Lecture Notes on Human Anatomy - Hryhorieva O.A., Svitlytskyi A.O. 2020

The Doctrine of Viscera (Splanchnology)
Anatomy of Internal Organs
Respiratory System

Lecture Outline:

1. Anatomy of the Respiratory system.

2. Nasal cavity.

3. Larynx and its Structure.

4. Trachea and Bronchi.

5. Lungs and their structure.

The respiratory Organs include the lungs, where gas exchange between air and Blood takes place, and the respiratory tract, through which air passes into the lungs and from the lungs to the external environment. Air from the environment enters the lungs sequentially through the nasal or Oral Cavity, Pharynx, larynx, trachea, and bronchi. A characteristic feature of the respiratory tract is that its walls do not collapse during changes in body position, bending, rotation, etc., because they are supported by a rigid cartilaginous framework.

The nasal cavity in the facial region is supplemented by the External Nose, which is supported by Cartilage. These cartilages prevent the nostrils from narrowing during inhalation and protect the tip of the nose, which protrudes above the face, from injury. The external nose communicates with the external environment through the nostrils. Most of the nasal mucosa is lined with ciliated epithelium, which traps dust entering the nasal cavity with the air. Goblet Cells within this epithelium and mucous glands moisten the mucosa. In the region of the superior nasal conchae, the mucosa contains olfactory epithelium. In the nasal cavity, air is cleaned, warmed, and humidified. From the nasal cavity, air passes through the choanae into the pharynx (or through the oral fauces during Mouth breathing), and from there into the larynx.

The larynx is located on the anterior surface of the neck at the level of the 4th–6th cervical vertebrae. It has an hourglass shape and is about 4 cm long. The larynx is a hollow, tubular organ. Its inner lining is a mucous membrane containing ciliated epithelium and mucus-producing glands. The middle layer is fibro-muscular-cartilaginous, consisting of cartilage, ligaments, and Muscles. The Skeleton of the larynx, its rigid foundation, is formed by cartilages: unpaired—thyroid, cricoid, and epiglottis; and paired—arytenoid, corniculate, and cuneiform. In men, the laryngeal prominence—the Adam's apple—is clearly visible on the thyroid cartilage and serves as a secondary sex characteristic.

The thyroid cartilage is the largest; its upper margin is connected to the Hyoid bone, which means that Movements of the hyoid bone, such as during swallowing, affect the larynx. Located in the upper part of the larynx is the epiglottis, which protects the respiratory tract from food entry during swallowing by closing the laryngeal inlet. The cricoid cartilage connects superiorly with the arytenoid cartilages and inferiorly with the trachea. The Main Functions of the larynx are conducting air into the trachea and sound production. Within the laryngeal lumen, the mucous membrane forms two pairs of folds on the lateral walls: the vestibular (false vocal) and vocal folds. Between the vestibular (upper) folds lies the rima vestibuli, while between the vocal (lower) folds lie the right and left vocal cords, which form the rima glottidis. When the laryngeal muscles contract, the rima glottidis narrows or widens, and the vocal cords are tensioned or relaxed. Sound is produced by the vibrational movements of the vocal cords in close coordination with the vocal muscles, which actively contract in Response to nerve impulses from the Central Nervous system.

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The trachea is a tube 9–12 cm long, located in the neck extending from the 6th cervical to the 5th thoracic vertebra; the Esophagus lies directly behind it. Its framework consists of 18–20 cartilaginous half-rings that remain open posteriorly on the side facing the esophagus.

At the level of the 4th–5th thoracic vertebrae, the trachea divides into two bronchi—right and left. The point of division is called the tracheal bifurcation, where a significant number of Lymph Nodes are located. The left and right main bronchi enter the lungs; the right bronchus is shorter and wider, which is why foreign bodies more frequently enter it. The main bronchi enter the hilum of their respective lung, where each bronchus undergoes repeated dichotomous branching (up to 23 times) to form the bronchial tree. Up to the 18th generation, the bronchi function strictly as conducting airways, whereas starting from the 19th generation, gas exchange begins to occur partially within the bronchioles.

The lungs are paired organs located in the thoracic cavity. Their apex reaches 2–3 cm above the first rib, while the inferior border extends to the 7th rib for the left lung and the 6th rib for the right lung. Each lung has a base, an apex, and three surfaces: diaphragmatic, costal, and medial. The medial surface features the lung hilum, through which the main bronchi, the pulmonary artery (carrying deoxygenated blood), and nerves enter. Fissures divide each lung into lobes: the right lung has three lobes, and the left has two. Each lung contains 10 bronchopulmonary segments. A bronchopulmonary segment is a portion of the lung supplied by a tertiary (segmental) bronchus and a corresponding branch of the pulmonary artery. Terminal bronchioles give rise to respiratory bronchioles, the walls of which bear pulmonary alveoli. Alveolar ducts branch off from each respiratory bronchiole and terminate in blind alveolar sacs; this entire structural unit is called a pulmonary acinus. The alveolar ducts and sacs belonging to a single terminal respiratory bronchiole form a primary lobule, of which there are about 16 in an acinus. Respiratory bronchioles, alveolar ducts, and alveolar sacs collectively form the alveolar tree, or the respiratory parenchyma of the lungs. The serous membrane enclosing the lungs is the Pleura, which consists of two layers: the visceral and parietal pleura. The visceral layer tightly covers the entire surface of the lung and reflects at the hilum to become the parietal layer, which firmly lines the thoracic wall. Between the visceral and parietal layers lies the pleural cavity, a potential space filled with a small amount of serous fluid (about 2 mL) that lubricates the pleural surfaces and reduces friction during respiratory movements. There is no air in this cavity, and the pressure within it is subatmospheric, which facilitates lung expansion during inhalation and increases the respiratory surface area of the lungs. Pathological conditions can allow infection to enter the pleural cavity, leading to inflammation (Pleurisy) accompanied by an accumulation of fluid or pus.