Respiratory Anatomy: From the Nose to the Alveoli

Air does not simply travel down a tube and arrive in the lungs. The airway changes as it descends because its job changes. Near the entrance, the priority is to warm, humidify and filter inspired air. Deeper in the chest, the passages become smaller and more specialised until they finally reach a surface thin enough for oxygen and carbon dioxide to move between air and blood.

For respiratory anatomy, it helps to follow that journey in order. The structures make much more sense when you can see why each one is shaped the way it is and what changes as the airway branches.

First, follow the route

Inspired air usually travels through the nose or mouth → pharynx → larynx → trachea → main bronchi → lobar bronchi → segmental bronchi → smaller bronchi → bronchioles → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveoli.

Functionally, this route is often divided into a conducting zone and a respiratory zone. The conducting zone carries and conditions air but is not the main site of gas exchange. It extends as far as the terminal bronchioles. The respiratory zone begins at the respiratory bronchioles, where alveoli start to appear in the airway walls, and continues through the alveolar ducts and alveolar sacs to the alveoli themselves.

This distinction is useful because it explains why a problem in a large conducting airway behaves differently from one affecting the gas-exchanging tissue. A mucus plug in a bronchus, bronchiolar narrowing and fluid filling alveoli are all respiratory problems, but anatomically they are happening in very different places.

The nose and pharynx prepare air before it reaches the chest

The nose is more than an opening for airflow. Inside the nasal cavity, the superior, middle and inferior nasal conchae, often called turbinates, increase the surface area of the mucosa and create turbulence. This brings inspired air into closer contact with a warm, moist, vascular lining.

Mucus traps particles and microorganisms, while ciliated epithelial cells help move trapped material backwards towards the pharynx. The nasal passages therefore contribute to filtration, humidification and warming before air reaches the lower airway. Air can also enter through the mouth, especially during exercise or when nasal airflow is obstructed, but this route provides less of that conditioning.

Posteriorly, the nasal cavity opens into the pharynx. The pharynx has three continuous regions: the nasopharynx behind the nose, the oropharynx behind the mouth and the laryngopharynx below them. The important point is that the pharynx is shared territory. Air is heading towards the larynx, while food and fluid need to continue towards the oesophagus.

That shared anatomy is why swallowing and airway protection are so closely linked. If the timing of swallowing, laryngeal closure or cough is impaired, material can enter the airway instead of passing safely into the oesophagus.

The larynx is the protected entrance to the lower airway

The larynx sits between the pharynx and trachea. Its framework is formed by cartilage, including the thyroid and cricoid cartilages, the epiglottis and paired arytenoid cartilages. The vocal folds lie within this framework.

The larynx has two jobs that are especially relevant here. It keeps an open route for breathing, and it helps protect that route during swallowing. Airway protection is not simply the epiglottis acting like a lid. Swallowing involves coordinated elevation of the larynx, closure of the vocal folds and movement of the epiglottis, alongside other pharyngeal and laryngeal movements.

That matters clinically because reduced consciousness, neurological disease or swallowing impairment can disturb this coordination. Understanding the anatomy makes the risk of aspiration easier to understand without reducing the process to a single structure.

The trachea stays open but remains flexible

Below the larynx, the airway becomes the trachea. In an adult it runs from the lower border of the cricoid cartilage into the upper thorax before dividing at the carina into the right and left main bronchi.

The tracheal wall is reinforced by C-shaped rings of hyaline cartilage. These rings help prevent airway collapse while still allowing the trachea to remain flexible. Their open ends face posteriorly towards the oesophagus and are bridged by the trachealis muscle. This arrangement also leaves space for the oesophagus to expand when a bolus is swallowed.

The lining of the trachea contains ciliated respiratory epithelium and mucus-producing cells. Together they form part of the mucociliary clearance system. Mucus traps inhaled material and coordinated ciliary movement carries it upwards towards the pharynx, where it can be swallowed or expectorated.

At the carina, around the level of the sternal angle and approximately the T4 to T5 vertebral level in an adult, the trachea divides. The mucosa around this region is particularly sensitive, which helps explain why stimulation near the carina can provoke a strong cough.

The bronchial tree branches with the lungs

The right and left main bronchi are not identical. The right main bronchus is generally shorter, wider and more vertically aligned with the trachea. The left has a longer and more oblique course as it passes towards the left lung. This difference helps explain why inhaled foreign material or aspirated contents may enter the right bronchial tree more readily, although final distribution also depends on body position and the circumstances of aspiration.

Each main bronchus then divides into lobar bronchi. There are three on the right, corresponding to the three lobes of the right lung, and two on the left. These divide again into segmental bronchi, each supplying a bronchopulmonary segment.

Bronchopulmonary segments are useful because they show that the lung is organised into smaller anatomical territories rather than functioning as one undivided organ. Disease, obstruction, collapse or consolidation can therefore be localised to a particular lobe or segment rather than affecting the whole lung equally.

The lungs have their own surface anatomy

The lungs are paired, cone-shaped organs that fill most of the thoracic cavity on either side of the mediastinum. Each has an apex that extends above the level of the first rib, a base resting on the diaphragm, and costal, mediastinal and diaphragmatic surfaces.

The right lung has three lobes: upper, middle and lower. These are separated by horizontal and oblique fissures. The left lung has two lobes: upper and lower, separated by an oblique fissure. The left lung also has a cardiac notch, where it accommodates the heart, and the lingula, which is part of the left upper lobe rather than a separate lobe.

On the mediastinal surface of each lung is the hilum, where structures enter and leave. The lung root contains the main bronchus, pulmonary artery, pulmonary veins, bronchial vessels, lymphatics and nerves.

This anatomy matters when you examine the chest. A large proportion of the lower lobes lies posteriorly, while the right middle lobe and lingula are better approached from the anterior and lateral chest. That is why a respiratory examination that includes auscultation should not stop at the front of the chest. Listening anteriorly, laterally and posteriorly gives you access to different parts of the lungs.

As bronchi become bronchioles, the airway wall changes

Branching continues deep into the lung. As the bronchi become smaller, the cartilage changes from larger supporting structures to smaller plates. Eventually the airway reaches the bronchioles, which no longer contain cartilage.

This is an important anatomical transition. Bronchioles are small airways with relatively prominent smooth muscle in their walls, so changes in smooth-muscle tone, mucosal swelling or secretions can make a substantial difference to their internal diameter. Because resistance rises sharply as an airway narrows, small changes in calibre can have a large effect on airflow.

The epithelial lining also becomes simpler as the airway gets smaller. Ciliated cells remain present for much of the bronchiolar tree, while goblet cells reduce and club cells become more prominent distally.

The final purely conducting airways are the terminal bronchioles. Beyond them, respiratory bronchioles contain scattered alveoli in their walls. The airway is now changing from a passage that mainly moves air into one that also participates directly in gas exchange.

The lungs move inside pleural sacs

Each lung is enclosed by its own pleural sac. The visceral pleura is closely applied to the lung surface and extends into the fissures. The parietal pleura lines the inner surface of the thoracic wall, diaphragm and mediastinum, with a small cervical extension at the top of the thorax.

Between these layers is the pleural cavity. In health, this is only a potential space containing a thin layer of serous fluid. The fluid allows the pleural surfaces to slide with very little friction while surface forces help keep the lung mechanically coupled to the moving chest wall.

The two pleural cavities are separate. That becomes easier to appreciate when thinking about a pneumothorax. If air enters one pleural space, normal pleural coupling is disrupted and the affected lung can recoil away from the chest wall. The problem is therefore outside the airway itself, even though the patient may present with severe respiratory symptoms.

The two pleural layers also have different sensory innervation. The parietal pleura is sensitive to pain, pressure and temperature, whereas the visceral pleura is relatively insensitive to these sensations. This is why pleuritic pain is associated with irritation of the parietal pleura rather than the lung tissue itself.

The respiratory zone starts before the final alveolar sacs

Once air passes beyond a terminal bronchiole, it enters the pulmonary acinus. An acinus is the gas-exchanging region supplied by a terminal bronchiole and includes respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli.

This is a gradual transition rather than a sudden change from “airway” to “alveolus”. Respiratory bronchioles still look like small airways, but their walls are interrupted by alveoli. Further along, alveolar ducts are lined by increasing numbers of alveolar openings until the architecture is dominated by gas-exchanging spaces.

That gradual structural change is the key idea. The deeper the airway travels, the less it is built for support and conduction and the more it is built to minimise the distance between air and blood.

Alveoli are built for exchange

Alveoli are microscopic air spaces surrounded by a dense capillary network. Their walls are extremely thin, creating a very short diffusion distance between alveolar gas and pulmonary capillary blood.

Most of the gas-exchanging surface is formed by flattened type I pneumocytes. These cells create a thin barrier across which oxygen and carbon dioxide can diffuse. Type II pneumocytes are more cuboidal and produce pulmonary surfactant. Surfactant reduces surface tension at the air-fluid interface and helps keep alveoli stable, particularly during expiration.

Type II cells are also important in repair because they can proliferate and contribute to regeneration of the alveolar epithelium after injury. Alveolar macrophages provide another layer of defence by phagocytosing particles and microorganisms that have travelled beyond the mucociliary clearance system.

For gas exchange to work, anatomy has to bring three things together: ventilated alveoli, an intact thin alveolar-capillary barrier and pulmonary blood flow. Air reaching an alveolus is only one half of the arrangement. The surrounding capillary circulation is what allows oxygen to enter the blood and carbon dioxide to leave it.

Bring the anatomy back to the bedside

Respiratory findings become easier to interpret when you ask where the problem is anatomically. Stridor points you towards the upper airway. Wheeze often reflects narrowed intrathoracic airways. A mucus plug in a main or lobar bronchus can affect ventilation beyond the obstruction. Alveolar filling changes the part of the lung where gas exchange should occur. A pneumothorax affects the pleural space rather than the airway lumen.

The same anatomy is underneath routine nursing assessment. Where you place the stethoscope determines which lung regions you are sampling. Chest expansion depends on the lungs moving with the thoracic cage. Cough and secretion clearance rely on an open conducting airway and functioning mucociliary system. Oxygenation ultimately depends on the distal respiratory zone where alveoli and pulmonary capillaries meet.

You do not need to memorise the respiratory tract as a disconnected list. Follow the route of the air, notice how the wall changes as the airway branches, and keep asking what each structure is built to do. That is the anatomy you will keep coming back to when respiratory assessment starts to make sense.

Summary

Air normally passes from the nose or mouth through the pharynx and larynx into the trachea, then through progressively smaller bronchi and bronchioles. The conducting zone ends at the terminal bronchioles. Respiratory bronchioles mark the beginning of the gas-exchanging region, leading into alveolar ducts and alveoli.

The right and left lungs differ in their lobar anatomy, and each lung sits within a separate pleural sac. Bronchi contain cartilage, while bronchioles do not. At the distal end of the system, type I pneumocytes form most of the thin gas-exchanging surface, type II pneumocytes produce surfactant and contribute to repair, and alveolar macrophages help defend the distal lung.

The most useful way to learn this anatomy is as one continuous journey from air entry to gas exchange, while keeping the clinical location of each structure in mind.

References

Brandt, J.P. and Mandiga, P. (2023) ‘Histology, Alveolar Cells’, StatPearls. NCBI Bookshelf. Available at: https://www.ncbi.nlm.nih.gov/books/NBK557542/

British Thoracic Society (2023) BTS Guideline for Pleural Disease. Available at: https://www.brit-thoracic.org.uk/clinical-resources/guidelines/pleural-disease/

Derricott, H., Jimenez, J. and La Rose, J. (2026) ‘Anatomy, Head and Neck, Trachea’, StatPearls. NCBI Bookshelf. Available at: https://www.ncbi.nlm.nih.gov/books/NBK448070/

Downey, R.P. and Samra, N.S. (2023) ‘Anatomy, Thorax, Tracheobronchial Tree’, StatPearls. NCBI Bookshelf. Available at: https://www.ncbi.nlm.nih.gov/books/NBK556044/

Mahabadi, N., Goizueta, A.A. and Bordoni, B. (2024) ‘Anatomy, Thorax, Lung Pleura and Mediastinum’, StatPearls. NCBI Bookshelf. Available at: https://www.ncbi.nlm.nih.gov/books/NBK519048/

Nata, S.C. and Launico, M.V. (2025) ‘Anatomy, Airway’, StatPearls. NCBI Bookshelf. Available at: https://www.ncbi.nlm.nih.gov/books/NBK459258/

Sparke, A. (2026) ‘The Pleurae’, TeachMeAnatomy. Available at: https://teachmeanatomy.info/thorax/organs/pleurae/

TeachMeAnatomy (2026) ‘The Lungs’. Available at: https://teachmeanatomy.info/thorax/organs/lungs/