider, shorter, and more vertical than the left.
2
Secondary (lobar) bronchi supply each lobe: 3 on the right, 2 on the left.
3
Tertiary (segmental) bronchi serve bronchopulmonary segments.
4
Bronchioles lack cartilage and rely on smooth muscle. Terminal bronchioles mark the end of the conducting zone.

Anatomy of the Alveolus

Type I Cell (Gas exchange) Type II Cell (Surfactant) Capillary network
\( O_2 \)
\( CO_2 \)
 Alveolar Macrophage Surfactant layer

Alveolar Cell Types

Type I Pneumocytes
Thin, flat squamous cells that cover ~95% of the alveolar surface. Their extreme thinness (~0.1 to 0.2 µm) allows rapid gas diffusion.
Type II Pneumocytes
Cuboidal cells that secrete pulmonary surfactant, reducing surface tension and preventing alveolar collapse. They can also regenerate Type I cells.
Alveolar Macrophages
Immune cells ("dust cells") that patrol the alveolar surface, engulfing bacteria, debris, and particles via phagocytosis.

The Respiratory Membrane

Gas exchange occurs across the respiratory membrane (also called the air-blood barrier), which is only about 0.5 µm thick. It consists of three layers:

1
Alveolar epithelium: the thin Type I pneumocyte wall lining the alveolus.
2
Fused basement membranes: a shared basement membrane between the alveolar epithelium and capillary endothelium.
3
Capillary endothelium: the single-cell wall of the pulmonary capillary.

Fick's Law of Diffusion

The rate of gas diffusion across the respiratory membrane is governed by Fick's Law:

Increases Rate
Larger surface area (A), greater pressure gradient (\( P_1 - P_2 \)), higher diffusion coefficient (D).
Decreases Rate
Increased membrane thickness (T), as seen in pulmonary edema or fibrosis.

Conducting vs Respiratory Zone

Conducting Zone Respiratory Zone
Function Air transport, warming, filtering Gas exchange
Structures Nose to terminal bronchioles Respiratory bronchioles to alveoli
Cartilage Present (rings/plates) Absent
Epithelium Pseudostratified ciliated columnar Simple squamous
Dead Space Yes (anatomical dead space ~150 mL) No

Surfactant and Surface Tension

Alveoli are lined with a thin layer of water, creating surface tension that tends to collapse them. Pulmonary surfactant, a phospholipid mixture secreted by Type II pneumocytes, reduces this surface tension.

Tracing Airflow: Step by Step

1
Nasal cavity / oral cavity: air enters, is warmed, humidified, and filtered.
2
Pharynx: air passes through the nasopharynx, oropharynx, and laryngopharynx.
3
Larynx: the epiglottis directs air past the vocal cords into the trachea.
4
Trachea: air travels down the windpipe, kept open by C-shaped cartilage rings.
5
Bronchi: the trachea splits at the carina into left and right primary bronchi, then lobar and segmental bronchi.
6
Bronchioles: progressively smaller airways lacking cartilage lead to terminal and respiratory bronchioles.
7
Alveolar ducts and alveoli: gas exchange occurs across the respiratory membrane into pulmonary capillaries.

Example: Partial Pressures

The partial pressure of a gas determines the direction and rate of diffusion. Consider the following typical values at sea level:

Alveolar Air Deoxygenated Blood Gradient
\( P_{O_2} \) 104 mmHg 40 mmHg 64 mmHg
\( P_{CO_2} \) 40 mmHg 45 mmHg 5 mmHg

Example: Applying Fick's Law

Scenario: A patient with pulmonary fibrosis has a respiratory membrane that is 3 times thicker than normal. How does this affect gas exchange rate?

Example: Anatomical Dead Space

Problem: A person has a tidal volume of 500 mL and an anatomical dead space of 150 mL. What volume of air actually reaches the alveoli per breath?

At a respiratory rate of 12 breaths/min, the alveolar ventilation rate is:

Common Mistakes

✕ Confusing Bronchi with Bronchioles
Bronchi have cartilage and submucosal glands; bronchioles do not. The loss of cartilage is the key structural distinction.
✕ Mixing Up Type I and Type II Cells
Type I cells are for gas exchange (thin, flat). Type II cells produce surfactant (cuboidal, granular). Their roles are not interchangeable.
✕ Forgetting Dead Space
Not all inhaled air reaches the alveoli. Approximately 150 mL remains in conducting airways and never participates in gas exchange.

Airway Summary Table

Structure Cartilage Epithelium Function
Trachea C-shaped rings Pseudostratified ciliated Conduct & filter
Primary Bronchi C-shaped rings Pseudostratified ciliated Conduct to lungs
Secondary Bronchi Plates Pseudostratified ciliated Conduct to lobes
Bronchioles None Simple columnar to cuboidal Conduct & regulate
Alveoli None Simple squamous Gas exchange

Question 1 of 5

Which cell type is primarily responsible for gas exchange in the alveoli?

Question 2 of 5

What is the primary function of pulmonary surfactant?

Question 3 of 5

The C-shaped cartilage rings of the trachea are open on the posterior side to allow:

Question 4 of 5

According to Fick's Law, which change would decrease the rate of gas diffusion across the respiratory membrane?

Question 5 of 5

If tidal volume is 500 mL, dead space is 150 mL, and respiratory rate is 15 breaths/min, what is the alveolar ventilation rate?