Lungs

The lungs are essential respiratory organs responsible for the exchange of gases between the atmosphere and the bloodstream in humans and many other animals. Situated within the thoracic cavity, they enable oxygen uptake and carbon dioxide elimination, forming a central component of the respiratory system. Their structure and function are intricately adapted to support efficient respiration, vocalisation, and metabolic demands across species. In humans, the lungs work in coordination with the diaphragm, rib cage, and airway network to sustain life through continuous and regulated breathing.

Evolutionary and Biological Background

The development of lungs represents a significant evolutionary adaptation among vertebrates, facilitating respiration in terrestrial environments. Early tetrapods relied on buccal pumping, a mechanism still observable in amphibians, which uses pharyngeal and buccal muscles to push air into the lungs. In contrast, mammals, reptiles, and birds evolved musculoskeletal systems that actively expand and contract the thoracic cavity to drive breathing.
In humans, the thoracic diaphragm is the principal respiratory muscle. Its rhythmic contractions alter thoracic pressure, allowing inhalation and exhalation. This mechanism enhances efficiency, supporting increased metabolic activity. Beyond respiration, the airflow generated by the lungs is central to sound production and speech in humans, as vocalisation depends on the controlled movement of air through the larynx.

Position and General Structure

The lungs occupy the thoracic cavity, lying on either side of the heart and protected by the rib cage. Conical in shape, each lung features a narrow apex extending into the root of the neck and a broad base resting on the diaphragm. The lungs extend posteriorly close to the vertebral column and anteriorly towards the sternum, filling most of the chest cavity.
The right lung is larger and heavier than the left, typically reflecting the asymmetrical placement of the heart. The combined weight of both lungs in adults is approximately 1.3 kilograms. The left lung is smaller due to the presence of the cardiac notch, an indentation accommodating the heart.
Each lung is enveloped in a pleural sac composed of two pleurae: the outer parietal pleura lining the rib cage and the inner visceral pleura covering the lung surface. Between these layers lies the pleural cavity, containing lubricating fluid that reduces friction during breathing.

Airway Organisation and Gas Exchange

Air enters the lungs through the lower respiratory tract, beginning with the trachea. The trachea divides into two primary bronchi, each entering a lung at the hilum. Within the lungs, the bronchi branch into smaller bronchioles through a series of divisions forming the conducting zone. Ultimately, air reaches the pulmonary alveoli, microscopic air sacs that serve as the primary sites of gas exchange.
The lungs contain approximately 2,400 kilometres of airways and between 300 and 500 million alveoli. These alveoli are richly supplied with capillaries, allowing oxygen to diffuse into the bloodstream while carbon dioxide diffuses out. This exchange sustains cellular respiration throughout the body.

Lobes, Fissures, and Segmental Anatomy

The lungs are divided into lobes, separated by fissures formed by invaginations of the visceral pleura during prenatal development. The right lung comprises three lobes—superior, middle, and inferior—divided by the horizontal and oblique fissures. The left lung has two lobes—superior and inferior—separated by an oblique fissure. The left lung also features the lingula, a tongue-like projection of the superior lobe, which is homologous to the middle lobe of the right lung.
Lobes are further divided into bronchopulmonary segments, each supplied by a segmental bronchus and accompanying artery. These segments represent discrete anatomical units, allowing for targeted surgical removal without compromising adjacent tissue. Clinically, segmental anatomy is essential for localising respiratory diseases such as infections, tumours, or obstructions.
Variations in fissures, such as incomplete formation or accessory fissures like the azygos fissure, are common. These anatomical differences can influence ventilation and may be relevant during procedures such as lung volume reduction surgery.

Mediastinal Relations and Surface Markings

Both lungs present specific impressions on their medial surfaces caused by contact with surrounding structures. On the right lung, the cardiac impression accommodates the heart. Superior to the hilum lies a groove for the azygos vein, and above this, a wider impression for the superior vena cava. Grooves for the esophagus, pulmonary vessels, and brachiocephalic vessels are also present.
The left lung has a larger and deeper cardiac impression due to the heart’s leftward orientation. A distinct groove for the aorta arches over the left lung, with additional impressions for the pulmonary artery and descending aorta.
The hilum on each lung serves as the entry and exit point for the bronchi, pulmonary vessels, lymphatics, and nerves. Together, these structures form the root of the lung. Tracheobronchial lymph nodes, situated near the hilum, play a vital role in immune defence.

Blood Supply and Circulation

The lungs possess a dual blood supply. Deoxygenated blood arrives via the pulmonary arteries from the right ventricle. Once oxygenated in the alveolar capillaries, it returns to the left atrium through the pulmonary veins. This pulmonary circulation ensures effective gas exchange.
In addition, the bronchial circulation delivers oxygenated blood from the systemic circuit to support lung tissue metabolism. This dual supply ensures that lung tissues receive both nutrient-rich and functional blood flow necessary for their operation.

Embryonic Development and Adaptation to Air Breathing

Lung development begins in early embryogenesis as an outpouching of the foregut, which later forms the upper digestive tract. As the foetus grows, the lungs develop within the fluid-filled amniotic sac and do not participate in gas exchange. Instead, oxygen is supplied through placental circulation, with blood bypassing the non-functional lungs via the ductus arteriosus.
At birth, exposure to air triggers the infant’s first breaths, initiating pulmonary ventilation. The ductus arteriosus closes, redirecting blood flow into the lungs. Full development of lung structure and function continues into early childhood.

Diseases and Medical Terminology

The lungs may be affected by a range of diseases. Infectious conditions such as pneumonia involve inflammation of alveoli, while chronic obstructive pulmonary disease and emphysema are strongly associated with smoking or environmental exposure to harmful substances. Bronchitis affects the bronchial tubes and may present as acute or chronic. Lung cancer remains one of the leading causes of cancer-related deaths.
In medical terminology, prefixes such as pulmo- (from Latin pulmonarius) and pneumo- (from Greek for “lung”) are commonly used in clinical contexts, appearing in terms such as pulmonology and pneumonia.