Lung Anatomy

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The lungs are composed of three main components, the bronchial tubes, the pulmonary arteries and veins, and the pulmonary parenchyma.

The respiratory system is arranged basically as an upside down tree. The trachea is the trunk of the tree and conducts air from the mouth and nostrils through the larynx or voicebox into the trachea and bronchi into the lungs. In the center of the chest, the trachea divides at the carina into two branches, the main bronchi. Each bronchus then further branches at the hilum giving off first lobar and then segmental bronchi (the limbs of the tree). There are three lobes and ten segments on the right side and two lobes with eight segments on the left.

In similar fashion, the main blood inflow is from the pulmonary artery which sends unoxygenated (dark red) blood from the right side of the heart into the lungs. In front of the carina, the main pulmonary artery divides into the right and left main pulmonary arteries, that further branch to supply arteries to each of the lobes and segments of the lung.

Blood flowing through these arteries reaches the alveoli of the lungs where the red blood cells are in close contact with air, separated only by a thin membrane of alveolar lining cells and endothelial cells of the capillaries. Oxygen is picked up and carbon dioxide discarded.

The oxygenated (bright red) blood returns to the left atrium, the collecting chamber of the left side of the heart via the pulmonary veins, two on each side.

Lymphatic vessels drain from each of the segments of the lung into lymph nodes in the hilum of the lung, N1 or hilar nodes. Lymphatic vessels then exit these nodes and pass upward into the mediastinum, a central space in the chest surrounding the trachea and major blood vessels. In the mediastinum, the lymphatic vessels empty into N2 or mediastinal lymph nodes. From there further lymphatics pass on to N3 nodes in the neck.

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