Respiratory System: Lung Anatomy

• The respiratory system's primary function is gas exchange, providing oxygen to the blood and removing carbon dioxide
• The lungs are the main organs of the respiratory system, responsible for this gas exchange

Lung Anatomy: Overview

• The lungs are located in the thoracic cavity, enclosed and protected by the rib cage, vertebral column, and sternum
• The diaphragm forms the floor of the thoracic cavity and is a major muscle of respiration
• The mediastinum, the space between the lungs, contains the heart, great vessels, trachea, esophagus, and other structures.

External Lung Anatomy

• The lungs are cone-shaped, with the apex extending superiorly above the clavicle and the base resting on the diaphragm.
• Each lung has a costal surface lying against the ribs, and a mediastinal surface facing the mediastinum
• The hilum is an indented area on the mediastinal surface where the bronchi, blood vessels, lymphatic vessels, and nerves enter and exit the lung
• The root of the lung refers to the structures that enter and exit at the hilum.

Lobes and Fissures

• The right lung is larger and has three lobes: superior, middle, and inferior.
• Two fissures separate these lobes: the horizontal fissure (separates superior and middle lobes) and the oblique fissure (separates middle and inferior lobes).
• The left lung has two lobes: superior and inferior.
• One fissure separates these lobes: the oblique fissure.
• The left lung also has a cardiac notch, an indentation on the mediastinal surface to accommodate the heart

Bronchial Tree

• The trachea (windpipe) descends from the larynx and divides into the right and left main (primary) bronchi
• Each main bronchus enters the lung at the hilum and branches into lobar (secondary) bronchi, each supplying a lobe of the lung (three on the right, two on the left)
• The lobar bronchi further divide into segmental (tertiary) bronchi, each supplying a bronchopulmonary segment
• Bronchopulmonary segments are functionally independent units of the lung tissue
• Segmental bronchi branch into smaller and smaller bronchi, eventually leading to bronchioles
• Bronchioles are small air passageways (less than 1 mm in diameter) that do not contain cartilage
• The terminal bronchioles are the final branches of the conducting zone, which only serves to conduct air
• Terminal bronchioles lead to respiratory bronchioles, which have alveoli budding from their walls and mark the beginning of the respiratory zone (where gas exchange occurs).

Alveoli

• Alveoli are tiny, sac-like structures that are the primary sites of gas exchange in the lungs
• Alveolar walls consist of a single layer of squamous epithelial cells (type I alveolar cells), surrounded by a network of capillaries
• Type II alveolar cells secrete surfactant, a substance that reduces surface tension in the alveoli and prevents their collapse
• Alveolar macrophages (dust cells) are phagocytic cells that patrol the alveoli and remove debris and pathogens.
• The respiratory membrane is the air-blood barrier formed by the alveolar wall and the capillary wall.

Blood Supply

• The lungs receive blood supply from both the pulmonary and systemic circulations
• The pulmonary arteries carry deoxygenated blood from the right ventricle to the lungs for oxygenation
• The pulmonary veins carry oxygenated blood from the lungs back to the left atrium of the heart
• The bronchial arteries, branches of the aorta, provide oxygenated blood to the lung tissue itself (except the alveoli, which are supplied by the pulmonary circulation)
• Bronchial veins drain some of the blood supplied by the bronchial arteries, but some blood also drains into the pulmonary veins.

Innervation

• The lungs are innervated by the autonomic nervous system (both sympathetic and parasympathetic fibers)
• Parasympathetic stimulation (from the vagus nerve) causes bronchoconstriction, increased mucus secretion, and vasodilation of pulmonary vessels.
• Sympathetic stimulation causes bronchodilation and vasoconstriction of pulmonary vessels.

Pleurae

• The lungs are enclosed by a double-layered serous membrane called the pleura
• The visceral pleura covers the surface of the lung
• The parietal pleura lines the thoracic cavity
• The pleural cavity is the space between the visceral and parietal pleurae, filled with a small amount of pleural fluid that reduces friction during breathing.
• The pleurae help to compartmentalize the lungs, preventing the spread of infection and allowing each lung to operate independently

Lymphatic Drainage

• The lungs have a rich network of lymphatic vessels that drain lymph fluid and help to remove pathogens and debris
• Lymphatic vessels follow the airways and blood vessels, draining into lymph nodes located in the hilum and mediastinum.