Lungs

The respiratory system organ located in the thoracic cavity and occupying a large part of it. The lungs are separated from each other by the mediastinum. Each lung is enveloped by the visceral pleura. The lungs are connected to the trachea by bronchi. The lung tissue consists of bronchi and respiratory tissue. Each lung lobe receives a secondary bronchus. In the lobe, tertiary bronchi branch out and form lung segments. Bronchi become smaller, their diameter decreases, but the total diameter increases, so the air flow velocity in them is lower than in larger bronchi.

The bronchi of a bronchial segment branch into bronchi of various diameters until the segmental bronchiole transitions into bronchioles. Each lung segment contains 30–60 bronchioles. In the lung bronchiole, the terminal bronchiole transitions into 1–3 respiratory bronchioles, and these transition into 2–11 alveolar ducts, from which alveolar sacs and alveoli emerge. Alveoli are open sac-shaped structures, 0.1–0.2 mm in diameter. The alveoli in the human adult lungs are separated by a very thin partition. There are approximately 300 million alveoli in the lungs of an adult, forming about 90 m² of respiratory surface area. The alveolar wall is composed of a single-layered flat epithelial and connective tissue with capillaries.

Source | Glossary of Most Commonly Used Biomedical Terms and Concepts | Lithuanian University of Health Sciences | Academician Professor Antanas Praškevičius, Professor Laima Ivanovienė