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These photographs will help you review the basic structures of the respiratory system. The trachea is the airway into the thoracic cavity. The bronchi (sing: bronchus) branch from the trachea and each leads into one of the two lungs, where they subdivide to become a series of bronchioles. Bronchioles, in turn, become alveolar_ducts, alveolar_sacs, and alveoli (sing: alveolus).

The diaphragm is a band of skeletal muscle. Its cycle of contraction and relaxation changes the pressure in the thoracic cavity, thereby facilitating breathing.

The cat's lungs have several lobes. The right lung has four lobes: anterior, middle, posterior, and mediastinal. The left lung has only three lobes: anterior, middle and posterior.

The trachea is supported by a series of cartilages. It lies below the larynx, or voicebox. The larynx is supported by nine cartilages. The largest and most superior is the thyroid cartilage. There is a projection from the thyroid cartilage's anterior medial surface called laryngeal_prominence, or "Adam's apple". Below the thyroid cartilage is the cricoid cartilage. All laryngeal cartilages are composed of hyaline cartilage except the epiglottic_cartilage (epiglottis), which is composed of elastic cartilage.

The trachea extends below the larynx to the level of the discs between the 4th and 5th thoracic vertebrae, where it divides to become the bronchi. The trachea is lined with ciliated pseudostratified columar epithelium. Goblet_cells in this epithelial layer produce mucus that helps rid the organ of unwanted debris.

The walls of the trachea are supported by a series of C-shaped cartilages. The open part of each "c" faces the esophagus, which is located posteriorly to the trachea. The "incomplete" shape of these cartilages allows expansion of the esophagus, which facilitates swallowing.

Trachea Larynx Thyroid cartilage Cricoid cartilage Trachea Left common carotid artery