Lung

A lung is a paired respiratory organ that exchanges oxygen and carbon dioxide between air and blood. Its branching airways end in alveoli surrounded by capillaries.

Bronchial tree: The branching network of airways that carries air from the trachea into the lungs. Its progressively smaller branches distribute inhaled air throughout each lung.
Respiratory epithelium: The specialized lining of much of the respiratory tract, including ciliated and mucus-secreting cells. Its cilia and mucus help clear inhaled particles before they reach delicate lung regions.
Spirometry: A lung-function test that measures the volume and flow of air during breathing maneuvers. It helps assess airflow limitation and changes in lung capacity.
Gills: Respiratory organs that extract dissolved oxygen from water, especially in fish and many aquatic animals. Gills exchange gases with flowing water rather than air in alveoli.
Alveolus: A microscopic air sac in the lung where gas exchange occurs across a thin wall. Alveoli provide the large, delicate surfaces where oxygen and carbon dioxide cross.
Pulmonary capillary: A tiny blood vessel that runs along alveoli and participates in gas exchange. Its thin wall places blood close to alveolar air.
Chronic obstructive pulmonary disease: A progressive lung disease characterized by persistent airflow obstruction, commonly including emphysema or chronic bronchitis. It damages airways or alveoli and makes breathing out difficult.
Tracheal system: A network of air-filled tubes that delivers oxygen directly to tissues in insects and some other arthropods. Unlike lungs, it can transport air to tissues without first oxygenating blood.
Pulmonary circulation: The blood flow from the heart to the lungs and back to the heart. It brings carbon-dioxide-rich blood to lung capillaries and returns oxygenated blood.
Surfactant: A mixture of lipids and proteins that reduces surface tension at the alveolar air–liquid interface. It helps prevent small alveoli from collapsing during exhalation.

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