Comprehensive Anatomy and Physiology of the Trachea, Lungs, and Respiratory Structures

Anatomy and Structural Characteristics of the Trachea

The trachea is a tubular air duct that measures approximately $12\,cm$ ($5\,\text{inches}$) in length and $2.5\,cm$ ($1\,\text{inch}$) in diameter. It is located anteriorly to the esophagus and extends from the larynx down to the superior border of the fifth thoracic vertebra ($T5$). At this specific level, the trachea bifurcates into the right and left primary bronchi. The internal point where this bifurcation occurs is known as the carina.

The trachea is strategically positioned in relation to other cervical and thoracic structures. It sits behind the lateral lobes of the thyroid gland in its superior portion. A superior view of a transverse section shows the trachea positioned directly in front of the esophagus, with the isthmus and lateral lobes of the thyroid gland surrounding the anterior and lateral aspects. The trachea's structural integrity is maintained by tracheal cartilages, which are connected by annular ligaments.

Histology of the Tracheal Wall

The wall of the trachea is composed of four distinct layers, organized from the most deep (internal) to the most superficial (external) as follows: the mucosa, the submucosa, the hyaline cartilage, and the adventitia.

The mucosa constitutes the innermost layer and consists of a ciliated pseudostratified columnar epithelium and an underlying layer of lamina propria. This lamina propria contains elastic and reticular fibers, providing both support and flexibility. The cilia function to move particles away from the lower respiratory tract.

The submucosa is the second layer, composed of areolar connective tissue. This layer contains seromucous glands and their respective ducts, which contribute to the moisture and protection of the airway. The third layer consists of hyaline cartilage. The trachea features between $16$ and $20$ horizontal, incomplete rings of hyaline cartilage. These rings are arranged in a shape resembling the letter "C" and are stacked upon one another. They are held together by dense connective tissue. The open part of the "C" faces the esophagus and is bridged by the fibromuscular membrane of the trachea, which contains the tracheal muscle (músculo traqueal).

The adventitia is the most superficial layer of the tracheal wall. It is made of areolar connective tissue that serves to anchor and connect the trachea to the surrounding tissues and structures.

The Bronchial Tree and its Branching Patterns

The bronchial tree begins at the trachea and terminates at the terminal bronchioles. At the upper border of the fifth thoracic vertebra ($T5$), the trachea divides into the right primary bronchus and the left primary bronchus. There are significant anatomical differences between the two: the right primary bronchus is more vertical, shorter, and wider than the left one. This anatomical orientation explains why aspirated foreign objects are more likely to enter and lodge in the right lung.

Once they enter the lungs, the primary bronchi undergo extensive branching. They divide into smaller lobar bronchi (secondary bronchi), with one lobar bronchus dedicated to each lobe of the lung. Consequently, the right lung has three lobar bronchi corresponding to its three lobes, while the left lung has two lobar bronchi for its two lobes. The lobar bronchi continue to branch into even smaller segmentary bronchi (tertiary bronchi), which then divide into bronchioles. These bronchioles further branch multiple times, and the smallest among them divide into even narrower tubes called terminal bronchioles.

The terminal bronchioles mark the end of the conduction zone of the respiratory system. This extensive branching system, from the trachea down to the terminal bronchioles, is referred to as the bronchial tree because of its resemblance to an inverted tree. The total hierarchy of the branching is as follows: Trachea $\rightarrow$ Primary Bronchi $\rightarrow$ Lobar Bronchi $\rightarrow$ Segmentary Bronchi $\rightarrow$ Bronchioles $\rightarrow$ Terminal Bronchioles.

Gross Anatomy of the Lungs and Pleural Membranes

The lungs are paired, cone-shaped organs located in the thoracic cavity. The term "pulmón" is derived from "light" because of their ability to float. Each lung is enclosed and protected by a double-layered serous membrane called the pleural membrane or pleura. The superficial layer is the parietal pleura, which lines the wall of the thoracic cavity. The deep layer, the visceral pleura, directly clothes the lungs themselves.

Between the visceral and parietal pleurae is a small space called the pleural cavity. This cavity contains a small volume of lubricating fluid secreted by the membranes, known as pleural fluid. This fluid is essential for reducing friction between the membranes, allowing them to slide smoothly against one another during the movements of respiration.

Anatomically, the lungs feature a superior apex (vértice) and a broad inferior base. Due to the space occupied by the heart, the left lung features a concavity on its medial surface called the cardiac notch (incisura cardíaca), making the left lung approximately $10\%$ smaller than the right lung. Conversely, while the right lung is thicker and wider, it is slightly shorter than the left lung because the diaphragm is higher on the right side to accommodate the liver located beneath it.

Lung Lobes, Fissures, and Surfaces

The lungs are divided into lobes by fissures. The right lung is divided into three lobes: the superior lobe, the middle lobe, and the inferior lobe. These are separated by the horizontal fissure (separating the superior and middle lobes) and the oblique fissure (separating the middle and inferior lobes). The left lung has only two lobes: the superior lobe and the inferior lobe, which are separated by a single oblique fissure.

Each lung has a medial surface that contains the hilum (hilio), through which the root (raíz) of the lung—consisting of bronchi, pulmonary blood vessels, lymphatic vessels, and nerves—enters and exits the organ. Internal views of the lungs show the complex arrangement of these structures. The right lung shows the superior, middle, and inferior lobes from a medial perspective, along with the fissures. The left lung clearly displays the cardiac notch (incisura cardíaca) on its medial aspect.

Alveoli and the Respiratory Membrane

Alveoli are the structural units where gas exchange occurs. The pathway leading to the alveoli proceeds from terminal bronchioles to respiratory bronchioles, then to alveolar ducts, and finally to alveolar sacs. An alveolar sac consists of two or more alveoli that share a common opening. An individual alveolus is a diverticulum-shaped evagination lined by simple squamous epithelium and supported by a thin elastic basement membrane.

The alveolar walls contain two primary types of epithelial cells. Type I alveolar cells are more numerous; they are simple squamous epithelial cells that form a nearly continuous lining of the alveolar wall and serve as the main site for gas exchange. Type II alveolar cells (also called septal cells) are fewer in number and are interspersed between the Type I cells. These rounded or cuboidal cells possess microvilli and secrete alveolar fluid, which keeps the surface between the cells and the air moist. This fluid contains surfactant, a substance that reduces surface tension. Alveolar macrophages (also known as dust cells) are also present to remove fine dust particles and debris from the alveolar spaces.

Gas exchange between the air spaces in the lungs and the blood occurs through diffusion across the respiratory membrane. This membrane is extremely thin to facilitate rapid diffusion and is composed of four layers: 1) The layer of Type I and Type II alveolar cells; 2) The epithelial basement membrane; 3) The capillary basement membrane; and 4) The capillary endothelium. The total structure involves the diffusion of oxygen ($O_2$) from the alveolus into the red blood cells (erythrocytes) in the pulmonary capillary, and the diffusion of carbon dioxide ($CO_2$) from the blood into the alveolus.

Mechanics of Respiration and Muscle Involvement

Breathing involves specific muscle groups for both inspiration (inhalation) and expiration (exhalation). During quiet inspiration, the primary muscles used are the diaphragm and the external intercostal muscles. The diaphragm contracts and moves downward, while the external intercostals contract to lift the ribs, increasing the volume of the thoracic cavity.

During forced or deep inspiration, accessory inspiratory muscles are recruited. These include the sternocleidomastoid, the scalenes, and the pectoralis minor. These muscles help to further expand the thoracic cage. In contrast, quiet expiration is largely a passive process resulting from the elastic recoil of the lungs and the relaxation of inspiratory muscles. However, forced expiration involves the contraction of the internal intercostal muscles and the abdominal muscles, including the external oblique, internal oblique, transversus abdominis, and rectus abdominis. These contractions increase intra-abdominal pressure and pull the ribs downward, forcing air out of the lungs.