You take about 20,000 breaths every day, usually without thinking about it at all. Breathing continues while you sleep, speeds up when you run and changes when you laugh, sing or speak. With each breath, your lungs bring in oxygen, which every cell needs to produce energy, and release carbon dioxide, a waste product. This exchange happens across a surface so large and delicate that, if spread out, it would cover a tennis court. This explainer describes how breathing works, from the muscles that move air to the microscopic sacs where gases are exchanged. It explains how the body works and is not medical advice.
Why we need to breathe
Cells produce energy through a process called cellular respiration, in which they combine glucose from food with oxygen to release energy, producing carbon dioxide and water as waste. Without a constant supply of oxygen, cells cannot produce enough energy, and brain cells in particular begin to be damaged within minutes. At the same time, carbon dioxide must be removed, because it makes the blood more acidic.
The respiratory system brings oxygen into the body and removes carbon dioxide. It works closely with the circulatory system, which carries these gases between the lungs and the rest of the body.
The path of air
When you breathe in, air travels through a series of passages:
- Nose and mouth: Air enters through the nose or mouth. The nose filters, warms and humidifies the air. Tiny hairs and sticky mucus trap dust and microorganisms.
- Pharynx: The throat, shared by air and food.
- Larynx: The voice box, containing the vocal cords. A flap called the epiglottis covers the larynx during swallowing, so food goes into the esophagus rather than the windpipe.
- Trachea: The windpipe, a tube reinforced with C-shaped rings of cartilage that keep it open.
- Bronchi: The trachea divides into two main bronchi, one for each lung, which branch into smaller and smaller tubes.
- Bronchioles: Tiny airways that branch further, like the twigs of a tree.
- Alveoli: At the end of the bronchioles are clusters of tiny air sacs called alveoli, where gas exchange takes place.
The airways are lined with cells that produce mucus and tiny hair-like structures called cilia. The cilia beat in coordinated waves, moving mucus with trapped particles up toward the throat, where it can be swallowed or coughed out. This system is sometimes called the mucociliary escalator.
The lungs
The lungs are two spongy, cone-shaped organs in the chest, protected by the rib cage. The right lung has three lobes, and the left lung has two, leaving room for the heart. Each lung is surrounded by a thin double membrane called the pleura, with a small amount of fluid between its layers that lets the lungs slide smoothly as they expand and contract.
The adult lungs contain roughly 300 to 500 million alveoli, giving a total surface area of around 50 to 75 square meters, comparable to a tennis court.
How we breathe in and out
The lungs do not have muscles of their own to move air. Breathing is driven mainly by the diaphragm, a dome-shaped sheet of muscle beneath the lungs, and by the muscles between the ribs, called intercostal muscles.
Inhalation
- The diaphragm contracts and moves downward, flattening.
- The external intercostal muscles contract, lifting the ribs up and outward.
- These movements increase the volume of the chest cavity.
- As the volume increases, the pressure inside the lungs falls below the air pressure outside the body.
- Air flows into the lungs from higher to lower pressure.
Exhalation
At rest, exhaling is mostly passive:
- The diaphragm and intercostal muscles relax.
- The elastic lungs and chest wall recoil, reducing the volume of the chest cavity.
- Pressure inside the lungs rises above outside air pressure.
- Air flows out.
During exercise or forceful breathing, other muscles, including the abdominal muscles, help push air out more quickly.
You can see this mechanism with a simple model: a balloon inside a bottle with a rubber sheet stretched across the bottom. Pulling down the sheet, like the diaphragm, makes the balloon inflate.
Gas exchange in the alveoli
The alveoli are where breathing achieves its purpose. Each alveolus is surrounded by a network of tiny blood vessels called capillaries. The walls of the alveoli and capillaries are extremely thin, only one cell thick each, so gases can pass through easily.
Gas exchange happens by diffusion, the movement of molecules from an area of higher concentration to an area of lower concentration:
- Air in the alveoli contains more oxygen than the blood arriving from the body. Oxygen diffuses from the alveoli into the blood.
- Blood arriving from the body contains more carbon dioxide than the air in the alveoli. Carbon dioxide diffuses from the blood into the alveoli and is exhaled.
The alveoli are coated with a substance called surfactant, which reduces surface tension and prevents them from collapsing. Premature babies may not produce enough surfactant, which can make breathing difficult.
How blood carries oxygen
Oxygen does not dissolve well in blood plasma. Most of it is carried by hemoglobin, a protein in red blood cells that contains iron. Each hemoglobin molecule can carry four oxygen molecules. When blood reaches tissues where oxygen levels are low, hemoglobin releases oxygen to the cells.
Carbon dioxide is carried in the blood in several forms: dissolved in plasma, bound to hemoglobin and, most of all, converted into bicarbonate ions.
Air in and air out
| Component | Inhaled air | Exhaled air |
|---|---|---|
| Nitrogen | about 78 percent | about 78 percent |
| Oxygen | about 21 percent | about 16 percent |
| Carbon dioxide | about 0.04 percent | about 4 percent |
| Water vapor | variable | saturated |
Exhaled air still contains a lot of oxygen, which is why rescue breathing can help a person who is not breathing.
What controls breathing
Breathing is controlled automatically by the respiratory center in the brainstem. It adjusts the rate and depth of breathing to the body's needs, mostly in response to the level of carbon dioxide in the blood, not oxygen.
When carbon dioxide levels rise, the blood becomes slightly more acidic. Sensors called chemoreceptors in the brainstem and in major arteries detect this change and signal the respiratory center to increase breathing. This is why holding your breath becomes uncomfortable: the urge to breathe comes mainly from the buildup of carbon dioxide. Sensors also respond to very low oxygen levels, but this is usually a secondary trigger.
You can also control your breathing voluntarily for a while, for example when speaking, singing, swimming or practicing breathing exercises. But automatic control eventually takes over.
Breathing during exercise
During exercise, muscles need more oxygen and produce more carbon dioxide. Breathing becomes faster and deeper. At rest, an adult typically breathes about 12 to 20 times per minute, moving about 5 to 8 liters of air per minute. During intense exercise, this can increase to more than 100 liters per minute in trained athletes. The heart also pumps faster, delivering more oxygen-rich blood to the muscles.
Breathing at high altitude
At high altitude, the air pressure is lower, so each breath contains fewer oxygen molecules, even though oxygen still makes up about 21 percent of the air. At the top of Mount Everest, about 29,000 feet (8,849 meters), the air pressure is about one-third of that at sea level.
The body responds by breathing faster and deeper and increasing heart rate. Over days and weeks, it produces more red blood cells to carry more oxygen, a process called acclimatization. Ascending too quickly can cause altitude sickness, with symptoms such as headache, nausea and fatigue, and in severe cases dangerous fluid buildup in the lungs or brain.
Protecting the lungs
The lungs are constantly exposed to the outside air and its pollutants. Smoking, air pollution, dust and some chemicals can damage the airways and alveoli over time. Health agencies recommend avoiding smoking, reducing exposure to polluted air when possible and staying physically active to support lung health. Anyone with persistent breathing problems should see a doctor.
Questions readers ask
Which muscle is most important for breathing? The diaphragm, a dome-shaped muscle beneath the lungs, does most of the work of breathing at rest.
Where does gas exchange happen? In the alveoli, tiny air sacs in the lungs surrounded by capillaries, where oxygen enters the blood and carbon dioxide leaves.
Why do we feel the urge to breathe when holding our breath? Mostly because carbon dioxide builds up in the blood, which sensors in the brain detect.
How much air do we breathe each day? Roughly 10,000 to 11,000 liters of air for an average adult at rest, more with activity.
Why is breathing harder at high altitude? The air pressure is lower, so each breath contains fewer oxygen molecules.
In short
Breathing brings oxygen into the body and removes carbon dioxide. The diaphragm and rib muscles expand the chest, lowering pressure in the lungs so air flows in; relaxing them pushes air out. Air travels through the nose, trachea and branching bronchi to millions of alveoli, where oxygen diffuses into the blood and carbon dioxide diffuses out. Hemoglobin carries oxygen to the cells, and the brainstem adjusts breathing mainly according to carbon dioxide levels. Silent and automatic, breathing connects every cell in the body to the air around us.