The respiratory system is made up of two groups of structures: the upper respiratory tract (nose, throat, and voice box) and the lower respiratory tract (windpipe, bronchial tubes, and lungs). Together, these organs form a continuous airway that brings oxygen into the body and removes carbon dioxide. Supporting the whole system are the breathing muscles, a layer of protective mucus, immune cells, and a network of nerves that keep you breathing without conscious effort.
The Upper Respiratory Tract
The upper tract includes everything from your nostrils down to your voice box. Air enters through the nasal cavity, where small hairs and bony ridges called turbinates create turbulence that forces incoming air to swirl. This turbulence isn’t random. It helps trap dust, pollen, and other particles while warming and humidifying the air before it travels deeper. Your mouth serves as a backup entrance when nasal airflow isn’t enough, like during heavy exercise.
From the nasal cavity, air passes through the pharynx, the shared passageway behind your nose and mouth that connects to both the esophagus and the airway below. The pharynx funnels air downward into the larynx, a small structure built from nine cartilages. The larynx houses your vocal folds, which vibrate to produce sound when you speak. It also contains the epiglottis, a flap of cartilage that snaps shut over the airway every time you swallow, preventing food and liquid from entering the lungs.
The Lower Respiratory Tract
The lower tract begins at the trachea, or windpipe, a tube reinforced by 16 to 20 C-shaped rings of cartilage stacked on top of one another. These rings are rigid enough to keep the airway open during exhaling but flexible enough to let the trachea narrow slightly when you swallow, giving the esophagus behind it room to expand around food. The inside of the trachea is lined with tiny hair-like projections called cilia, which constantly sweep mucus and trapped debris upward toward the throat so you can swallow or cough it out.
At its base, the trachea splits into two main bronchi, one entering each lung. The right bronchus branches into three smaller passages (serving the right lung’s three lobes), and the left branches into two (serving the left lung’s two lobes). These secondary bronchi keep dividing into progressively smaller tubes. By the time the branching reaches its finest level, the tubes are called bronchioles, and they no longer contain cartilage. Instead, smooth muscle controls their diameter, widening or narrowing them to regulate airflow.
At the very end of each bronchiole sit clusters of tiny air sacs called alveoli. This is where the actual work of breathing happens. Oxygen passes through the paper-thin walls of the alveoli into surrounding capillaries, while carbon dioxide moves in the opposite direction to be exhaled. The walls separating alveolar air from capillary blood are extraordinarily thin, and each alveolus is wrapped in a dense mesh of blood vessels. Estimates of the total surface area available for gas exchange vary, but most fall between 70 and 130 square meters. That is roughly half the area of a tennis court, all folded and packed inside your chest.
The Breathing Muscles
Your lungs cannot inflate on their own. They depend on muscles that change the size of the chest cavity to create pressure differences that pull air in and push it out. The primary muscle is the diaphragm, a dome-shaped sheet that sits below the lungs and separates the chest from the abdomen. When you inhale, the diaphragm contracts and flattens downward, expanding the chest cavity. Your lungs stretch to fill the extra space, and air rushes in.
The intercostal muscles, located between your ribs, assist by contracting to pull the rib cage upward and outward during inhalation. When you exhale, both the diaphragm and the intercostal muscles relax. The chest cavity shrinks, the lungs deflate, and air flows out, similar to releasing a pinched balloon. Under normal resting conditions, exhaling is largely passive. During exercise or heavy breathing, additional muscles in the abdomen and neck kick in to speed up both phases.
How the Brain Controls Breathing
You don’t have to think about breathing because a cluster of nerve cells in the brainstem handles it automatically. The respiratory center sits in the medulla oblongata, the lowest part of the brainstem, and manages the minute-to-minute rhythm of inhalation and exhalation. A neighboring region in the pons fine-tunes the transitions between breathing in and breathing out, helping to keep the rhythm smooth.
The system adjusts itself based on chemistry. Specialized sensor cells in the brainstem detect rising levels of carbon dioxide in the blood, which makes the blood slightly more acidic. When CO2 climbs, these sensors signal the breathing muscles to work harder, increasing the rate and depth of each breath until CO2 drops back to normal. This is why you breathe faster during exercise and why holding your breath eventually becomes unbearable. The urge to inhale is driven primarily by CO2 buildup, not by a lack of oxygen.
Built-In Defense Systems
Every breath carries potential threats: bacteria, viruses, dust, and chemical irritants. The respiratory system has layered defenses to deal with them. The first line is the mucociliary escalator, a continuous conveyor belt made of sticky mucus and cilia. Specialized cells lining the airways secrete gel-like mucus that traps inhaled particles. The cilia beneath this mucus layer beat in coordinated waves, pushing the contaminated mucus upward toward the throat at a steady pace. From there, you swallow it (usually without noticing) and stomach acid neutralizes whatever it carried.
Particles small enough to slip past the mucus and reach the alveoli encounter a second line of defense: immune cells called alveolar macrophages. These cells patrol the air sacs, engulfing and digesting bacteria, fungal spores, and fine particulate matter. Under normal conditions, the mucociliary escalator and macrophages together clear inhaled contaminants effectively and quietly, without triggering the kind of inflammation you would feel as symptoms. One mucus protein in particular, known as Muc5b, plays a central role in routine microbial clearance within the lungs.
Beyond Breathing: Other Roles
The respiratory system does more than exchange gases. By adjusting how much carbon dioxide you exhale, it helps regulate blood pH. Carbon dioxide dissolved in blood forms an acid, so breathing faster lowers acidity and breathing slower raises it. This makes the lungs a rapid-response partner to the kidneys in keeping blood chemistry stable.
The nasal cavity also houses olfactory nerve endings responsible for your sense of smell. And the larynx, beyond protecting the airway, is your primary instrument for speech. The vocal folds vibrate as air passes over them, producing sound waves that your mouth and tongue then shape into words. Even the simple act of sighing or laughing depends on precise coordination between the breathing muscles and the structures of the upper airway.
Lung Capacity at a Glance
A healthy adult’s lungs hold about 6 liters of air at maximum inflation. You never use all of that in a normal breath. A resting breath moves roughly half a liter in and out, a volume called tidal volume. Even after the deepest possible exhale, about a liter of air remains trapped in the lungs to keep the alveoli from collapsing completely. During intense physical activity, you tap into your reserve capacity, breathing deeper and faster to move several liters per breath, but you still never fully empty or fully fill the lungs.