What Are Lungs?

Lungs are the paired primary organs of the respiratory system in mammals, responsible for facilitating the exchange of oxygen and carbon dioxide between inhaled air and the circulating blood. Located within the thoracic cavity on either side of the mediastinum, the two lungs together form a gas exchange interface with a combined alveolar surface area estimated at 70 to 140 square meters in a healthy adult, housed within a volume roughly the size of a soccer ball. Their structural design balances two competing engineering requirements: minimizing the diffusion distance between air and blood to maximize gas exchange efficiency, and providing a mechanically compliant structure that can inflate and deflate approximately 15 times per minute for an entire lifetime.

The two lungs are asymmetric. The right lung is larger, heavier, and divided into three lobes: superior, middle, and inferior, separated by the horizontal and oblique fissures. The left lung has two lobes, superior and inferior, with the cardiac notch on its mediastinal surface accommodating the position of the heart. Each lobe is subdivided into bronchopulmonary segments, each supplied by its own segmental bronchus and a separate arterial branch from the pulmonary circulation, a feature that allows surgical resection of individual segments without compromising adjacent lung tissue.

Structural Organization

The airway tree within the lungs follows a hierarchical branching pattern, with the main bronchi entering at the hilum and dividing approximately 23 times before reaching the alveolar sacs. The first 16 generations constitute the conducting zone, lined with ciliated epithelium and mucus-secreting cells that filter and humidify inhaled air. The remaining generations form the respiratory zone, where the alveolar walls become progressively thinner and gas exchange occurs across the blood-gas barrier, a membrane less than 0.5 micrometers thick consisting of the alveolar epithelium, the shared basement membrane, and the capillary endothelium. The NCBI Bookshelf anatomy reference for the thorax and lungs provides a detailed anatomical description of the bronchovascular segments and their surgical significance.

Pulmonary Vasculature and Pleura

The lungs receive two distinct blood supplies. The pulmonary circulation carries deoxygenated blood from the right ventricle via the pulmonary arteries; this blood is oxygenated and returned to the left atrium via the pulmonary veins, completing the gas exchange circuit. The bronchial circulation, a branch of the systemic aorta, supplies oxygenated blood to the conducting airways, pleura, and lung parenchyma itself. The pleura is a serous membrane enclosing each lung in two layers, the visceral pleura adhering to the lung surface and the parietal pleura lining the thoracic wall; the pleural space between them is maintained at slightly negative pressure, which keeps the lungs expanded against the chest wall. The NHLBI overview of how the lungs and respiratory system work describes the interplay between the pulmonary vasculature, the airways, and respiratory mechanics.

Surfactant and Mechanical Properties

The alveolar surface is coated with pulmonary surfactant, a mixture of phospholipids and surfactant-associated proteins secreted by type II alveolar epithelial cells. Surfactant reduces surface tension at the air-liquid interface within the alveoli, preventing alveolar collapse during exhalation and equalizing the work of inflation across alveoli of different sizes. Without surfactant, as in neonatal respiratory distress syndrome in premature infants, alveoli collapse and cannot be re-expanded by normal respiratory effort. Research on lung anatomy and the cellular biology of the alveolar epithelium from the National Cancer Institute's SEER program covers the tissue-level anatomy relevant to pathological staging of lung tumors.

Applications

Knowledge of lung anatomy and physiology has applications in a wide range of engineering and clinical domains, including:

• Design of mechanical ventilators and anesthesia delivery systems
• Pulmonary imaging using CT, MRI, and ultrasound for diagnosis and screening
• Inhaled drug delivery and aerosol therapy device design
• Surgical planning for lobectomy, segmentectomy, and lung transplantation
• Tissue engineering and ex vivo lung perfusion for transplantable organ preparation