Lung: Tissue Function

Upper Respiratory Tract

The upper respiratory tract consists of the nasal cavity, pharynx, and larynx; the lower respiratory tract comprises the trachea, bronchi, and lungs. The nasal cavity has a large surface area and its function is to adjust the temperature of the inspired air to the body’s temperature. There are special cartilaginous bony prominences in our nose called nasal conchae, which contribute to increasing the surface area of the nose and also support olfactory mucous membranes involved in distinguishing different odors.

The high vascularity of the nasal mucosa helps to humidify the air before it further progresses along the respiratory tract. The mucous glands of the nose secrete mucous, which traps air particles, while small cilia beat toward the pharynx and transport any trapped particles upward, eventually to be coughed up or swallowed into the stomach.

Pharynx

The pharynx is commonly known as the throat. It stretches from the posterior part of the nasal [Page 711]cavity up to the level of the larynx and esophageal opening. The pharynx is divided into three parts: the nasopharynx, oropharynx, and laryngopharynx. Air is directed from the nasal cavity to the nasopharynx, which is located at the posterior part of the nasal cavity. Air is then directed downward into the oropharynx, located at the posterior part of oral cavity, and finally enters the lowest part of the pharynx, the laryngopharynx. Here, the air passes through the epiglottis, which is a cartilaginous gateway of the larynx that separates the esophagus and trachea. The epiglottis makes sure that our food enters through the esophagus only; it does this by covering the trachea during swallowing.

Larynx

The larynx lies between the pharynx and the trachea. The anterior portion of the larynx contains a prominent cartilage called the thyroid cartilage. The anterior part of the thyroid cartilage is known as the Adam’s apple, which can easily be seen and felt in front of the neck. The larynx performs a special function called phonation. Phonation is performed by two vocal folds arranged at the center of the larynx. The posterior portion of each vocal cord is attached to small arytenoid cartilages, which are capable of movement with the help of muscles. The moment air passes through the vocal cords, the arytenoid cartilages adjust them accordingly to create sound through vibration. The larynx is continuous with the trachea and it has a signet-ring-shaped cricoid cartilage at the lower portion connecting it with the first tracheal ring.

Trachea

The trachea starts from the lower portion of the larynx and courses in front of the neck. It ends at the upper part of the sternum and it is made up of approximately 20 cartilaginous rings that keep it open all the time. The luminal side of trachea has an epithelial lining that secretes mucus, trapping particles and debris from the inspired air. There are minute cilia that continuously beat and propel air particles and debris upward toward the larynx and then pharynx.

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