The respiratory system is the network of airways, lungs and supporting structures that brings air to the body's gas-exchanging surfaces. Air travels from the nose through branching passages to tiny air sacs called alveoli. The surrounding chest wall and breathing muscles move air into and out of these spaces.
What is the order of the respiratory airways?
The respiratory tract is the continuous series of passages through which air travels. During breathing through the nose, air follows this route:
- Nasal cavity: the air-filled space within the nose.
- Pharynx: the muscular passage behind the nasal and oral cavities.
- Larynx: the cartilaginous airway containing the vocal folds.
- Trachea: the main airway descending towards the chest.
- Bronchi: the branching airways that enter and distribute air within the lungs.
- Bronchioles: smaller branches that lack cartilage in their walls.
- Alveolar ducts and alveoli: the terminal passages and air sacs involved in gas exchange.
The upper airways include the nose, nasal cavity and pharynx. Definitions differ over whether the larynx belongs to the upper or lower airways, so its position should be stated explicitly. Here, the lower respiratory tract includes the larynx, trachea and airways within the lungs.
How do the nose and pharynx conduct air?
The nasal septum, a partition made from bone and cartilage, separates the nasal cavity into right and left sides. Curved bony shelves called nasal conchae project from its lateral walls. They increase the surface over which incoming air passes.
Most of the nasal cavity has a mucous membrane, a moist lining that helps warm and humidify air. Mucus traps inhaled particles, while cilia, microscopic hair-like projections on lining cells, move mucus towards the pharynx. The upper part of the cavity contains specialised tissue for smell.
The pharynx has named regions according to the cavity or airway lying in front of it: the nasopharynx behind the nasal cavity, the oropharynx behind the mouth, and the laryngopharynx behind the larynx. Air from the nose passes through these regions towards the larynx. Air entering through the mouth joins this route at the oropharynx.
What is the larynx and where are the vocal folds?
The larynx lies in the neck between the pharynx and trachea. Its framework includes the thyroid cartilage at the front, the ring-shaped cricoid cartilage below it, and the arytenoid cartilages behind. Muscles and ligaments connect these cartilages and control the opening of the airway.
The vocal folds are paired folds containing a ligament and muscle beneath their lining. The rima glottidis is the opening between the vocal folds anteriorly and the arytenoid cartilages posteriorly; the folds and this opening together form the glottis. During inspiration, the folds normally move apart to widen the passage, while during phonation their vibration produces voice.
The epiglottis is a leaf-shaped cartilage at the entrance to the larynx. During swallowing, coordinated movement of the larynx and surrounding structures helps separate the airway from the route taken by food. The epiglottis participates in this protection rather than acting as an isolated lid.
How do the trachea and bronchial tree branch?
The trachea descends through the neck into the thorax, the chest region bounded by the ribs, sternum and thoracic vertebral column, with the diaphragm forming its floor. C-shaped cartilage rings support its front and sides. Its posterior wall contains smooth muscle, a type of muscle not under voluntary control, and lies immediately in front of the oesophagus, the passage carrying food towards the stomach.
At its lower end, the trachea divides into the main bronchi. A bronchus is a conducting airway supported by cartilage. In adults, the right main bronchus is usually wider, shorter and more vertical than the left main bronchus.
Each main bronchus divides into lobar bronchi, which supply the lobes, and then segmental bronchi. A bronchopulmonary segment is a region supplied by a segmental bronchus and an accompanying pulmonary artery branch. Together, these divisions form the bronchial tree.
Within the lungs, cartilage rings give way to cartilage plates and eventually disappear in the bronchioles. Bronchioles retain smooth muscle in their walls. Terminal bronchioles are the final purely conducting branches; they lead into respiratory bronchioles, whose walls begin to contain alveoli.
What is the difference between conducting and respiratory zones?
These zones describe function rather than simply dividing the airway into upper and lower portions. The conducting zone moves and conditions air without providing alveolar gas-exchanging surfaces. The respiratory zone begins where alveoli first appear in airway walls.
| Feature | Conducting zone | Respiratory zone |
|---|---|---|
| Extent | Nose to terminal bronchioles | Respiratory bronchioles to alveoli |
| Main structures | Nasal cavity, pharynx, larynx, trachea, bronchi and conducting bronchioles | Respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli |
| Main role | Transport, warming, humidification and particle removal | Gas exchange between air and blood |
| Alveoli in the walls | Absent | Present |
Alveolar ducts lead into alveolar sacs, groups of alveoli opening into a shared space. Thin alveolar walls lie close to pulmonary capillaries, the smallest blood vessels carrying blood through the lungs. This close relationship provides the surface across which oxygen and carbon dioxide move.
How many lobes and fissures do the lungs have?
Each lung has an apex, its upper tip, and a base resting on the diaphragm. Its outer surface faces the chest wall, while its medial surface faces the mediastinum, the central compartment between the lungs. The hilum is the area on the medial surface where airways, vessels and nerves enter or leave.
The right lung has three lobes:
- Upper lobe of right lung.
- Middle lobe of lung.
- Lower lobe of right lung.
The left lung has two lobes:
- Upper lobe of left lung.
- Lower lobe of left lung.
Fissures are deep clefts separating the lobes, although they may be incomplete. Typically, the right lung has horizontal and oblique fissures; the left lung has an oblique fissure. Individual variation can include an absent fissure or an additional fissure. The left lung also has a cardiac notch, an indentation along its anterior border accommodating the heart, and a lingula, a tongue-shaped projection of its upper lobe below that notch.
What is the pleura and how does it surround the lungs?
The pleura is a thin, moist membrane arranged around each lung. Visceral pleura covers the lung surface and extends into the fissures. Parietal pleura lines the inner chest wall, covers the upper surface of the diaphragm and borders the mediastinum.
The pleural cavity is the potential space between these layers: normally they lie closely opposed rather than enclosing an open air-filled gap. A thin film of fluid allows the surfaces to slide during breathing. Each lung has its own pleural cavity, separate from the other side.
The layers become continuous around the root of the lung, the bundle of airways, vessels and other structures passing through the hilum. In the adjacent 3D model, rotate and zoom to compare the lungs, then isolate a structure to examine its shape.
How do the diaphragm and intercostal muscles move air?
The diaphragm is a dome-shaped sheet of muscle and tendon separating the thorax from the abdomen. Its muscle fibres converge on a central tendon, a strong sheet of connective tissue. During inspiration, or breathing in, contraction lowers its domes and increases the vertical dimension of the chest.
Intercostal muscles occupy the spaces between adjacent ribs. Each rib contributes to the chest wall, and rib elevation increases its front-to-back and side-to-side dimensions. The external intercostal muscles assist this expansion during inspiration.
During quiet expiration, or breathing out, the inspiratory muscles relax and the net elastic recoil of the respiratory system reduces chest volume towards its resting level. During forceful expiration, abdominal muscles and the interosseous portions of the internal intercostal muscles, the parts between the bony ribs, help reduce that volume. These movements change pressure within the lungs and move air through the airway passages.