Basic Anatomy: Parts and Structure of the Respiratory System

The respiratory system divides into two main sections: the upper respiratory tract and the lower respiratory tract. The upper tract includes everything from your nose to your larynx, while the lower tract encompasses structures from the trachea down to the tiny air sacs in your lungs. Together, these components create an elaborate pathway for air to travel deep into your body while being warmed, humidified, and cleaned along the way.

Your nose serves as the primary entrance for air and performs crucial preparatory functions. The external nose, supported by bone and cartilage, houses two nostrils (nares) separated by the nasal septum. Inside, the nasal cavity extends back to the nasopharynx. The nasal cavity's walls feature three shelf-like projections called turbinates or conchae, which create turbulent airflow. This turbulence ensures incoming air contacts the warm, moist mucous membranes, adding heat and humidity while trapping particles.

The nasal cavity's lining contains specialized cells performing different functions. Goblet cells produce mucus that traps dust, bacteria, and other particles. Ciliated cells have tiny hair-like projections that beat in coordinated waves, moving mucus and trapped particles toward the throat for swallowing or expulsion. The roof of the nasal cavity houses olfactory receptors responsible for your sense of smell. Rich blood vessel networks warm incoming air to body temperature, preventing cold shock to delicate lung tissues.

The pharynx, commonly called the throat, serves as a shared pathway for both air and food. This muscular tube divides into three regions: the nasopharynx (behind the nose), oropharynx (behind the mouth), and laryngopharynx (above the larynx). The pharynx contains lymphoid tissue, including tonsils and adenoids, providing immune defense against inhaled pathogens.

The larynx, or voice box, marks the transition between upper and lower respiratory tracts. This complex structure, composed of nine cartilages connected by muscles and ligaments, serves three critical functions: maintaining an open airway, directing food and air to proper passages, and producing sound. The thyroid cartilage forms the prominent "Adam's apple," more visible in males due to testosterone's effects during puberty.

Within the larynx, the vocal cords (properly called vocal folds) stretch across the airway. These paired membranes vibrate as air passes through, producing sound waves. Intrinsic laryngeal muscles precisely control vocal cord tension and position, enabling the remarkable range of human speech and song. The epiglottis, a leaf-shaped cartilage, acts like a trap door, closing over the larynx during swallowing to prevent food from entering the airway.

The trachea, or windpipe, extends about 4-5 inches from the larynx to its division into the main bronchi. This tube maintains its shape through 16-20 C-shaped cartilage rings stacked vertically. The open part of each C faces posteriorly, allowing the esophagus to expand when swallowing food. The tracheal lining continues the respiratory mucosa pattern, with mucus-producing and ciliated cells creating the "mucociliary escalator" that moves debris upward for removal.

At the trachea's lower end, it divides into right and left main bronchi at the carina, an internal ridge extremely sensitive to touch—even light contact triggers vigorous coughing. The right main bronchus is wider, shorter, and more vertical than the left, making it the more common site for inhaled foreign objects to lodge. Each main bronchus enters its respective lung at the hilum, along with blood vessels, lymphatics, and nerves.

Within the lungs, bronchi continue branching like an inverted tree. Main bronchi divide into lobar bronchi (three right, two left), which further divide into segmental bronchi, then smaller and smaller branches. This branching pattern, called the bronchial tree, creates enormous surface area while fitting into the limited chest cavity space. As bronchi branch and narrow, their walls change—cartilage decreases and smooth muscle increases, allowing fine control of airway diameter.

The smallest airways, bronchioles, lack cartilage entirely and are surrounded by smooth muscle. Terminal bronchioles, the smallest airways that only conduct air, lead to respiratory bronchioles that have scattered alveoli and participate in gas exchange. These lead to alveolar ducts and finally alveolar sacs, grape-like clusters of individual alveoli.

Alveoli, the respiratory system's functional units, number approximately 300-500 million per person. These tiny air sacs, each about 0.2 millimeters in diameter, provide roughly 750 square feet of surface area—about the size of a tennis court. Alveolar walls are incredibly thin (0.5 micrometers) and composed primarily of two cell types. Type I pneumocytes, covering 95% of the surface area, facilitate gas exchange. Type II pneumocytes produce surfactant, a soap-like substance that reduces surface tension and prevents alveolar collapse.