Conducting Zone vs Respiratory Zone

What Is the Difference Between the Conducting Zone and the Respiratory Zone?

The difference is function: the conducting zone is a set of pipes that delivers air, and the respiratory zone is the surface where that air finally meets the blood. Every airway from the nostrils to the terminal bronchioles belongs to the conducting zone, and every airway that has alveoli opening from its wall belongs to the respiratory zone.

The two zones also differ in their walls, their volume and the way air moves through them. Conducting airways have thick walls lined with ciliated, mucus-covered epithelium, and air flows through them in bulk, pushed by the pressure changes of breathing. Respiratory airways have walls so thin that gases diffuse straight through them, and the air inside barely moves forward, because the total width of the airway tree becomes enormous at that depth.

The verdict in one line

If an airway can exchange oxygen and carbon dioxide with the blood, it is part of the respiratory zone; if it cannot, it is part of the conducting zone. The first airway that passes this test is the respiratory bronchiole, which is why the border between the zones sits just after the last terminal bronchiole.

Explained simply

Picture a city water system. The big mains and the street pipes only carry water, and that is the conducting zone. The taps where water is actually used are the respiratory zone. In the lungs, the taps are hundreds of millions of tiny air sacs, and what they hand over is oxygen.

What Is the Conducting Zone?

The conducting zone is the part of the respiratory tract that carries air between the outside world and the gas exchange surfaces without taking part in gas exchange itself. Because the air in it never reaches the blood, its volume, about 150 mL in an adult, is called the anatomical dead space.

Conducting zone definition in anatomy

Anatomy textbooks define the conducting zone as all the airways from the nose to the terminal bronchioles. Lung physiology often uses a narrower definition that starts at the trachea: in the classic model of the airway tree, the trachea is generation 0, and each division adds one generation until the terminal bronchioles at about generation 16. Both definitions agree on where the zone ends.

Parts of the conducting zone

• Nose and nasal cavity: filter, warm and humidify incoming air over the folds of the nasal conchae.
• Pharynx: the shared throat passage for air and food.
• Larynx: guards the airway and houses the vocal folds.
• Trachea: the windpipe, held open by C-shaped rings of hyaline cartilage.
• Bronchi: main, lobar and segmental bronchi, supported by cartilage rings and plates.
• Bronchioles: small airways about 1 mm wide or less, with smooth muscle but no cartilage.
• Terminal bronchioles: the last and smallest purely conducting airways.

The whole branching system from the trachea down is described on our bronchial tree page.

What Is the Respiratory Zone? (Respiratory Zone Anatomy)

The respiratory zone is the part of the lungs where gas exchange takes place, made of the respiratory bronchioles, alveolar ducts, alveolar sacs and the alveoli. In the airway tree model it covers roughly generations 17 to 23, the last few divisions before the airways end in blind sacs.

Respiratory bronchioles

Respiratory bronchioles are short airways with a few alveoli budding from their walls, so they both conduct air and exchange gases. They are the transition point between the two zones, which is why some textbooks call them the transitional zone. Our page on terminal and respiratory bronchioles compares the two neighbors in detail.

Alveolar ducts, sacs and alveoli

Each respiratory bronchiole opens into several alveolar ducts and alveolar sacs, whose walls are made almost entirely of alveoli. An adult has about 300 to 500 million alveoli, giving a gas exchange surface of roughly 50 to 75 square meters. The alveolar wall is lined mostly by flat type I pneumocytes, with type II cells making surfactant.

The pulmonary acinus

All the respiratory tissue supplied by one terminal bronchiole forms a unit called the acinus, the basic working unit of gas exchange. Despite its importance, an acinus is tiny: the distance from a terminal bronchiole to its farthest alveolus is only a few millimeters. Acini group together into the larger pulmonary lobule.

What Is the Function of the Conducting Zone and the Respiratory Zone?

The conducting zone function is to deliver clean, warm, moist air to the lungs and carry stale air back out, while the respiratory zone function is to exchange oxygen and carbon dioxide between that air and the blood.

Conducting zone function

1. Air transport: a low-resistance path for air in and out with each breath.
2. Filtering: nasal hairs, mucus and the branching shape of the airways trap dust, pollen and microbes, and cilia sweep the mucus up toward the throat in the mucociliary escalator described on our cilia and mucus page.
3. Warming and humidifying: by the time air reaches the deep lung, it is close to body temperature and fully saturated with water vapor.
4. Control of airflow: smooth muscle in the bronchi and bronchioles can narrow or widen the airways.
5. Protection and voice: the larynx closes during swallowing, triggers the cough reflex and produces sound.

Respiratory zone function

The respiratory zone has one main job: gas exchange. Oxygen diffuses from alveolar air across the respiratory membrane into the pulmonary capillaries, and carbon dioxide diffuses the opposite way. Alongside that job, alveolar macrophages clean up the fine particles that slip past the conducting zone, and type II pneumocytes make pulmonary surfactant, which keeps the small air sacs from collapsing.

How Do the Airway Walls Change From the Conducting to the Respiratory Zone?

Moving from the conducting zone to the respiratory zone, the airway walls become thinner and simpler: cartilage disappears first, then mucus glands and goblet cells, then cilia, until only a single layer of very flat cells separates air from blood.

Epithelium

The trachea and bronchi are lined by pseudostratified ciliated columnar epithelium with many goblet cells, the classic respiratory epithelium. In the bronchioles the lining becomes simple columnar and then simple cuboidal, goblet cells fade out, and club cells take over the job of protecting the lining. In the alveoli, the lining is simple squamous epithelium, formed mostly by type I pneumocytes only a fraction of a micrometer thick.

Cartilage, glands and smooth muscle

• Cartilage rings and plates support the trachea and bronchi but are absent from all bronchioles.
• Submucosal glands are found in the trachea and bronchi and are missing from bronchioles.
• Smooth muscle forms a strong layer in the bronchioles and shrinks to small bundles around the openings of the alveolar ducts.
• Cilia continue as far as the respiratory bronchioles but are absent from the alveoli, where macrophages do the cleaning.

Our respiratory system histology page shows these tissues level by level.

What Do the Conducting Zone and Respiratory Zone Have in Common?

The two zones are parts of one continuous airway tree, so they share the same air, the same branching pattern and the same aim of supplying the body with oxygen.

• One continuous tube system: every respiratory zone airway is the direct continuation of a conducting airway, with no valve or gap between them.
• Branching by division: both zones grow by repeated division of each airway into two or more smaller ones.
• Lined by epithelium: both are covered by an unbroken layer of epithelial cells, which only changes shape along the way.
• Moved by the same muscles: both fill and empty because the diaphragm and the muscles of respiration change the size of the chest.
• Develop from the same bud: both grow from the lung bud of the embryo, with the respiratory zone forming last, as described on our respiratory system development page.
• Protected by defenses: each zone has its own cleaning system, cilia and mucus in one, macrophages and surfactant in the other.

How Do the Conducting and Respiratory Zones Work Together?

The conducting zone prepares and delivers each breath, and the respiratory zone uses it, so neither works without the other. Air that is not cleaned, warmed and humidified would dry out and damage the delicate alveolar walls, and air that never reached the alveoli would be useless to the blood.

Bulk flow, then diffusion

In the conducting zone, air moves by bulk flow, like water through a hose, driven by the pressure changes of inhalation and exhalation. With every division the number of airways multiplies faster than each one narrows, so the total cross-sectional area of the airway tree grows enormously in the respiratory zone. As a result, the forward speed of the air drops so low that the final short distance to the alveolar wall is covered mainly by diffusion of the gas molecules themselves.

Why dead space matters

Of a typical resting breath of about 500 mL, about 150 mL stays in the conducting zone and is breathed out unchanged, so only about 350 mL of fresh air reaches the respiratory zone. That is why slow, deep breaths refresh the alveoli more effectively than fast, shallow ones of the same total volume; our tidal volume page explains the numbers.

Two blood supplies

The walls of the conducting airways are fed by the bronchial arteries of the systemic circulation, while the respiratory zone exchanges gases with blood from the pulmonary circulation. For the full route of a breath, see the pathway of air.

Common Conditions That Affect the Conducting and Respiratory Zones

Diseases of the conducting zone mainly narrow, inflame or widen the airways and make airflow harder, while diseases of the respiratory zone damage or flood the gas exchange surface. Our respiratory diseases section covers each one, and anyone with breathing symptoms should see a doctor.

• Asthma (conducting zone): inflamed bronchi and bronchioles tighten and fill with mucus.
• Bronchitis (conducting zone): inflammation of the bronchial lining with extra mucus and coughing.
• Bronchiectasis (conducting zone): permanently widened bronchi that clear mucus poorly.
• Emphysema (respiratory zone): alveolar walls break down, shrinking the exchange surface.
• Pneumonia (respiratory zone): infection fills alveoli with fluid and cells.
• Pulmonary fibrosis (respiratory zone): scar tissue thickens the walls between alveoli.

COPD usually affects both zones at once, combining narrowed small airways with emphysema.