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Respiratory System

The respiratory system is the interface between the organism and the environment that provides the venue for gas exchange to occur. Not only does the respiratory system serves as the gas exchange surface, but it also humidifies, warms or cools, and cleans the air that enters the respiratory tract. Two major divisions of the respiratory system exist: the conducting portion and the respiratory portion. The former is responsible for conduction of gases in and out of the respiratory portion, which is the true exchange surface.

Please read before proceeding: Click on one of the images or titles below for a larger image and begin the tutorial. Within the tutorial, click on either the legends or the labels for descriptions and functions of the structure.

	Trachea 1: The trachea is the main conducting portion of the respiratory system. It is lined with pseudostratified ciliated columnar epithelium. The dense adventitia underneath the submucosa contains rings of C-shaped hyaline cartilage that continue further down the respiratory tract to the primary bronchi. Flexible yet strong, the cartilage protects the air passageway from injury and collapse.
	Trachea 2: The trachea is the main conducting portion of the respiratory system. It is lined with pseudostratified ciliated columnar epithelium. Notice the large goblet cells. These cells secrete mucous which serves to trap foreign matter.
	Trachea 3: The trachea is the main conducting portion of the respiratory system. It is lined with pseudostratified ciliated columnar epithelium. Notice the cilia that project into the lumen. These cilia beat in synchrony to remove debris trapped by the mucous.
	Trachea (TEM): The electron micrograph shows very clearly two goblet cells containing the mucous granules. Note that the labeled cell, on the left, is releasing mucous into the lumen of the trachea. Also notice the cilia on top of the epithelium.
	Bronchus: The trachea branches into two primary brochi. Like the trachea, the primary bronchi are also lined with rings of hyaline cartilage for support.
	Bronchiole: The bronchi branch into successively smaller bronchioles. This cross section through a bronchiole shows simple columnar epithelium lining the lumen, as well as smooth muscle in the lamina propria. As the diameter of the lumen decreases from the trachea to the alveoli, the amount of cartilage is reduced, the height of the epithelial cells is reduced, and the degree of ciliation and the present of goblet cells are decreased. Compare this picture of the bronchiole with the trachea and the bronchus.
	Respiratory Bronchiole: The terminal bronchioles branch into the respiratory bronchioles which terminate into the alveolar ducts which are shown in the section of this lung parenchyma.
	The Alveoli: This section shows lung parenchyma with alveolar ducts and numerous alveoli.
	Alveoli with Macrophages (Dust Cells): The alveoli are the true respiratory surface of the lungs. The very thin walls of the alveoli allow gases to be readily exchanged. Their arrangement in numerous pockets serves to maximize surface area for gas exchange. The walls of the alveoli are composed of two types of cells: Type I and Type II pneumocytes.
	Pneumocyte II: Sixty percent of the alveolar cells are Type II alveolar cells which specialize in the production of surfactant.

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