The Lungs are the principal organs of the human respiratory system and contain the specialised surfaces through which oxygen enters the blood and carbon dioxide leaves it. They occupy most of the thoracic cavity on either side of the heart and are enclosed by thin membranes called pleurae. The right lung is divided into three lobes, while the left lung has two major lobes and a region accommodating the position of the heart.
Air reaches the lungs through the trachea and branching bronchi, which progressively divide into smaller airways. The terminal regions contain alveoli surrounded by dense networks of pulmonary capillaries. The lungs are elastic organs whose volume changes as the thoracic cavity expands and contracts. Their internal architecture creates an enormous gas exchange surface while keeping the diffusion distance between air and blood extremely small. Lung function therefore depends on coordinated airway structure, elastic properties, pulmonary circulation, alveolar stability and ventilation.
The lungs are covered by visceral pleura and lie within a pleural cavity that contains a small amount of lubricating fluid. The pleural membranes allow the lungs to move smoothly relative to the chest wall while maintaining mechanical coupling between the lungs and thoracic cage. Pressure relationships within the pleural space help keep the lungs expanded. When the thoracic cavity enlarges, the lungs are pulled outward and their internal pressure falls, allowing air to enter.
If air enters the pleural space under inappropriate conditions, this mechanical relationship can be disrupted and part or all of a lung may collapse. The pleural system therefore has a structural and mechanical role rather than simply acting as a protective covering. The elastic properties of lung tissue and the surface tension of alveolar fluid also influence the effort required for breathing. Surfactant produced by specialised alveolar cells reduces surface tension and contributes to alveolar stability.
The internal architecture of the lungs is dominated by repeated branching of the airways. The trachea divides into primary bronchi, which branch into progressively smaller airways before reaching bronchioles and gas exchange regions. This branching distributes inhaled air across a very large number of alveoli. The conducting portions of the airway also modify the physical properties of incoming air by warming and humidifying it and by removing particles. Ciliated epithelial cells and mucus contribute to clearance of inhaled material.
The smaller airways contain less cartilage and greater relative importance of smooth muscle, making their diameter sensitive to physiological and pathological influences. At the alveolar level, the walls become extremely thin and specialised for diffusion. The transition from conducting structures to gas exchange surfaces therefore represents a gradual shift from air transport and protection towards rapid exchange of respiratory gases.
Alveoli are the functional gas exchange units of the lungs. Each alveolus is closely associated with pulmonary capillaries, allowing oxygen to diffuse from alveolar air into blood while carbon dioxide diffuses in the opposite direction. The diffusion barrier is extremely thin, comprising alveolar epithelium, associated extracellular material and capillary endothelium. The enormous total number of alveoli provides a large surface area for exchange.
Alveolar ventilation and pulmonary blood flow must be appropriately matched for efficient oxygen uptake and carbon dioxide removal. Surfactant reduces the tendency of alveoli to collapse by lowering surface tension at the air liquid interface. Alveolar macrophages provide additional defence by engulfing particles and microorganisms that reach the distal lung. The alveoli therefore combine respiratory exchange, mechanical stability and immune protection within a highly specialised microscopic environment.
Pulmonary circulation supplies the lungs with blood from the right side of the heart. Blood arriving through the pulmonary arteries has relatively low oxygen content and elevated carbon dioxide content compared with blood leaving the lungs. It passes through dense capillary networks surrounding alveoli, where diffusion changes its gas composition. Oxygenated blood then returns through pulmonary veins to the left side of the heart.
The pulmonary circulation operates at lower pressures than systemic circulation, which helps accommodate the extensive capillary network within the delicate lung tissue. Blood flow is influenced by gravity, lung volume and local oxygen concentration. Regions of the lung that receive inadequate ventilation can undergo local vascular responses that redirect some blood towards better ventilated areas. This contributes to matching between airflow and blood flow, although the match is not perfectly uniform.
Lung function depends on mechanical properties including compliance, elasticity, airway resistance and surface tension. Compliance describes the change in lung volume produced by a given change in pressure. Excessive stiffness can increase the effort required to expand the lungs, whereas excessive loss of elastic recoil can impair expiration and air movement. Airway resistance is strongly influenced by airway radius and can increase when smooth muscle contracts, mucus accumulates or airway walls become swollen.
Surfactant reduces surface tension and therefore lowers the mechanical work needed to inflate alveoli. These properties interact during breathing. A person may have adequate gas exchange surfaces but still experience impaired ventilation if airway resistance becomes very high or if respiratory muscles cannot generate sufficient pressure changes. Lung function must therefore be understood as both a mechanical and a gas exchange process.
Lung disease can affect airways, alveoli, pleurae, blood vessels or supporting tissue. Airway disorders can narrow the passages and reduce airflow, while destructive processes can reduce alveolar surface area. Inflammatory or infectious conditions can alter the thickness or permeability of gas exchange surfaces. Fluid accumulation within or around the lungs can interfere with normal ventilation and diffusion. Pulmonary vascular disorders can reduce effective blood flow through regions of the lung.
Long term exposure to inhaled pollutants or tobacco smoke can produce structural and functional changes. Diagnostic approaches include spirometry, imaging, oxygen measurements, carbon dioxide measurements and examination of respiratory symptoms and signs. Different diseases produce distinct patterns of impairment, so diagnosis requires identifying the physiological mechanism rather than relying solely on a general description of breathing difficulty.
The lungs have additional roles beyond oxygen uptake and carbon dioxide removal. They participate in the metabolism of certain circulating substances and contain enzymes and cellular systems that modify biologically active molecules. Pulmonary immune defences help prevent inhaled microorganisms and particles from reaching deeper tissues. The lungs also contribute to vocalisation indirectly by supplying controlled airflow through the upper respiratory structures. Their large surface area creates extensive interaction with the environment, making effective defence essential.
Lung development begins before birth and continues after birth as respiratory structures mature and functional capacity changes. Age, physical activity, environmental exposure and disease can influence lung performance. The lungs therefore represent highly specialised organs in which branching anatomy, microscopic exchange surfaces, mechanical properties, vascular supply and immune protection operate together to support continuous respiration.
