Human Anatomy and Physiology - I. V. Gayvoronsky 2011

Anatomy and Physiology of the Respiratory System
Lower Respiratory Tract

The Larynx

Structure. The larynx is located in the anterior neck region. Superiorly, it is connected to the Hyoid bone by ligaments, and inferiorly, it continues into the Trachea (Fig. 8.4). The upper boundary of the larynx lies at the level of the intervertebral disc between the IV and V cervical vertebrae. The lower boundary is at the level of the VII cervical vertebra. Anteriorly, the larynx is covered by the Neck Muscles. Posteriorly, it is adjacent to the Pharynx, and laterally, it is flanked by the carotid Arteries, the Internal jugular vein, and the Vagus nerve.

The laryngeal cavity can be divided into three sections: the upper section (vestibule), the middle section, and the lower section (infraglottic cavity). The boundaries between these sections are formed by the paired vestibular folds (false vocal cords) and vocal folds (true vocal cords), which define two openings known as the vestibular and vocal rimae (slits). The lumen of the rima glottidis is narrower and can be altered by the action of the laryngeal muscles.

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Fig. 8.4. Larynx (anterior view):

1 — hyoid bone; 2 — superior horn of thyroid Cartilage; 3 — lamina of thyroid cartilage; 4 — inferior horn of thyroid cartilage; 5 — cricoid cartilage; 6 — tracheal cartilages; 7 — annular ligaments of trachea; 8 — cricothyroid joint; 9 — elastic cone (cricovocal membrane); 10 — superior thyroid notch; 11 — thyrohyoid membrane

Fig. 8.5. Cytology/practical/72.html">Cross section of the larynx (posterior view):

I — laryngeal vestibule; II — middle part; III — infraglottic cavity; 1 — epiglottis; 2 — thyroid cartilage; 3 — vestibular fold; 4 — laryngeal ventricle; 5 — vocal Muscle; 6 — cricothyroid muscle; 7 — cricoid cartilage; 8 — tracheal cartilage; 9 — vocal fold

The upper division of the larynx is quite wide, extending from the laryngeal inlet to the vestibular folds. The middle part represents the narrowest section; this space is bounded superiorly by the vestibular folds and inferiorly by the vocal folds. In the middle part, between the folds on each side, lies a depression known as the laryngeal ventricle (ventricle of Morgagni). The laryngeal ventricles act as air resonators during phonation and also help warm the inhaled air. Below the vocal folds lies the infraglottic cavity, which gradually widens inferiorly and continues into the tracheal lumen. Due to the varying widths of the laryngeal compartments, the coronal and sagittal sections give it an hourglass shape (Fig. 8.5).

The framework of the organ is formed by cartilages, which are classified into paired and unpaired. The unpaired cartilages are the thyroid, cricoid, and epiglottic cartilages (Fig. 8.6), while the paired ones include the arytenoid, cuneiform, corniculate, and triticeal cartilages.

The shield-shaped thyroid cartilage anteriorly protects the other structures. It consists of two laminae joined at an acute angle, forming the laryngeal prominence, which can be easily felt (palpated) under the Skin of the neck as a firm elevation. In men, this structure is prominent and is commonly known as the Adam's apple. Superior and inferior horns project from each lamina. The thyrohyoid membrane spans between the hyoid bone and the thyroid cartilage.

The epiglottic cartilage lies posterior to the ROOT of the Tongue, just above the laryngeal inlet. It features a wide upper portion—the lamina—which tapers inferiorly into a stalk or petiole. Covered by mucous membrane, this cartilage is called the epiglottis. Its primary function is to prevent Water and food from entering the lower respiratory tract.

Fig. 8.6. Cartilages and Muscles of the larynx:

a, d — lateral view; b — anterior view; c, e — posterior view; f — sagittal section; 1 — cricoid cartilage; 2 — laryngeal prominence; 3 — epiglottis; 4 — superior horn of thyroid cartilage; 5 — inferior horn of thyroid cartilage; 6 — arch of cricoid cartilage; 7 — cricothyroid joint; 8 — arytenoid cartilage; 9 — cricoarytenoid joint; 10 — lamina of cricoid cartilage; 11 — lateral cricoarytenoid muscle; 12 — aryepiglottic muscle; 13 — thyroarytenoid muscle; 14 — posterior cricoarytenoid muscle; 15 — oblique arytenoid muscle; 16 — transverse arytenoid muscle; 17 — vestibular fold; 18 — laryngeal ventricle; 19 — vocal fold

The cricoid cartilage is situated inferior to the others and forms the Base of the larynx. It is named after its distinct signet-ring shape and is divided into an arch and a lamina.

The arytenoid cartilage is paired and sits posteriorly on the lamina of the cricoid cartilage. It possesses vocal and muscular processes. The vocal ligament is stretched between the thyroid cartilage and the vocal process, while the muscular process serves for the attachment of several laryngeal muscles. The remaining paired cartilages are small: the cuneiform and corniculate cartilages are embedded in the mucous membrane near the laryngeal inlet, whereas the triticeal cartilage lies within the lateral part of the thyrohyoid membrane.

The laryngeal cartilages are interconnected by ligaments and joints. The thyroid and cricoid cartilages are joined by two cricothyroid joints. The cricoarytenoid joints lie between the cricoid cartilage and the bases of the arytenoid cartilages. In these joints, the arytenoid cartilage rotates around a vertical axis, leading to the widening or narrowing of the rima glottidis.

The laryngeal muscles are striated and under voluntary control. They are classified into extrinsic and intrinsic muscles. The extrinsic muscles move the larynx upward or downward during swallowing and vocalization; anatomically, they belong to the Infrahyoid muscles of the neck (sternothyroid and thyrohyoid). The intrinsic laryngeal muscles are functionally divided into four groups:

1) muscles affecting the width of the laryngeal inlet: the aryepiglottic muscle, which closes the laryngeal inlet;

2) muscles affecting THE POSITION OF the epiglottis: the thyroepiglottic muscle, which elevates the epiglottis;

3) muscles affecting the width of the rima glottidis:

✵ dilatator (posterior cricoarytenoid);

✵ constrictors (lateral cricoarytenoid, thyroarytenoid, transverse and oblique arytenoid muscles);

4) muscles affecting the state of the vocal cord:

✵ tensors (cricothyroid muscle);

✵ relaxers (vocalis muscle).

The interior of the larynx is covered with a mucous membrane, The surface of which is lined with ciliated epithelium. Stratified squamous non-keratinized epithelium is found only in the region of the vocal fold.

The mucous membrane, except for the area of the vocal folds, is loosely fused with the submucosa. This is particularly characteristic of the vestibular fold region. Edema may occur in these areas, causing breathing difficulties. This condition is known as "false croup", which commonly affects young children.

Functions of the larynx. The larynx belongs to the lower respiratory tract and ensures airflow. Numerous receptors are located in the mucous membrane of the larynx and trachea; when irritated, they trigger the so-called cough reflex, which serves as a protective mechanism against the inhalation of large amounts of dust particles. At the same time, the larynx acts as The Organ of voice production.

Voice production is carried out by means of the vocal cords located within the folds of the same name. Sound formation is influenced by the degree of their tension, as well as the width of the rima glottidis (Fig. 8.7). During quiet breathing, it is 5 mm, while during deep breathing and loud shouting, it reaches 15 mm. During speech, the width of the rima glottidis changes constantly, narrowing and widening. The degree of tension of the vocal cords plays a crucial role in sound articulation. They tense and relax under the Influence of the respective muscles. Upon expiration, a jet of air passing through the rima glottidis sets the cords and folds into oscillatory motion. This generates sounds that depend on the frequency and amplitude of cord oscillation. The oscillation frequency determines the pitch of the voice, while the amplitude determines its volume. In addition, the voice depends on the position of the tongue, Lips, soft palate, and the patency of the Nasal cavity and its Paranasal Sinuses. Men have longer vocal folds compared to women. Therefore, the male voice is generally lower than the female voice.

Fig. 8.7. Shapes of the rima glottidis under various functional states (diagram):

a — rima glottidis during phonation; b — rima glottidis during quiet breathing; c — rima glottidis during deep breathing; 1 — vocal cord; 2 — membranous part of the rima glottidis; 3 — cartilaginous part of the rima glottidis; 4 — muscular process of the arytenoid cartilage; 5 — cricoid cartilage; 6 — vocal process of the arytenoid cartilage; 7 — thyroid cartilage

Trachea and main Bronchi

The trachea (windpipe), trachea, is a hollow cylindrical tube 11 - 13 cm in length. It begins from the larynx at the level of the VII cervical vertebra. Between the IV and V thoracic vertebrae, it divides into two main bronchi, forming the tracheal bifurcation. The cervical and thoracic PARTS OF THE trachea are distinguished. In the cervical region, The Thyroid Gland lies adjacent to it. In the thoracic cavity, the trachea is located in the Mediastinum, dividing it into anterior and posterior compartments. Here, large Blood Vessels, including the aorta, are adjacent to it. Posterior to the trachea along its entire length lies the Esophagus.

The mucous membrane of the trachea is lined with ciliated epithelium and contains numerous glands. The framework of the organ consists of 15 — 20 cartilaginous half-rings interconnected by ligaments. The posterior wall lacks cartilage tissue, forming the membranous part of the trachea. Its framework is composed of Connective Tissue and smooth muscle fibers arranged transversely. Due to the presence of cartilaginous half-rings, the trachea does not collapse during Respiration. Externally, the organ is covered by an adventitial layer.

The main bronchi, bronchi principales, diverge at an angle of 70°. The right main bronchus is shorter and wider, measuring 3 cm in length; it lies more vertically and is a direct continuation of the trachea. Due to this anatomical feature, foreign bodies enter this bronchus more frequently (in 70—80% of cases). The left main bronchus is 4—5 cm long.

The main bronchi enter the hilum of the Lungs, where they branch out, giving rise to the bronchial tree. The structural principles of the walls of the main bronchi and the trachea are similar. Like the trachea, they consist of cartilaginous half-rings. The inner mucous membrane is lined with ciliated epithelium, and the exterior of the main bronchi is covered by an adventitial layer.