The trachea, commonly known as the windpipe, serves as a fundamental airway within the human respiratory system. This tube-like structure acts as a passageway for air, connecting the upper respiratory tract to the lungs. Its primary function involves transporting oxygen-rich inhaled air to the lungs and carbon dioxide-laden exhaled air away from them. The trachea’s structure ensures a consistently open route for the continuous flow of gases.
The Windpipe’s Pathway
The trachea begins at the inferior border of the larynx, or voice box, at the sixth cervical vertebra (C6) in the neck. From this point, it extends downwards into the chest cavity, measuring about 9 to 15 centimeters long and 2 to 3 centimeters in diameter in adults. This tube is reinforced by 16 to 20 C-shaped rings of hyaline cartilage, which provide structural support and prevent the airway from collapsing. The open, posterior part of these C-shaped rings faces the esophagus, allowing for its expansion during swallowing.
As it descends, the trachea passes behind the sternum and between the lungs, generally occupying a midline position. The inner lining of the trachea features a specialized mucous membrane composed of ciliated pseudostratified columnar epithelium and goblet cells. Goblet cells produce mucus that traps inhaled particles and debris. Cilia, tiny hair-like projections, sweep this mucus upwards to the pharynx for expulsion, protecting the lower airways from foreign substances.
Branching Towards the Lungs
At its lower end, at the fifth thoracic vertebra (T5), the trachea divides at a point called the carina. Here, it bifurcates into two main airways: the right and left primary (main) bronchi, one leading into each lung. The right main bronchus is generally shorter, wider, and more vertically oriented than the left main bronchus, a characteristic that can influence the path of aspirated foreign objects.
Upon entering the lungs, these primary bronchi branch extensively, forming the bronchial tree. Each primary bronchus divides into smaller secondary, or lobar, bronchi; the right lung, with three lobes, receives three secondary bronchi, while the left lung, with two lobes, receives two. These lobar bronchi then subdivide into tertiary, or segmental, bronchi, which supply specific lung segments. The branching continues with progressively smaller airways called bronchioles, which lack cartilage and rely on smooth muscle for structural integrity.
Terminal bronchioles mark the end of the conducting zone. They transition into respiratory bronchioles, where gas exchange begins. Respiratory bronchioles lead into tiny alveolar ducts, terminating in clusters of microscopic air sacs called alveoli. Approximately 480 million alveoli provide a vast surface area for gas exchange. Within these alveoli, oxygen from inhaled air diffuses into the bloodstream, and carbon dioxide from the blood diffuses into the alveoli for exhalation, across a thin blood-air barrier in contact with capillaries.