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Respiratory System Overview

• The respiratory system is responsible for gas exchange, bringing oxygen into the body and removing carbon dioxide.
• Learning outcomes include describing the gross anatomy of the larynx, ventilation and lung mechanics, and factors affecting airflow.

Larynx

• The larynx is a hollow, musculo-ligamentous structure with a cartilaginous framework that caps the lower respiratory tract.
• It's composed of three large unpaired cartilages (cricoid, thyroid, and epiglottis), and three pairs of smaller cartilages (arytenoid, corniculate, and cuneiform).
• It also contains a fibroelastic membrane and intrinsic muscles.
• The larynx is located in the anterior midline of the neck, extending from the root of the tongue to the trachea.

Laryngeal Inlet

• The laryngeal inlet is formed by the superior margin of the epiglottis and aryepiglottic folds.
• Lateral borders are composed of mucosal folds (aryepiglottic folds).
• The posterior part is formed by a transverse mucosal fold between the two arytenoids.

Cavity of Larynx

• The larynx's interior contains vestibular and vocal folds.
• The space between the two vestibular folds is called the rima vestibuli.
• The space between the two vocal folds is called the rima glottidis.
• The space between vestibular and vocal folds is the ventricle of the larynx or sinus of the larynx.

Summary of Larynx Cavities

• The larynx's compartments are defined by folds: the spaces between the aryepiglottic folds form the laryngeal inlet; the spaces between the vestibular folds form the rima vestibuli; and the spaces between the vocal folds form the rima glottidis.
• The vestibular folds and vocal folds bound the superior and inferior parts of the laryngeal cavity.
• The sinus of the larynx is located between the vestibular and vocal folds.
• The space below the vocal folds is called the infraglottic cavity.

Nerve Supply

• Sensory nerves above the vocal cord are supplied by the internal laryngeal nerve.
• Sensory nerves below the vocal cord are supplied by the recurrent laryngeal nerve.
• Motor nerves for all muscles besides the cricothyroid are supplied by the recurrent laryngeal nerve.
• Cricothyroid is supplied by the external laryngeal nerve.

Mechanics of Breathing

• Breathing mechanics involve pressure, volume, and flow changes during the breathing cycle.
• Breathing mechanics are the interplay of forces generated by pressure, volume, and flow changes.
• Important components include respiratory muscles, alveolar, pleural, and transpulmonary pressures; lung compliance, and lung recoil.
• Work of breathing is also important.

Muscles of Respiration

• Muscles of inspiration include the diaphragm and external intercostals.
• Accessory muscles include the sternocleidomastoid, scalenes, serratus anterior, and pectoralis minor.
• Muscles of expiration include the internal intercostals, external oblique abdominis, internal oblique abdominis, rectus abdominis, and transversus abdominis.
• Normal expiration is due to elastic recoil of the lungs and associated structures.
• Forced expiration requires additional abdominal muscles.

Functions of Respiratory Muscles

• Accessory muscles elevate the sternum, and fix ribs during inspiration.
• Principal external intercostal muscles elevate the ribs, increasing the width of the thoracic cavity.
• The diaphragm's dome descends, increasing the thoracic cavity's longitudinal dimension.

Alveolar Pressure

• Alveolar pressure is the pressure inside the alveoli, vital for inspiration and expiration.
• Normal alveolar pressure is equal to atmospheric pressure (760 mm Hg).
• During inspiration, alveolar pressure is lower than atmospheric pressure.
• During expiration, alveolar pressure is higher than atmospheric pressure.

Pleural and Transpulmonary Pressure

• Pleural pressure is the pressure in the space between the lung and chest wall pleura.
• Negative pleural pressure prevents lung collapse.
• Transpulmonary pressure is the difference between alveolar and pleural pressure.

Pulmonary Ventilation

• Pulmonary ventilation involves moving air in and out of the lungs.
• Inspiration requires active contractions of respiratory muscles.
• Expiration can be passive or active, depending on the need.

Thoracic Volume

• Thoracic volume changes during breathing, due to movements of the diaphragm and ribs.
• Inspiration increases thoracic volume.
• Expiration decreases thoracic volume.