The pulmonary system is the network of organs, airways, blood vessels, and muscles that moves air into and out of your body and delivers oxygen to your blood. It starts at your nose and mouth and ends deep inside your lungs, where hundreds of millions of tiny air sacs exchange oxygen for carbon dioxide with every breath. Beyond breathing, the system also defends against inhaled particles, helps regulate your blood’s pH, and works in lockstep with your heart to keep every organ supplied with oxygen.
Structures of the Pulmonary System
Air enters through your nose or mouth, passes through the throat, and reaches the larynx (voice box) at the top of the trachea, or windpipe. The trachea is a tube reinforced by C-shaped rings of cartilage that keep it open. After about 10 to 12 centimeters, the trachea splits into two main bronchi, one for each lung.
Inside the lungs, each main bronchus divides into smaller lobar bronchi (three on the right, two on the left, matching the lobes of each lung), then into even smaller segmental bronchi, and eventually into thousands of tiny tubes called bronchioles. At the end of the bronchioles sit clusters of alveoli, the microscopic air sacs where gas exchange actually happens. You have roughly 300 million alveoli, and their combined surface area falls somewhere between 70 and 140 square meters, roughly the floor space of a small apartment. That enormous surface is packed into a pair of organs that together hold only about 4 to 6 liters of air.
How Breathing Works
Breathing depends on pressure changes created by muscles. Inhalation is the active phase: your diaphragm, a dome-shaped muscle beneath the lungs, contracts and flattens, while the intercostal muscles between your ribs pull the rib cage upward and outward. This expands the chest cavity, drops the air pressure inside your lungs below atmospheric pressure, and air rushes in.
Exhalation at rest is mostly passive. The diaphragm relaxes, the elastic tissue of the lungs naturally recoils, and the chest cavity shrinks. This raises pressure inside the lungs above atmospheric pressure, pushing air back out. During exercise or heavy breathing, abdominal muscles and internal intercostal muscles contract to force air out more quickly.
A healthy adult at rest breathes 10 to 20 times per minute. Children breathe faster: a one-year-old typically takes 26 to 60 breaths per minute, while teenagers settle into an adult-like range of 12 to 22. Newborns breathe the fastest, at 30 to 60 breaths per minute.
Gas Exchange in the Alveoli
The entire purpose of moving air in and out is to get oxygen into the bloodstream and carbon dioxide out. This happens in the alveoli through simple, passive diffusion. There is no active pumping involved, and the process requires no energy from your body. Oxygen moves from the air inside each alveolus across a thin membrane (only about one micrometer thick in places) into the surrounding capillaries, while carbon dioxide moves in the opposite direction.
Several factors determine how efficiently this exchange occurs: the thickness of the membrane between air and blood, the total surface area available, and the solubility of the gas. Carbon dioxide is about 20 times more soluble in the membrane tissue than oxygen, which is why your body can offload carbon dioxide efficiently even when lung function is somewhat compromised. Oxygen transfer is more sensitive to damage. Conditions that thicken the membrane or destroy alveoli, like pulmonary fibrosis or emphysema, reduce diffusion and make it harder to get enough oxygen into the blood.
Pulmonary Circulation
The pulmonary system has its own dedicated blood circuit, separate from the circulation that supplies the rest of your body. The right ventricle of your heart pumps oxygen-poor blood into the main pulmonary artery, a short vessel only about 5 centimeters long before it splits into right and left branches heading to each lung. These arteries branch into progressively smaller vessels until they form a dense web of capillaries wrapped around the alveoli.
Pulmonary arteries are built differently from the arteries in the rest of your body. Their walls are about one-third the thickness of comparable systemic arteries, and they have a larger diameter. This makes them more flexible and allows them to operate at much lower pressures, which is important because the blood only needs to travel a short distance through the lungs. After picking up oxygen and releasing carbon dioxide, the blood flows into pulmonary venules, then into larger pulmonary veins, and finally back into the left atrium of the heart, ready to be pumped to the rest of the body.
How Your Brain Controls Breathing
You rarely think about breathing because a cluster of neurons in the lower brainstem handles it automatically. This respiratory center adjusts your breathing rate and depth from moment to moment based on feedback from chemical sensors. The most important signal is the level of carbon dioxide in your blood. Specialized cells in the brainstem detect rising carbon dioxide concentrations and respond by increasing both the rate and depth of breathing to expel the excess.
This system ties directly into your body’s acid-base balance. Carbon dioxide dissolved in blood combines with water to form carbonic acid, which releases hydrogen ions and lowers pH. By breathing faster, you exhale more carbon dioxide, reduce the amount of acid forming in the blood, and bring pH back up. By breathing more slowly, you retain carbon dioxide and allow pH to drop. This respiratory adjustment can kick in within minutes, making it the body’s fastest tool for correcting pH imbalances. The kidneys handle longer-term pH correction, but the lungs are the first responders.
Built-In Defense Mechanisms
Every breath carries potential threats: dust, pollen, bacteria, viruses, and other particles. The pulmonary system has a layered defense to deal with them. The nose filters and warms incoming air. The branching structure of the airways forces air to change direction repeatedly, which causes heavier particles to collide with mucus-coated walls and stick there.
The primary defense deeper in the airways is mucociliary clearance. The lining of the airways is covered in a thin layer of mucus that traps inhaled particles and pathogens. Beneath that mucus, millions of microscopic hair-like structures called cilia beat in coordinated waves, pushing the contaminated mucus upward toward the throat at a steady pace. Once it reaches the throat, you swallow it or cough it out, usually without noticing. A thin, watery layer beneath the mucus keeps the cilia lubricated so they can beat freely.
When this clearance system fails, the consequences are significant. People born with defective cilia (a condition called primary ciliary dyskinesia) develop chronic lung infections and progressive lung damage because trapped particles and pathogens simply sit in the airways instead of being swept out.
Three Categories of Lung Disease
Diseases that affect the pulmonary system generally fall into three broad categories, though many conditions involve a combination.
- Airway diseases narrow or block the tubes that carry air. Asthma and COPD are the most common examples. People with these conditions often describe the sensation as trying to breathe out through a straw.
- Lung tissue diseases damage the structure of the lungs themselves, typically through scarring or inflammation. This prevents the lungs from fully expanding, a pattern called restrictive lung disease. Pulmonary fibrosis is one example. People with these conditions often feel as though they’re wearing a too-tight vest and can’t take a deep breath.
- Lung circulation diseases affect the blood vessels within the lungs. Clotting, scarring, or inflammation in those vessels impairs gas exchange and can strain the heart. Pulmonary hypertension is the most recognized example. Shortness of breath during physical activity is the hallmark symptom.
Because the pulmonary system involves airways, lung tissue, and blood vessels working together, damage to any one component ripples through the others. A circulation problem reduces how much oxygen reaches the blood even if the airways and alveoli are healthy. An airway disease traps stale air in the lungs and limits how much fresh oxygen reaches the alveoli. Understanding which component is affected is the starting point for treatment.