The lungs work by providing a large, thin and highly vascularised surface where oxygen from inhaled air enters the blood and carbon dioxide leaves it, while mechanical ventilation continually renews the air within the gas exchange regions. The lungs are located within the thoracic cavity and are enclosed by pleural membranes. The right lung normally has three lobes and the left lung has two lobes, with the left side accommodating the position of the heart.
Air enters through the upper airways, travels through the trachea and bronchi, passes through progressively smaller bronchioles and eventually reaches alveoli. The lungs themselves contain extensive branching airways and millions of microscopic alveolar structures, producing a very large total surface area for gas exchange. Pulmonary capillaries form a dense network around the alveoli. When the respiratory muscles expand the thorax, air flows into the lungs because pressure inside the alveoli falls below atmospheric pressure. During expiration, elastic recoil helps reduce lung volume and drive air outward.
Ventilation depends on pressure relationships rather than the lungs actively sucking air into themselves. The diaphragm is the principal muscle of quiet inspiration. When it contracts, it flattens and increases the vertical dimension of the thoracic cavity. External intercostal muscle contraction also elevates and expands the ribs.
The resulting increase in thoracic volume lowers pressure within the pleural space and causes the lungs to expand because the visceral and parietal pleural surfaces are closely coupled by a thin layer of fluid. Expansion of the lungs increases alveolar volume and lowers alveolar pressure below atmospheric pressure, producing inward airflow. When the diaphragm and inspiratory muscles relax, elastic recoil of the lungs and chest wall reduces thoracic volume, alveolar pressure rises above atmospheric pressure and air flows out. During exercise or respiratory distress, additional muscles can increase the force and volume of breathing. The mechanical behaviour of the lungs is therefore governed by muscle activity, thoracic movement, pleural pressure and elastic properties.
Once air reaches the alveoli, gas exchange occurs across the alveolar capillary membrane. Oxygen moves from alveolar air into pulmonary capillary blood because the partial pressure of oxygen is higher in the alveoli than in incoming venous blood. Carbon dioxide moves from blood into the alveoli because its partial pressure is higher in venous blood than in alveolar air. The respiratory membrane is extremely thin, consisting principally of alveolar epithelium, fused or closely associated basement membranes and capillary endothelium.
Its thinness reduces diffusion distance and allows gases to cross rapidly. Pulmonary surfactant, produced by specialised type II alveolar cells, reduces surface tension at the air liquid interface and helps prevent alveoli from collapsing. Surfactant also improves lung compliance, meaning less pressure is required to achieve a given change in lung volume. Without adequate surfactant, particularly in premature infants, alveoli are more difficult to keep open and severe respiratory impairment can occur.
The efficiency of pulmonary gas exchange depends on matching ventilation with perfusion. Ventilation describes the movement of air into and out of alveoli, while perfusion describes blood flow through pulmonary capillaries. An alveolus that receives air but little blood cannot transfer much oxygen into the circulation, while blood flowing through a poorly ventilated region cannot acquire adequate oxygen or eliminate carbon dioxide efficiently. The body possesses local mechanisms that help improve matching.
Low alveolar oxygen can cause pulmonary vasoconstriction in nearby vessels, redirecting blood towards better ventilated regions. This response differs from systemic tissues, where low oxygen generally promotes vasodilation. Gravity also influences pulmonary perfusion, causing regional differences in blood flow when a person is upright. Ventilation and perfusion therefore vary across the lungs, but overall coordination between them is essential for efficient oxygen uptake and carbon dioxide removal.
The lungs also participate in the regulation of blood chemistry through control of carbon dioxide elimination. Cellular metabolism continuously produces carbon dioxide, which is transported in the blood to the lungs. A large proportion is carried as bicarbonate, with additional amounts dissolved in plasma or bound to haemoglobin and other proteins. In pulmonary capillaries, carbon dioxide is released from blood and diffuses into alveoli. Ventilation then removes it from the body.
If ventilation increases relative to carbon dioxide production, arterial carbon dioxide tension tends to fall. If ventilation decreases, carbon dioxide tends to accumulate. Because carbon dioxide is linked chemically to hydrogen ion concentration through the carbonic acid bicarbonate system, changes in ventilation can rapidly influence blood pH. This makes the lungs an important component of acid base homeostasis. The respiratory centres in the brainstem continuously adjust ventilation in response to chemical feedback so that carbon dioxide remains within an appropriate physiological range under ordinary conditions.
Lung function is influenced by mechanical properties including compliance, airway resistance and elastic recoil. Compliance describes the change in lung volume produced by a given change in pressure. A highly compliant lung expands relatively easily, whereas a stiff lung requires greater pressure changes. Pulmonary fibrosis decreases compliance because scar tissue reduces lung elasticity and distensibility. Emphysema can increase lung compliance while reducing elastic recoil because destruction of alveolar walls removes elastic tissue.
Airway resistance is strongly influenced by airway radius, so bronchoconstriction can markedly increase the effort required to move air. Asthma illustrates how airway inflammation, smooth muscle contraction and mucus can increase resistance. The lungs also contain elastic fibres that help return them towards their resting volume after inspiration. The balance between lung recoil and chest wall mechanics contributes to the functional residual capacity, the volume remaining in the lungs after a normal quiet expiration.
The lungs are protected by several mechanisms that maintain the integrity of their delicate gas exchange surfaces. The conducting airways trap particles in mucus and use ciliary movement to transport material towards the throat. Coughing can rapidly expel secretions and foreign material. Alveolar macrophages remove particles and microorganisms that reach the deepest lung regions. The epithelial lining and local immune mechanisms provide additional defence. Despite these protections, inhaled pollutants, tobacco smoke, occupational dusts and infectious organisms can damage lung tissue.
Chronic exposure to cigarette smoke can cause inflammation, impaired ciliary function, airway remodelling and destruction of alveolar structures. Infectious diseases such as pneumonia can fill alveoli with inflammatory material and reduce the effectiveness of gas exchange. Pulmonary oedema can similarly introduce excess fluid into or around alveolar spaces, increasing diffusion distance and interfering with oxygen transfer. Lung function therefore depends not only on mechanical ventilation but also on maintaining clean, open and structurally intact exchange surfaces.
The lungs work as part of a larger respiratory and circulatory system rather than independently. The heart sends deoxygenated blood from the right ventricle through the pulmonary arteries to the lungs. Pulmonary capillaries surrounding the alveoli receive oxygen and release carbon dioxide. Oxygenated blood then travels through pulmonary veins to the left atrium and subsequently enters systemic circulation.
At the tissue level, oxygen leaves systemic capillary blood and enters cells, while carbon dioxide generated by metabolism enters the blood and is transported back to the lungs. The lungs therefore establish the pulmonary part of a continuous gas transport circuit linking the atmosphere, pulmonary circulation, heart, systemic circulation and tissues. Their effectiveness depends on airway patency, respiratory muscle function, alveolar ventilation, pulmonary perfusion, diffusion capacity and blood haemoglobin. A failure in any major component can impair oxygen delivery or carbon dioxide elimination.
