The main parts of the respiratory system include the nose and nasal cavity, mouth, pharynx, larynx, trachea, bronchi, bronchioles and lungs. The lungs contain alveoli, which are the principal sites of gas exchange. The diaphragm and intercostal muscles provide much of the mechanical force required for ventilation, while the pulmonary blood vessels bring blood into close contact with alveolar air. The respiratory system is therefore more than the lungs themselves: it includes the conducting airways, respiratory muscles, gas-exchange surfaces and circulation required to move air and exchange gases efficiently.
The nose and nasal cavity form an important entrance to the respiratory tract. Nasal structures help warm and humidify incoming air and trap particles in mucus. Air can also enter through the mouth, particularly during exercise or when nasal passages are obstructed. From the nasal or oral passages, air passes through the pharynx, a shared passage behind the nose and mouth, before reaching the larynx. The arrangement of these structures allows the respiratory tract to condition inhaled air while also connecting the airway to the structures involved in swallowing and voice production.
The larynx, commonly called the voice box, lies between the pharynx and trachea. It contains the vocal folds and contributes to sound production, but it also helps protect the lower airway during swallowing. The trachea, or windpipe, is a reinforced airway supported by cartilage that helps prevent collapse. It divides into the right and left main bronchi, which carry air into the corresponding lungs. The bronchi continue to divide into progressively smaller branches, forming an extensive bronchial tree that distributes air throughout the lungs.
The bronchioles are smaller airway branches arising from the bronchi. Unlike the larger bronchi, their walls contain less cartilage and their diameter is strongly influenced by smooth muscle. Bronchioles ultimately lead to respiratory regions containing alveoli. This transition from conducting airways to gas-exchanging structures is functionally important because air must be distributed widely while maintaining sufficiently low resistance. Conditions such as asthma can cause bronchiolar smooth-muscle contraction, mucosal swelling and mucus production, narrowing these passages and making airflow more difficult.
The lungs are the principal organs of the respiratory system and occupy most of the thoracic cavity on either side of the heart. The right lung has three lobes, while the left lung has two because the heart occupies part of the left side of the chest. Within the lungs, the branching bronchi and bronchioles terminate in alveolar structures. The alveoli are surrounded by capillaries supplied by the pulmonary circulation, creating the thin interface required for oxygen and carbon-dioxide exchange. The lungs are enclosed by the pleura, whose two layers and lubricating fluid allow relatively friction-free movement during breathing.
The diaphragm is the principal muscle of breathing. It is a dome-shaped sheet of muscle separating the thoracic and abdominal cavities. When it contracts, it moves downward and increases the volume of the chest cavity, causing the lungs to expand. Intercostal muscles between the ribs also contribute by changing the dimensions of the rib cage. During vigorous breathing, abdominal muscles and accessory muscles in the neck and upper chest can contribute to ventilation. Weakness or paralysis affecting these muscles or their nerves can substantially impair breathing even when the lungs themselves are relatively normal.
The respiratory system is closely connected to the pulmonary circulation, which is not an airway but is essential for respiratory function. The pulmonary artery carries oxygen-poor blood from the right side of the heart to capillaries surrounding the alveoli, while pulmonary veins return oxygenated blood to the left side of the heart. The blood therefore provides the transport pathway between the lungs and the rest of the body. The nervous system supplies another essential component by controlling the rhythm and depth of breathing and responding to changing oxygen and carbon-dioxide requirements.
Functionally, the respiratory system can be divided into a conducting zone and a respiratory zone. The conducting zone transports, warms, humidifies and filters air but does not normally perform the main gas exchange. The respiratory zone contains respiratory bronchioles, alveolar ducts and alveoli where oxygen and carbon dioxide cross between air and blood. This distinction is useful because diseases can affect different compartments: asthma primarily involves airway narrowing, pneumonia can fill alveoli with inflammatory fluid, pulmonary fibrosis can thicken the gas-exchange barrier, and neuromuscular disorders can impair the muscles that ventilate otherwise structurally normal lungs.
