The respiratory system is made up of two groups of structures: the upper respiratory tract (nose, mouth, throat, and voice box) and the lower respiratory tract (windpipe, bronchial tubes, and lungs). Supporting these airways are muscles that power breathing, protective membranes that wrap the lungs, and a built-in defense system that filters out debris before it reaches deep lung tissue.
The Upper Respiratory Tract
Air enters through your nose or mouth, and from there it passes through a series of connected passages before reaching the lungs. The nasal cavity is the first major stop. Its lining warms and humidifies incoming air while tiny hairs and sticky mucus trap dust, pollen, and other particles. Breathing through the nose filters air far more effectively than breathing through the mouth, though the mouth serves as a backup route when you need more airflow during exercise or when nasal congestion blocks the way.
From the nasal cavity (or mouth), air moves into the pharynx, the shared passageway commonly called the throat. The pharynx connects to both the digestive and respiratory systems, which is why swallowing and breathing need careful coordination. At the bottom of the pharynx sits the larynx, or voice box. The larynx contains your vocal cords and, critically, the epiglottis: a flap of tissue that folds backward over the airway entrance every time you swallow. This prevents food and liquid from slipping into the lungs. The larynx actually closes at three levels during a swallow, with the vocal cords pressing together, a second set of tissue folds closing above them, and the epiglottis capping the whole opening and directing food toward the esophagus.
The Windpipe and Bronchial Tree
Below the larynx, air enters the trachea, a tube about 10 to 12 centimeters long reinforced by C-shaped rings of cartilage that keep it from collapsing. The trachea splits into two primary bronchi, one heading into each lung. Those bronchi divide again into secondary and then tertiary bronchi, branching over and over into progressively smaller tubes called bronchioles.
This branching pattern is often called the bronchial tree because it resembles an upside-down tree. The airways divide roughly 15 to 16 times on average before reaching the terminal bronchioles, though the number of generations varies across different regions of the lung, ranging from about 8 to 23. With each split the tubes get narrower, and by the time air reaches the smallest bronchioles, those passages are less than a millimeter wide. Every generation of branching from the trachea to the terminal bronchioles serves as a conducting pipe, moving air deeper without any gas exchange taking place.
Alveoli and Gas Exchange
At the very ends of the bronchioles sit the alveoli, tiny balloon-like air sacs where the actual work of breathing happens. Each lung contains millions of alveoli, and their combined surface area is enormous. Estimates across the scientific literature place the total alveolar surface area somewhere between 70 and 140 square meters, roughly the floor space of a small apartment. That massive surface exists because gas exchange depends on contact area: more surface means more oxygen can cross into the blood with each breath.
The walls of the alveoli are extraordinarily thin and share a membrane with a dense network of capillaries, the smallest blood vessels in the body. Oxygen from inhaled air diffuses across this shared membrane into the bloodstream, while carbon dioxide moves in the opposite direction, passing from the blood into the alveoli to be exhaled. The whole exchange happens passively, driven by differences in gas concentration on each side of the membrane, and it takes place in a fraction of a second.
The Pleural Membranes
Each lung is wrapped in a double-layered membrane called the pleura. The inner layer, the visceral pleura, clings directly to the lung surface. The outer layer, the parietal pleura, lines the inside of the chest wall. Between these two layers is a thin space filled with a small amount of fluid that acts as a lubricant, letting the lungs slide smoothly against the chest wall as they expand and contract. Without this fluid, every breath would create friction between the lung surface and the ribcage. The pleural layers also help maintain the slight vacuum around the lungs that keeps them inflated.
Muscles That Power Breathing
The lungs themselves have no muscle. They’re spongy and elastic, but they can’t expand on their own. Instead, breathing depends on muscles in the chest and abdomen that change the size of the chest cavity to pull air in and push it out.
The diaphragm is the primary breathing muscle. It sits beneath the lungs like a dome-shaped floor separating the chest from the abdomen. When you inhale, the diaphragm contracts and flattens, pulling downward to create more space in the chest cavity. This slight vacuum draws air into the lungs. When you exhale, the diaphragm relaxes back into its dome shape, and the elastic lungs deflate on their own, much like a balloon left open.
During physical activity or heavy breathing, additional muscles kick in. The intercostal muscles between your ribs help expand and compress the ribcage. Your abdominal muscles assist with forceful exhalation by pushing the diaphragm upward. Muscles in the neck and around the collarbone help lift the upper chest during deep inhalation. At rest, though, quiet breathing is almost entirely the diaphragm’s job.
Built-In Defense Systems
Every breath carries potential threats: dust, bacteria, viruses, and other particles. The respiratory system has a layered defense to deal with them. The nose handles the largest particles with its hair and mucus. Deeper in the airways, a system called mucociliary clearance takes over. Goblet cells in the airway lining produce a layer of sticky mucus that traps smaller pathogens and debris. Beneath that mucus, millions of tiny hair-like structures called cilia beat in coordinated waves, sweeping the contaminated mucus upward toward the throat where it can be swallowed or coughed out.
This mucus escalator runs continuously in healthy lungs, clearing foreign material before it can reach the delicate alveoli. When the system is overwhelmed or damaged, by smoking, infection, or chronic illness, pathogens can penetrate deeper into the lungs and cause disease. The coordinated action between mucus production and cilia movement is one of the most important innate defense mechanisms in the entire body.