Lungs

IWBCA
Feb 15
14 min read

The lungs are paired organs in the chest that bring oxygen into the body, remove carbon dioxide, and work with the heart, blood vessels, and brain to match breathing to what the body needs at rest, during sleep, and during activity.

Overview

What are the lungs and what do they do?

The lungs are paired, spongy, air-filled organs that occupy most of the thoracic cavity on either side of the heart, enclosed by the rib cage and resting on the diaphragm. Their primary function is pulmonary gas exchange. With each respiratory cycle, air is conducted through the tracheobronchial tree into millions of alveoli, where oxygen diffuses across the alveolar–capillary membrane into pulmonary capillary blood and carbon dioxide diffuses in the opposite direction to be eliminated in exhaled air. This continuous exchange maintains arterial oxygen and carbon dioxide within tightly regulated physiologic ranges and is essential for normal tissue oxygen delivery and acid–base balance.

At a microscopic level, the lungs provide an exceptionally large interface between air and blood. The combined surface area of the alveoli is on the order of tens of square meters, comparable to the area of a tennis court, yet it is compacted within the confines of the thorax. The alveolar–capillary barrier is extremely thin, which allows rapid equilibration of gases even when cardiac output and ventilation increase substantially during exertion.

Beyond gas exchange, the lungs function as a dynamic filter and endocrine–metabolic organ. The pulmonary microcirculation traps small thrombi, cellular aggregates, and particulate material, allowing their degradation before they reach critical systemic vascular beds, such as the cerebral or coronary circulation. Endothelial surfaces within the pulmonary vasculature participate in hormone activation and inactivation, including the conversion of angiotensin I to angiotensin II, which contributes directly to systemic blood pressure regulation and volume homeostasis. Minute-to-minute adjustments in ventilation modulate carbon dioxide excretion and therefore play a central role in maintaining systemic pH.

The lungs are also a major immunologic and barrier organ. The conducting airways are lined by mucus and a ciliated epithelium that traps and clears inhaled particles and pathogens. The distal airspaces and interstitium contain resident macrophages, lymphocytes, and other immune cells that survey the alveolar surface and mount localized responses to infectious and inflammatory stimuli. Together, these integrated mechanical, vascular, metabolic, and immune functions position the lungs as a central regulator of gas exchange, cardiovascular physiology, and host defense.

Anatomy

What are the main structures and regions of the lungs?

Each lung has lobes, segments, and a branching airway network that ends in microscopic gas-exchange surfaces. Surrounding structures help protect the lungs and allow them to move smoothly with each breath.

Location and Relationship to Other Organs: The lungs fill most of the chest cavity, separated from each other by the heart and central structures in the mediastinum. The diaphragm forms the floor beneath them, and the ribs and chest wall surround them. The right lung is usually slightly larger than the left because the heart lies closer to the left side.
Lobes and Segments: The right lung has three lobes, and the left lung has two lobes and a region called the lingula. Each lobe is subdivided into bronchopulmonary segments. These segments have their own air and blood supply, which helps limit the spread of infection or localized disease and guides surgical planning when lung tissue must be removed.
Pleura and Pleural Space: The lungs are covered by a thin membrane called the visceral pleura, and the inside of the chest wall is lined by the parietal pleura. A thin layer of lubricating fluid lies between them in the pleural space. This arrangement allows the lungs to slide smoothly as they expand and contract while maintaining close contact with the chest wall.
Airways and Supporting Structures: The lungs are attached to the central airways and blood vessels at a region called the hilum. Here, the main bronchi, pulmonary arteries, pulmonary veins, and lymphatic vessels enter and leave the lung. The bronchi provide structural support for the larger airways, whereas progressively smaller branches carry air deep into the lung parenchyma.

Airways and Alveoli

How does air move through the lungs and reach the blood?

Air travels through a branching system of tubes that start at the nose and mouth and end in microscopic sacs called alveoli. The structure of this system allows air to be filtered, warmed, and evenly distributed to large surfaces where gas exchange occurs.

Conducting Airways: The conducting airways include the nose, pharynx, larynx, trachea, bronchi, and bronchioles that deliver air to the gas-exchange areas but do not themselves exchange gases with the blood. The trachea branches into right and left main bronchi, which divide repeatedly into smaller bronchi and bronchioles. Their walls contain cartilage and smooth muscle that help maintain patency and regulate airflow.
Mucus and Ciliary Clearance: Cells lining the conducting airways produce mucus and have tiny hairlike structures called cilia. Mucus traps inhaled particles and microorganisms, and cilia move this mucus upward toward the throat, where it can be swallowed or cleared. This “mucociliary escalator” is a key defense against respiratory infection and particulate injury.
Terminal and Respiratory Bronchioles: At the ends of the smallest conducting bronchioles are terminal bronchioles and then respiratory bronchioles. Respiratory bronchioles have alveolar outpouchings and represent the transition from purely conducting airways to true gas-exchange regions.
Alveolar Sacs and Alveoli: Clusters of alveolar sacs sit at the ends of the airway tree. Each alveolus is a thin-walled pouch lined by a single layer of cells and closely surrounded by capillaries. The combined surface area of these alveoli is enormous relative to body size, thereby providing the lungs with a high capacity for gas exchange during rest and exercise.
Alveolar–Capillary Barrier: The wall between air in the alveolus and blood in the capillary is extremely thin. It consists of alveolar cells, a shared basement membrane, and capillary endothelial cells. Oxygen and carbon dioxide diffuse across this barrier rapidly, allowing the lungs to adjust blood gas levels with each breath.

Mechanics of Breathing

How do the lungs fill and empty with each breath?

Breathing depends on coordinated movements of the diaphragm, chest wall, and airways that change the pressure inside the chest and draw air in or expel it. The lungs are passive structures that respond to these pressure changes.

Diaphragm and Chest Wall Movement: The diaphragm is the primary muscle of inspiration. When it contracts, it moves downward, enlarging the chest cavity and lowering the pressure inside the lungs, thereby allowing air to flow in. Muscles between the ribs and the accessory muscles of the neck and chest assist during deep breaths or increased demand. When these muscles relax, the chest cavity recoils and air moves out.
Pressure Gradients and Airflow: Air flows from regions of higher pressure to regions of lower pressure. During inhalation, pressure in the alveoli becomes slightly lower than atmospheric pressure, and air moves into the lungs. During exhalation, elastic recoil of the lungs and chest wall increases alveolar pressure above atmospheric pressure, thereby expelling air.
Lung Compliance and Elastic Recoil: Lung compliance refers to the ease with which the lungs expand under pressure. Elastic fibers and structural proteins in the lung tissue, along with surface tension forces, determine how much effort is required to take a breath. Elastic recoil is the tendency of the lungs to spring back after being stretched and is essential for passive exhalation.
Surfactant and Alveolar Stability: Specialized cells in the alveoli produce surfactant, a substance that lowers surface tension. Surfactant helps prevent alveoli from collapsing during exhalation and reduces the effort required to inflate them during the next inhalation. Without adequate surfactant, breathing becomes harder and gas exchange less efficient.
Airway Resistance: The diameter of the airways, especially the medium-sized bronchi, influences how easily air flows. Smooth muscle surrounding these airways can contract or relax in response to neural signals, chemical mediators, and inflammation. Increased airway resistance, as in asthma, impairs airflow and can cause wheezing and shortness of breath.

Gas Exchange and Transport

How do the lungs exchange gases with the blood and support the rest of the body?

Gas exchange in the lungs ensures that blood leaving the lungs has enough oxygen and appropriate carbon dioxide levels. The heart and circulation then deliver this blood to all organs.

Oxygen Uptake: When air reaches the alveoli, oxygen concentration is higher in the alveoli than in the deoxygenated blood arriving through the pulmonary arteries. Oxygen diffuses across the alveolar–capillary barrier into capillary blood, where it binds to hemoglobin in red blood cells and is transported to the tissues.
Carbon Dioxide Removal: Carbon dioxide produced by body tissues is carried in the blood to the lungs. Concentration is higher in the blood entering the pulmonary capillaries than in the alveolar air, so carbon dioxide diffuses into the alveoli and is exhaled. This process helps maintain appropriate blood pH.
Matching Ventilation And Perfusion: For efficient gas exchange, the amount of air reaching each region of the lung needs to be matched with the blood flow in that region. The lungs and blood vessels adjust local airway tone and vascular tone so that poorly ventilated areas receive less blood and well-ventilated areas receive more, as far as possible.
Acid–Base Balance: Carbon dioxide is a major factor in acid–base status. By increasing or decreasing ventilation, the lungs alter the amount of carbon dioxide exhaled. This helps correct small shifts in blood pH and works in concert with the kidneys, which regulate acid-bicarbonate balance over longer periods.

Regulation and Interaction With Other Systems

How is breathing controlled and how do the lungs interact with other organs?

Breathing is regulated by centers in the brainstem that integrate signals from the body and adjust respiratory rate and depth. The lungs also interact with the heart, kidneys, immune system, and endocrine system.

Neural Control of Breathing: Networks in the brainstem generate a rhythmic pattern of inspiration and expiration. These centers receive input from sensors that monitor oxygen, carbon dioxide, and pH in the blood and cerebrospinal fluid. They also integrate signals related to emotion, pain, and voluntary control, such as speaking or singing.
Chemoreceptors and Feedback: Chemoreceptors in the carotid bodies and aortic bodies sense oxygen and carbon dioxide levels in arterial blood. When oxygen falls, or carbon dioxide rises, these receptors send signals that increase ventilation. Central chemoreceptors in the brainstem respond primarily to changes in carbon dioxide and pH.
Interaction with the Heart and Blood Vessels: The lungs and heart are tightly linked. The right ventricle pumps blood into the pulmonary circulation, and the resistance in the lung vessels influences right ventricular workload. Changes in lung volume and pressure also affect venous return and left ventricular filling, especially in disease states.
Immune and Barrier Functions: The lungs form a large surface in contact with the external environment. Immune cells, such as macrophages, patrol the airways and tissues to remove inhaled particles and microorganisms. Lymphatic vessels drain fluid and help clear debris from the lungs, and local immune responses can be rapid and intense when pathogens are detected.
Hormonal and Metabolic Roles: The lungs participate in the activation and breakdown of certain hormones and signaling molecules. For example, enzymes in the pulmonary circulation convert angiotensin I to angiotensin II, which influences blood pressure and fluid balance. The lungs also clear various mediators from the blood as it passes through the pulmonary capillaries.

Common Lung Conditions

What are some common disorders that affect the lungs?

Many conditions can alter how well the lungs move air, exchange gases, or protect against infection. Disorders can primarily affect the airways, the lung parenchyma, the pulmonary vasculature, or the pleural space. In clinical practice, several of these processes occur together, with airway disease, parenchymal injury, and vascular or thrombotic complications interacting to determine symptoms and prognosis.

Obstructive Airway Diseases: Obstructive airway diseases narrow the conducting airways, increase airflow resistance, and impair exhalation. Asthma, chronic obstructive pulmonary disease, chronic bronchitis, and emphysema are key examples. Patients typically report wheeze, exertional or resting shortness of breath, chest tightness, and cough, often with symptom variation in response to infections, allergens, cold air, or inhaled irritants.
Restrictive Lung Diseases: Restrictive processes limit the ability of the lungs and thoracic cage to expand. Causes include interstitial lung diseases such as idiopathic pulmonary fibrosis, connective tissue disease–associated fibrosis, sarcoidosis, chest wall deformities, obesity hypoventilation, neuromuscular disorders, and pleural fibrosis. People often describe rapid, shallow breathing, difficulty taking a deep breath, and reduced exercise tolerance despite a relatively normal airway examination.
Infections of the Lungs and Airways: Infectious processes such as community-acquired and hospital-acquired pneumonia, bronchitis, influenza, respiratory syncytial virus infection, and other viral or bacterial pneumonias inflame the airways and alveoli. Typical features include cough, sputum production, fever, pleuritic chest pain, and shortness of breath, with severity ranging from mild, self-limited illness to acute respiratory distress syndrome and respiratory failure requiring intensive care.
Pulmonary Vascular and Thrombotic Diseases: Pulmonary vascular disease encompasses a broad spectrum of conditions that affect blood flow through the pulmonary arteries and veins and frequently involve abnormalities in blood clotting. Acute pulmonary embolism from deep vein thrombosis is the most recognized example and occurs when thrombus from systemic veins lodges in pulmonary arteries, causing sudden dyspnea, pleuritic chest pain, tachycardia, and, in severe cases, hemodynamic collapse. Recurrent emboli or incomplete resolution of clot can lead to chronic thromboembolic pulmonary hypertension, in which organized thrombus and secondary vascular remodeling cause sustained elevation of pulmonary artery pressure, progressive exertional dyspnea, and right heart strain. In situ pulmonary artery thrombosis can occur in severe pulmonary hypertension, advanced chronic lung disease, or acute inflammatory states. Microvascular thrombosis in the pulmonary capillary bed is a recognized feature of acute respiratory distress syndrome and severe viral pneumonias, including COVID-19–associated lung injury, and contributes to refractory hypoxemia. Pulmonary vein thrombosis, although less common, may complicate thoracic surgery, atrial fibrillation ablation, or malignancy. Across this spectrum, thrombotic obstruction of pulmonary vessels impairs gas exchange, increases dead space ventilation, elevates right ventricular afterload, and raises the risk of right ventricular failure and sudden deterioration.
Pulmonary Hypertension and Other Pulmonary Vascular Disorders: Pulmonary hypertension represents a group of conditions characterized by chronically elevated pressure in the pulmonary arterial circulation. Etiologies include idiopathic and heritable pulmonary arterial hypertension, chronic thromboembolic pulmonary hypertension, pulmonary hypertension associated with connective tissue disease, congenital heart disease, portal hypertension, chronic lung disease, and long-standing left heart disease. Symptoms typically include exertional dyspnea, fatigue, chest discomfort, syncope, and signs of right heart failure such as peripheral edema and abdominal distension. Additional pulmonary vascular entities include pulmonary veno-occlusive disease and pulmonary capillary hemangiomatosis, which predominantly affect postcapillary vessels and can present with severe hypoxemia and rapid clinical decline.
Pulmonary Hemorrhagic and Vasculitic Disorders: Diffuse alveolar hemorrhage and pulmonary involvement in systemic vasculitis represent critical vascular complications within the lung. Small-vessel vasculitides such as granulomatosis with polyangiitis, microscopic polyangiitis, eosinophilic granulomatosis with polyangiitis, anti–glomerular basement membrane disease, and Behçet disease can inflame and damage pulmonary capillaries and arterioles, leading to hemoptysis, anemia, new infiltrates on imaging, and hypoxemic respiratory failure. Pulmonary capillaritis may present with subtle initial symptoms that then progress abruptly, and it often coexists with renal involvement and systemic features of vasculitis. Prompt recognition of these vascular inflammatory processes is essential because they require immunosuppressive and supportive therapy rather than standard antibacterial treatment alone.
Pulmonary Artery Aneurysms, Malformations, and Structural Vascular Lesions: Pulmonary artery aneurysms, pseudoaneurysms, and arteriovenous malformations are less common but clinically important structural vascular abnormalities. Pulmonary artery aneurysms can arise in the setting of connective tissue disease, vasculitis, congenital heart disease, or severe pulmonary hypertension and may present with dyspnea, chest pain, or life-threatening hemoptysis if they rupture. Pulmonary arteriovenous malformations create direct connections between pulmonary arteries and veins, bypassing the capillary bed, and can lead to hypoxemia, paradoxical emboli, and stroke. These lesions highlight that the pulmonary circulation can be affected by both thrombotic and nonthrombotic structural pathology, each with distinct diagnostic and therapeutic implications.
Pleural Diseases: Pleural disorders affect the thin membranes surrounding the lungs and can significantly impact lung mechanics even when the lung tissue itself is relatively normal. Pleural effusion is an accumulation of fluid in the pleural space that can result from heart failure, infection, malignancy, pulmonary embolism, or systemic inflammatory disease and often causes dyspnea and pleuritic pain. Pneumothorax is air in the pleural space and can be spontaneous, traumatic, or iatrogenic; it may present with sudden chest pain and breathlessness and can progress to tension pneumothorax if not treated. Chronic pleural thickening or fibrothorax restricts lung expansion and contributes to a restrictive ventilatory pattern.
Lung Cancer and Other Structural Lesions: Primary lung cancers, metastatic lesions to the lung, and benign tumors such as hamartomas can arise from airway epithelium, alveolar structures, or supporting tissue. These lesions may narrow or obstruct bronchi, invade adjacent structures, and shed tumor emboli into the pulmonary vasculature. Early-stage disease may be asymptomatic and detected incidentally, whereas advanced disease commonly causes chronic cough, hemoptysis, weight loss, chest pain, progressive dyspnea, and paraneoplastic syndromes. Endobronchial lesions, airway stenosis, and post-obstructive pneumonias frequently coexist and further compromise lung function.

Diagnosis and Testing

How do clinicians evaluate lung structure and function?

Assessment of the lungs combines information from symptoms, physical examination, imaging, and tests that measure airflow and gas exchange. The choice of tests depends on the suspected problem.

Clinical Evaluation and Physical Examination: Clinicians begin by asking about cough, sputum, wheezing, shortness of breath, chest pain, exercise tolerance, and environmental or occupational exposures. During the physical exam, they observe breathing patterns, listen to breath sounds, and assess for signs of respiratory distress or chronic lung disease.
Chest X-Ray: A chest X-ray is often the first imaging test. It provides a general overview of lung fields, heart size, and the pleural space. It can suggest pneumonia, fluid accumulation, collapsed lung areas, mass lesions, and certain patterns of chronic disease.
Computed Tomography of the Chest: Computed tomography offers detailed cross-sectional images of the lungs and surrounding structures. It can identify small nodules, subtle areas of scarring, airway abnormalities, and pulmonary emboli when used with contrast. High-resolution techniques help characterize interstitial lung disease.
Pulmonary Function Tests: Pulmonary function tests measure how much air the lungs can hold, how quickly air can be moved, and how well gases transfer across the alveolar–capillary barrier. Spirometry evaluates airflow, lung volumes describe overall size and compliance, and diffusion capacity reflects gas transfer efficiency.
Arterial Blood Gases and Oximetry: Arterial blood gas analysis measures oxygen, carbon dioxide, and pH directly in arterial blood. Pulse oximetry estimates oxygen saturation noninvasively through the skin. These tests help determine whether gas exchange is adequate and whether ventilation matches metabolic demand.
Bronchoscopy and Airway Sampling: Bronchoscopy involves inserting a flexible endoscope into the airways to visualize the trachea and bronchi, collect samples, remove mucus plugs or foreign bodies, and, in some cases, perform targeted treatments. Tissue or fluid obtained during bronchoscopy can guide the diagnosis of infection, cancer, or inflammatory disease.

Protection and Prevention

How can people support healthy lungs and reduce the risk of lung disease?

Although some lung diseases arise from inherited or immutable factors, many risks can be reduced through long-term lifestyle choices and environmental protection.

Avoiding Tobacco and Inhaled Toxins: Avoiding cigarette smoking and secondhand smoke is one of the most important steps for lung health. Reducing exposure to occupational dusts, fumes, and other airborne irritants through ventilation, protective equipment, and regulatory controls further protects the lungs.
Vaccination and Infection Prevention: Vaccines against influenza, pneumococcal disease, and other respiratory infections reduce the risk of severe pneumonia and complications. Hand hygiene, appropriate mask use in high-risk settings, and staying away from others when ill help limit transmission of respiratory pathogens.
Indoor and Outdoor Air Quality: Good ventilation, reducing indoor pollutants, controlling mold and humidity, and using cooking fuels safely support healthier air in the home. Awareness of outdoor air quality alerts can inform activity levels on days with elevated particulate or ozone levels, particularly for individuals with existing lung conditions.
Physical Activity and Respiratory Conditioning: Regular physical activity supports lung and heart function, improves endurance, and can reduce breathlessness during everyday tasks. Pulmonary rehabilitation programs provide structured exercise and education for individuals with chronic lung disease.
Early Attention to Symptoms: Persistent cough, unexplained breathlessness, wheezing, coughing up blood, or chest discomfort should not be ignored. Early evaluation supports prompt diagnosis and treatment, thereby slowing disease progression and improving outcomes.

Living With Lung Disease

What should people know if they have a lung condition or are at increased risk?

Many individuals live for years with stable lung disease when they understand their condition, follow treatment plans, and respond promptly to changes. Daily management focuses on maintaining function and preventing exacerbations.

Understanding The Specific Diagnosis and Triggers: Each lung disease has characteristic patterns, triggers, and treatment options. Knowing whether the primary problem involves airflow obstruction, restriction, vascular issues, or infection helps people recognize early warning signs and avoid known triggers such as smoke, allergens, or specific activities.
Medication Use and Inhaler Technique: Many lung conditions require inhaled medications, such as bronchodilators and inhaled corticosteroids, as well as oral or injectable therapies. Correct inhaler technique is essential for medications to reach the airways. Regular review of technique and adherence to the care plan by the care team improves symptom control.
Monitoring Symptoms and Exacerbations: Changes in baseline cough, sputum color or volume, wheezing, breathlessness, or exercise tolerance can signal an exacerbation. Early contact with clinicians often allows adjustment of therapy before severe deterioration occurs and may prevent hospitalization.
Oxygen Therapy and Devices: Some individuals with advanced lung disease require supplemental oxygen to maintain adequate blood oxygen saturation, particularly during exertion or sleep. Oxygen systems are prescribed and monitored carefully, and education on safe use is crucial.
Coordination With The Care Team: Pulmonologists, primary care clinicians, respiratory therapists, nurses, and rehabilitation specialists all contribute to long-term management. Education, action plans, and follow-up schedules help align day-to-day choices with treatment goals and provide support when the disease course changes.

By understanding how the lungs are built, how they function, and what can impair them, individuals and clinicians can work together to maintain lung health, respond early to problems, and limit the impact of lung disease on daily life.

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