The respiratory system is made up of two main sections: the upper respiratory tract (nose, mouth, nasal cavity, sinuses, and larynx) and the lower respiratory tract (trachea, bronchi, and lungs). Together with the muscles that power breathing and the membranes that protect the lungs, these structures move air into your body, extract oxygen, and push carbon dioxide back out.
Upper Respiratory Tract
Every breath starts in your nose or mouth. Air enters through the nostrils into the nasal cavity, a warm, moist space lined with tiny hairs and a sticky mucus layer. This lining traps dust, allergens, bacteria, and other particles before they travel deeper. The nasal cavity also heats and humidifies incoming air so it doesn’t irritate the delicate tissues further down.
Connected to the nasal cavity are the sinuses, hollow pockets inside your cheekbones and forehead. Sinuses lighten the weight of your skull, contribute to the resonance of your voice, and produce mucus that drains into the nasal passages to keep filtering air.
From the nasal cavity, air moves into the pharynx, the shared passageway at the back of your throat that handles both air and food. The pharynx funnels air downward toward the larynx, commonly called the voice box. The larynx sits at the top of your windpipe and contains the vocal cords that vibrate when you speak. It also houses a flap of tissue called the epiglottis, which snaps shut over the airway every time you swallow, preventing food and liquid from entering your lungs.
Lower Respiratory Tract
Below the larynx, air enters the trachea, or windpipe. The trachea is a tube roughly 10 to 12 centimeters long, reinforced by C-shaped rings of cartilage that keep it open the way wire rings keep a vacuum hose from collapsing. The inner wall of the trachea is lined with cilia, microscopic hair-like projections that sweep mucus and trapped debris upward toward the throat, where you swallow or cough it out.
At its base, the trachea splits into two bronchi, one leading to each lung. These primary bronchi branch into smaller and smaller tubes inside the lungs, forming what’s often called the bronchial tree. The bronchial tubes divide into thousands of thinner passages called bronchioles. Each round of branching produces narrower tubes with thinner walls, gradually transitioning from rigid cartilage-supported airways to flexible, muscle-wrapped channels that can widen or tighten to control airflow.
Alveoli: Where Gas Exchange Happens
At the very tips of the bronchioles sit the alveoli, tiny air sacs clustered like bunches of grapes. This is where the real work of breathing takes place. Oxygen passes through the ultra-thin walls of the alveoli into surrounding capillaries, entering the bloodstream. At the same time, carbon dioxide moves in the opposite direction, from the blood into the alveoli, ready to be exhaled.
The numbers here are striking. Your lungs contain roughly 300 million alveoli, and their combined surface area measures about 118 square meters, roughly the size of a singles tennis court. That enormous surface, packed into a pair of organs that fit inside your rib cage, is what makes gas exchange efficient enough to keep every cell in your body supplied with oxygen.
The Pleura: Protective Wrapping
Each lung is enclosed in a double-layered membrane called the pleura. The inner layer, the visceral pleura, clings directly to the surface of the lungs, blood vessels, and bronchi. The outer layer, the parietal pleura, attaches to the inside of the chest wall. Between these two layers sits a thin film of pleural fluid. This fluid acts as a lubricant, letting the layers slide smoothly against each other every time you inhale and exhale. Without it, the friction of roughly 20,000 breaths a day would cause constant irritation.
One notable difference between the layers: the parietal pleura contains sensory nerves and is sensitive to pain, while the visceral pleura does not. That’s why conditions that inflame the outer layer, like pleurisy, can produce sharp chest pain with every breath.
Muscles That Power Breathing
Your lungs can’t move on their own. They depend on surrounding muscles to create the pressure changes that pull air in and push it out.
The diaphragm is the primary breathing muscle, a dome-shaped sheet of muscle sitting beneath the lungs and separating the chest cavity from the abdomen. When you inhale, the diaphragm contracts and flattens downward, expanding the chest cavity. That expansion creates a slight vacuum inside the lungs, and air rushes in to fill it. When you exhale during quiet breathing, the diaphragm simply relaxes back into its dome shape, and air flows out passively.
The intercostal muscles, layered between your ribs, assist the diaphragm. The external intercostals pull the ribs upward and outward during inhalation, further expanding the rib cage and increasing the volume inside the lungs. The internal intercostals do the opposite: they contract to shrink the rib cage during active exhalation. A deeper set, the innermost intercostals, supports the internal layer when you need to force air out, like during a cough, a sigh, or intense exercise.
During a normal resting breath, you move about 500 milliliters of air (roughly half a liter) in and out. This is called your tidal volume. During heavy exercise or a deep breath, additional muscles in the neck and abdomen kick in to increase that volume significantly.
Beyond Breathing: pH Balance
The respiratory system does more than deliver oxygen. It plays a critical role in keeping your blood at the right pH, which hovers around 7.4. The mechanism is surprisingly simple: carbon dioxide dissolved in your blood reacts with water to form an acid. The more carbon dioxide in your blood, the more acidic it becomes. The less carbon dioxide, the more alkaline.
Your brain monitors blood pH through specialized sensors in the brainstem and in blood vessels near the heart. If your blood becomes too acidic, these sensors signal your respiratory centers to speed up and deepen breathing. Faster breathing expels more carbon dioxide, which reduces acidity and brings pH back toward normal. If your blood drifts too alkaline, the opposite happens: breathing slows and becomes shallower, allowing carbon dioxide to accumulate and lower the pH. This feedback loop runs continuously and adjusts within seconds, making it one of the fastest regulatory systems in the body.
How All the Parts Work Together
Air enters through the nose or mouth, gets filtered, warmed, and humidified in the nasal cavity, then travels through the pharynx and larynx into the trachea. The trachea funnels it into the bronchi, which branch into progressively smaller bronchioles, ending at the alveoli. There, oxygen crosses into the bloodstream while carbon dioxide crosses out. The diaphragm and intercostal muscles drive the entire cycle by changing the volume of the chest cavity, and the pleura ensures the lungs can expand and contract without friction.
Each part has a specific job, but they function as a single integrated chain. A problem at any point, whether it’s nasal congestion restricting airflow, inflamed bronchioles narrowing the passages (as in asthma), or damaged alveoli reducing gas exchange surface area (as in emphysema), affects the efficiency of the whole system.