What is the significance of the respiratory membrane

The oropharynx contains two distinct sets of tonsils, the palatine and lingual tonsils. A palatine tonsil is one of a pair of structures located laterally in the oropharynx in the area of the fauces.

The lingual tonsil is located at the base of the tongue. Similar to the pharyngeal tonsil, the palatine and lingual tonsils are composed of lymphoid tissue, and trap and destroy pathogens entering the body through the oral or nasal cavities. The laryngopharynx is inferior to the oropharynx and posterior to the larynx. It continues the route for ingested material and air until its inferior end, where the digestive and respiratory systems diverge.

The stratified squamous epithelium of the oropharynx is continuous with the laryngopharynx. Anteriorly, the laryngopharynx opens into the larynx, whereas posteriorly, it enters the esophagus. The larynx is a cartilaginous structure inferior to the laryngopharynx that connects the pharynx to the trachea and helps regulate the volume of air that enters and leaves the lungs.

The structure of the larynx is formed by several pieces of cartilage. Three large cartilage pieces—the thyroid cartilage anterior , epiglottis superior , and cricoid cartilage inferior —form the major structure of the larynx. The thyroid cartilage is the largest piece of cartilage that makes up the larynx. The thick cricoid cartilage forms a ring, with a wide posterior region and a thinner anterior region.

Three smaller, paired cartilages—the arytenoids, corniculates, and cuneiforms—attach to the epiglottis and the vocal cords and muscle that help move the vocal cords to produce speech. Figure 6. The larynx extends from the laryngopharynx and the hyoid bone to the trachea. Figure 7.

The true vocal cords and vestibular folds of the larynx are viewed inferiorly from the laryngopharynx. The epiglottis , attached to the thyroid cartilage, is a very flexible piece of elastic cartilage that covers the opening of the trachea. The glottis is composed of the vestibular folds, the true vocal cords, and the space between these folds. A vestibular fold, or false vocal cord, is one of a pair of folded sections of mucous membrane. A true vocal cord is one of the white, membranous folds attached by muscle to the thyroid and arytenoid cartilages of the larynx on their outer edges.

The inner edges of the true vocal cords are free, allowing oscillation to produce sound. The size of the membranous folds of the true vocal cords differs between individuals, producing voices with different pitch ranges. Folds in males tend to be larger than those in females, which create a deeper voice. The act of swallowing causes the pharynx and larynx to lift upward, allowing the pharynx to expand and the epiglottis of the larynx to swing downward, closing the opening to the trachea.

These movements produce a larger area for food to pass through, while preventing food and beverages from entering the trachea. Continuous with the laryngopharynx, the superior portion of the larynx is lined with stratified squamous epithelium, transitioning into pseudostratified ciliated columnar epithelium that contains goblet cells.

This specialized epithelium produces mucus to trap debris and pathogens as they enter the trachea. The cilia beat the mucus upward towards the laryngopharynx, where it can be swallowed down the esophagus. The mucociliary escalator runs from the bronchi to the larynx, and serves to efficiently clean, moisten, and warm incoming air.

It consists of pseudostratified columnar epithelium that has tiny hairs called cilia on their apical surface. Also included in this epithelial layer are specialized mucus-producing cells called goblet cells. Similar to the nasal cavity, the mucus serves to moisten air, while trapping pathogens and debris that is in the air. The mucus sits on top of the cilia, which continually beat upwards, carrying the mucus and any debris that it traps away from the lungs.

This cleaning process also serves to eliminate bacteria from the air, thereby protecting the lungs from infection. Figure 8. Mucociliary escalator: Debris is trapped in mucus that is made by goblet cells. The mucus and debris are brushed upwards, away from the lungs, thereby ensuring that air is clean prior to entry into the lungs. The trachea windpipe extends from the larynx toward the lungs. The trachea is formed by 16 to 20 stacked, C-shaped pieces of hyaline cartilage that are connected by dense connective tissue.

The trachealis muscle and elastic connective tissue together form the fibroelastic membrane, a flexible membrane that closes the posterior surface of the trachea, connecting the C-shaped cartilages. The fibroelastic membrane allows the trachea to stretch and expand slightly during inhalation and exhalation, whereas the rings of cartilage provide structural support and prevent the trachea from collapsing. In addition, the trachealis muscle can be contracted to force air through the trachea during exhalation.

The trachea is lined with pseudostratified ciliated columnar epithelium, which is continuous with the larynx. The esophagus borders the trachea posteriorly. Figure 9. The trachea branches into the right and left primary bronchi at the carina. These bronchi are also lined by pseudostratified ciliated columnar epithelium containing mucus-producing goblet cells. The carina is a raised structure that contains specialized nervous tissue that induces violent coughing if a foreign body, such as food, is present.

Rings of cartilage, similar to those of the trachea, support the structure of the bronchi and prevent their collapse. The primary bronchi enter the lungs at the hilum, a concave region where blood vessels, lymphatic vessels, and nerves also enter the lungs.

The bronchi continue to branch into bronchial a tree. A bronchial tree or respiratory tree is the collective term used for these multiple-branched bronchi. The main function of the bronchi, like other conducting division structures, is to provide a passageway for air to move into and out of each lung. In addition, the mucous membrane traps debris and pathogens. A bronchiole branches from the tertiary bronchi. Bronchioles, which are about 1 mm in diameter, further branch until they become the tiny terminal bronchioles, which lead to the structures of gas exchange.

There are more than terminal bronchioles in each lung. The muscular walls of the bronchioles do not contain cartilage like those of the bronchi. This muscular wall can change the size of the tubing to increase or decrease airflow through the tube. In contrast to the conducting division, the respiratory division includes structures that are directly involved in gas exchange.

The respiratory division begins where the terminal bronchioles join a respiratory bronchiole , the smallest type of bronchiole, which then leads to an alveolar duct , opening into a cluster of alveoli. Figure Bronchioles lead to alveolar sacs in the respiratory division, where gas exchange occurs. An alveolar duct is a tube composed of smooth muscle and connective tissue, which opens into a cluster of alveoli.

Alveoli are small, grape-like sacs that are attached to the alveolar ducts. An alveolar sac is a cluster of many individual alveoli that are responsible for gas exchange. An alveolus is approximately mm in diameter with elastic walls that allow the alveolus to stretch during air intake, which greatly increases the surface area available for gas exchange.

Alveoli are connected to their neighbors by alveolar pores, which help maintain equal air pressure throughout the alveoli and lung. The alveolar wall consists of three major cell types: type I alveolar cells, type II alveolar cells, and alveolar macrophages.

A type I alveolar cell is a squamous epithelial cell of the alveoli, which constitute up to 97 percent of the alveolar surface area. Type 1 alveolar cells are very thin- only about 25 nm thick and are highly permeable to gases. A type II alveolar cell is interspersed among the type I cells and secretes pulmonary surfactant , a substance composed of phospholipids and proteins that reduces the surface tension of the alveoli, preventing these tiny air sacs from collapsing during expiration. Roaming around the alveolar wall is the alveolar macrophage , a phagocytic cell of the immune system that removes debris and pathogens that have reached the alveoli.

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