The development of the lungs begins in the embryonic period and continues until approximately 8 years of age. The left lung shares its space with the heart, which it accommodates in the cardiac notch. The right lung consists of 3 lobes (upper, middle, lower), while the left lung consists of 2 lobes (upper, lower) and the lingula, a structure that is homologous to the middle lobe of the right lung. Gas exchange takes place in the alveoli of the lungs. Hyaline cartilage in the form of C-shaped rings ( trachea) and plates ( bronchi) provides structural protection and integrity.
The entire respiratory tract down to the bronchioles is covered in ciliated epithelium, which provides immunologic protection by helping clear the airways of dust and microorganisms. The respiratory system is furthermore divided into an upper tract (structures from the larynx upwards) and a lower tract (structures below the larynx). The conducting zone is composed of nonrespiratory tissue and provides the passage for ventilation of the respiratory zone, where the O 2 and CO 2 exchange takes place. The respiratory system consists of a conducting zone ( anatomic dead space i.e., the airways of the mouth, nose, pharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles) and a respiratory zone ( lung parenchyma i.e., respiratory bronchioles, alveolar ducts, alveolar sacs).
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