What Is Lung?

The lung is the paired organ of the respiratory system responsible for gas exchange between the atmosphere and the bloodstream, and it is a primary subject of study in pulmonology, physiology, and biomedical engineering. Each lung consists of a hierarchically branching airway tree that conducts air to progressively smaller structures, culminating in hundreds of millions of alveoli, the thin-walled gas-exchange sacs where oxygen and carbon dioxide diffuse across the blood-gas barrier. In the engineering sciences, the lung is studied as a mechanical and transport system: its structural properties determine breathing mechanics, and quantitative models of pressure, flow, and volume relationships underpin the design of diagnostic instruments, ventilators, and respiratory therapeutics.

The lung's architecture is divided into two functional zones. The conducting zone, comprising airway generations 1 through 16 from the trachea down to the terminal bronchioles, conveys inhaled air without participating in gas exchange. The respiratory zone, beginning at generation 17 with the respiratory bronchioles and extending to the alveolar ducts and alveolar sacs, performs the actual exchange of oxygen and carbon dioxide between air and pulmonary capillary blood. The combined alveolar surface area in a healthy adult is approximately 70 to 140 square meters.

Pulmonary Mechanics

Lung mechanics describes the relationships among gas pressures, flows, and volumes during breathing. The key mechanical parameters are compliance, the ratio of volume change to pressure change, and resistance, the ratio of pressure drop to flow rate in the airways. Compliance reflects the elastic properties of the alveolar walls and the surface tension at the air-liquid interface, which is regulated by pulmonary surfactant. Resistance depends on airway caliber and is elevated in obstructive diseases such as asthma and chronic obstructive pulmonary disease (COPD). Spirometry, body plethysmography, and impulse oscillometry are the principal clinical tools for measuring these mechanical parameters. A detailed treatment of pulmonary compliance physiology by the NCBI Bookshelf covers how compliance is measured and how it changes in restrictive and obstructive disease.

Computational and Biomedical Engineering Models

Engineering analysis of lung function treats the organ as a system subject to identification: measurements of pressure and flow at accessible sites are interpreted through mathematical models to infer internal mechanical properties. Finite-element and multiscale computational models reconstruct the lung's geometry from CT imaging data and simulate both large-airway fluid dynamics and alveolar mechanics. These models support the design of patient-specific ventilator settings and predict regional deposition of inhaled therapeutics. A review of computational lung modeling in respiratory medicine surveys the range of approaches from one-dimensional airway network models to full three-dimensional fluid-structure interaction simulations. The bioengineering and clinical translation of human lung tissue is an active research area focused on producing functional lung tissue constructs for transplantation and in vitro drug testing.

Measurement and Imaging

Quantitative measurement of regional lung function has advanced with imaging modalities beyond conventional spirometry. Electrical impedance tomography (EIT) generates cross-sectional maps of ventilation distribution at the bedside using electrodes placed around the thorax and small alternating currents, without ionizing radiation. Hyperpolarized gas MRI using helium-3 or xenon-129 visualizes airspace morphometry and regional ventilation at high spatial resolution. CT-derived airway segmentation and density analysis provides quantitative phenotyping for COPD and fibrosis research. These measurement techniques connect structural anatomy directly to the mechanical and transport functions of the lung.

Applications

The lung as a subject of study and engineering has applications in a wide range of domains, including:

• Mechanical ventilation and respiratory support design in critical care
• Pulmonary drug delivery and inhaled aerosol targeting
• Diagnosis and monitoring of pulmonary diseases such as asthma, COPD, and fibrosis
• Lung transplantation and bioengineered tissue research
• Occupational health assessment for airborne exposure and particulate inhalation