Human Anatomy - Kotsan I. Y. 2009

Respiratory System
Trachea

The Trachea is a direct continuation of the Larynx (Fig. 166). It is a tube measuring 9–13 cm in length and 1.5–2.7 cm in diameter. The trachea begins at the level of the VI–VII cervical vertebrae and terminates at the level of the IV–V thoracic vertebrae, where it divides into the right and left main Bronchi. The site of this division is referred to as the tracheal bifurcation (bifurcatio tracheae). At the bifurcation, a crescent-shaped ridge of the wall projects into the tracheal lumen, known as the tracheal carina (carina tracheae). Because the trachea is situated in both the neck and the thoracic cavity, it is conventionally divided into two parts: the cervical part (pars cervicalis) and the thoracic part (pars thoracica). Anteriorly, the cervical part of the trachea is covered by the sternothyroid and sternohyoid Muscles and the isthmus of The Thyroid Gland. In the thoracic cavity, the anterior relations of the trachea include the aortic arch, brachiocephalic trunk, left brachiocephalic vein, Water/144.html">Origin of the left common carotid artery, and Thymus gland. Posteriorly, the Esophagus runs along the length of the trachea, while the right and left neurovascular bundles (comprising the common carotid artery, Internal jugular vein, and Vagus nerve) lie laterally.

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Fig. 166. Trachea and Bronchi (anterior view)

1 — trachea; 2 — esophagus; 3 — aorta; 4 — left main bronchus; 5 — left pulmonary artery; 6 — left superior lobar bronchus; 7 — segmental bronchi of the superior lobe of the left lung; 8 — left inferior lobar bronchus; 9 — azygos vein; 10 — segmental bronchi of the inferior and middle lobes of the right lung; 11 — right inferior lobar bronchus; 12 — right middle lobar bronchus; 13 — right superior lobar bronchus; 14 — right main bronchus; 15 — tracheal bifurcation

The wall of the trachea consists of a mucosa, submucosa, fibro-musculo-cartilaginous layer, and adventitia.

The mucosa of the trachea is lined with pseudostratified ciliated columnar epithelium containing numerous goblet Cells. The lamina propria of the mucosa is rich in elastic fibers, lymphocytes, and lymphoid nodules (follicles).

The submucosa is formed of loose irregular Connective Tissue that transitions gradually into the Cytology/practical/45.html">Dense connective tissue of the tracheal perichondrium. Situated within the submucosa are mixed seromucous tracheal glands (glandulae tracheales), whose excretory ducts dilate in a bulbous fashion and open onto the mucosal surface. During Respiration, small dust particles adhere to the moistened mucous membrane, and the beating cilia of the epithelium propel them upward toward the exit of the respiratory tract.

The fibro-musculo-cartilaginous layer of the trachea is composed of 16–20 hyaline Cartilage rings enclosed by perichondrium and interconnected by annular ligaments (ligg. anularia). Each cartilage spans only two-thirds of the tracheal circumference; the posterior third is formed by the membranous wall (paries membranaceus), which contains bundles of smooth Muscle cells oriented predominantly circularly as well as longitudinally. The annular ligaments connecting the cartilaginous rings consist of connective tissue fibers running parallel to the longitudinal axis of the organ and continuing directly into the perichondrium. The alternation of cartilages and ligaments gives the trachea the appearance of a corrugated tube. The absence of cartilage in the posterior wall allows for the unobstructed passage of food through the esophagus, which lies immediately posterior to the trachea and can bulge into the tracheal lumen during swallowing. At the same time, because of its cartilages—bound together by dense fibrous tissue—and its membranous wall, the trachea is highly resilient and elastic, enabling it to resist external pressure and keep the airway permanently open.

The adventitia of the trachea is formed of loose irregular connective tissue.

The Blood supply to the trachea is derived from Branches of the inferior thyroid and internal thoracic Arteries, as well as bronchial Branches of the Thoracic Aorta. Venous drainage occurs via the tracheal plexus into the azygos and hemiazygos Veins and the thyroid veins. Efferent Lymphatic vessels drain into the paratracheal, superior tracheobronchial, and partially into the lateral deep cervical and anterior mediastinal Lymph Nodes.

The trachea is innervated by branches of the vagus nerve and the Sympathetic trunk.

Age-related Features of the trachea. The newborn's trachea is funnel-shaped, measuring 3.2–4.5 cm in length, with a lumen width in its middle portion of about 0.8 cm. Postnatally, the trachea grows proportionally with the body, with the most rapid growth occurring During the first year of life and at Puberty. In the newborn, the trachea is located high and slightly to the right of the midline. Its origin lies at the level of the II–IV cervical vertebrae, and its bifurcation corresponds to the II–III thoracic vertebrae. In a 1- to 2-year-old child, the upper margin of the trachea is located at the level of the IV–V cervical vertebrae; at 5–6 years, anterior to the V–VI cervical vertebrae; and in adolescence, at the level of the VI cervical vertebra. In elderly individuals, THE ORIGIN OF the trachea descends to the level of the VII cervical vertebra. In women, the upper margin of the trachea lies slightly higher than in men. Swallowing or HEAD movements alter THE POSITION OF the upper margin of the trachea. Up to 7 years of age, the tracheal bifurcation is situated anterior to the IV–V thoracic vertebrae, after which it gradually assumes the adult position at the level of the V thoracic vertebra.

The mucosal lining of the tracheal wall in the newborn is thin and delicate, and the glands are poorly developed. The membranous wall of the newborn's trachea is relatively wide, and the cartilages are poorly developed, thin, and soft. After 60–70 years of age, the tracheal cartilages become dense and brittle, fracturing easily under compression.