The diaphragm moves down and the ribs lift, so the chest gets bigger and air flows in (blue). Then they relax and air flows out (purple). Press “Follow an oxygen molecule” to trace one breath all the way into the blood.
Rotate the body and click any part — or use the colored legend — to see its job. Air travels nose → pharynx → larynx → trachea → bronchi → bronchioles → alveoli, where oxygen passes into the blood. The diaphragm and rib muscles do the pumping; the lungs themselves have no muscle to inflate.
Every cell in your body releases energy from food by cellular respiration, which uses oxygen and produces carbon dioxide. The job of the respiratory system is to bring fresh air deep into the body, let oxygen pass into the blood and let carbon dioxide leave it. That swap of gases is called gas exchange, and it happens in millions of tiny air sacs called alveoli. Breathing (ventilation) is just the pumping that moves air in and out so the swap can keep going.
Air enters the nose, where it is filtered, warmed and moistened, then passes the pharynx (throat) and the larynx (voice box) into the trachea, a tube held open by C-shaped rings of cartilage. The trachea splits into two bronchi, one for each lung, and these branch again and again — about 23 times in all — into ever narrower bronchioles. The last branches end in grape-like clusters of alveoli. Along the way, mucus traps dust and microbes and tiny hairs called cilia sweep it back up to the throat.
The lungs have no muscle of their own. To breathe in, the diaphragm contracts and flattens downward and the external intercostal muscles lift the ribs up and out. The chest gets bigger, so the lungs, which are stuck to the chest wall by the pleural fluid, stretch with it. By Boyle’s law ($pV = \text{constant}$), a bigger volume means a lower pressure: air pressure in the lungs drops a little below the air outside, and air flows in. To breathe out quietly, the muscles simply relax, the elastic lungs recoil, the pressure rises slightly above outside pressure, and air flows out.
Gas exchange works by diffusion, which is fastest across a big, thin surface with a steep difference in concentration. The lungs have all three: about 480 million alveoli giving roughly 70 m² of surface (a large part of a tennis court), a barrier between air and blood only about 0.5 µm thick, and a dense net of capillaries whose flowing blood keeps carrying oxygen away and bringing carbon dioxide in. Breathing keeps renewing the air, so the gradient never runs down.
Watch the Lung vs outside air readout: it goes slightly negative while you breathe in and positive while you breathe out — that pressure difference is what moves the air. Switch to Exercise and compare Air per minute with Rest (about 6 L/min). Press Follow an oxygen molecule and see it turn red as it is picked up by blood in an alveolus. Turn off the rib cage and rotate to see how the right bronchus is steeper than the left, and why the left lung has a notch for the heart.
| Part | What it is | Main job | Key fact |
|---|---|---|---|
| Nose & nasal cavity | Entrance lined with hairs and mucus | Filters, warms and moistens air | Mouth breathing skips this |
| Pharynx | The throat | Passage for both air and food | Shared with the digestive system |
| Larynx | Cartilage box with vocal cords | Makes sound; epiglottis guards the airway | “Adam’s apple” is its front |
| Trachea | Windpipe | Carries air to the chest | 10–12 cm long, 16–20 C-shaped rings |
| Bronchi | Two main branches | Take air into each lung | Right one is wider and steeper |
| Bronchioles | Small airways, no cartilage | Carry air deep into the lung; control airflow | Smooth muscle narrows them in asthma |
| Alveoli | Tiny air sacs, wall one cell thick | Gas exchange with the blood | ~480 million, ~70 m² |
| Lungs | Two spongy, elastic organs in pleura | House the airways and alveoli | Right: 3 lobes · Left: 2 lobes |
| Diaphragm | Dome of muscle under the lungs | Main muscle of breathing in | Contracts → flattens → air in |
| Ribs & intercostals | Rib cage and the muscles between ribs | Protect the chest; lift ribs to help breathing | Used more in deep and fast breathing |
| Breathing in (inspiration) | Breathing out (expiration, at rest) | |
|---|---|---|
| Diaphragm | Contracts, flattens, moves down | Relaxes, domes back up |
| External intercostals | Contract — ribs move up and out | Relax — ribs move down and in |
| Chest volume | Increases | Decreases |
| Pressure in lungs | Falls slightly below outside air | Rises slightly above outside air |
| Air flow | In | Out |
| Energy | Active (muscles work) | Passive at rest (elastic recoil) |
| Gas | Inhaled air | Exhaled air |
|---|---|---|
| Oxygen (O₂) | ~21% | ~16% |
| Carbon dioxide (CO₂) | ~0.04% | ~4% |
| Nitrogen (N₂) | ~78% | ~78% (unchanged) |
| Water vapour | Varies | Saturated |
| Temperature | Room temperature | Close to body temperature |
At rest an adult breathes about 12–20 times a minute, moving about 0.5 L each time (the tidal volume) — around 6–8 L per minute. The biggest breath out after the biggest breath in, the vital capacity, is about 4–5 L, and the lungs can hold about 6 L in total. Even after breathing out as hard as you can, about 1–1.5 L stays behind (the residual volume), which keeps the alveoli from collapsing. In hard exercise, breathing can rise above 100 L per minute.
Breathing is automatic, run by the brainstem (the medulla and pons), which sends rhythmic signals down the phrenic nerves to the diaphragm. The main trigger to breathe more is not low oxygen but rising carbon dioxide: extra CO₂ makes the blood more acidic, and sensors in the brainstem and in the arteries speed up and deepen breathing. That is why you can only hold your breath for a limited time.
The respiratory system is the group of organs that brings air into the body and lets oxygen pass into the blood while carbon dioxide passes out. It includes the nose, pharynx, larynx, trachea, bronchi, bronchioles and alveoli inside the lungs, plus the diaphragm and rib muscles that pump air in and out.
Key takeaway: airways carry air to the alveoli; muscles pump it; the alveoli swap gases with the blood.Air flows nose (or mouth) → pharynx → larynx → trachea → left or right bronchus → smaller bronchi → bronchioles → alveoli. Breathing out follows the same path in reverse.
Key takeaway: nose, pharynx, larynx, trachea, bronchi, bronchioles, alveoli.The diaphragm is a dome of muscle under the lungs. When it contracts it flattens and moves down, making the chest cavity bigger. The pressure inside the lungs falls slightly below the air outside, so air rushes in. When it relaxes it springs back up, the chest gets smaller, the pressure rises and air flows out.
Key takeaway: diaphragm down → bigger chest → lower pressure → air in.Gas exchange happens in the alveoli. Oxygen diffuses from the air in the alveolus, where its concentration is high, across a wall about 0.5 micrometres thick into the blood in the surrounding capillaries, where it is low. Carbon dioxide diffuses the opposite way. About 480 million alveoli give a surface of roughly 70 square metres, which makes this fast.
Key takeaway: diffusion across a huge, very thin, well-supplied surface in the alveoli.Breathing (ventilation) is the physical movement of air in and out of the lungs. Cellular respiration is the chemical reaction inside every cell that uses oxygen to release energy from glucose, producing carbon dioxide and water. Breathing supplies the oxygen for respiration and removes the carbon dioxide it makes.
Key takeaway: breathing moves air; respiration releases energy in cells.Working muscles respire faster, so they use more oxygen and make more carbon dioxide. The rising carbon dioxide makes the blood slightly more acidic, which the brainstem detects, so it makes you breathe faster and deeper. Air per minute can rise from about 6 litres at rest to more than 100 litres in hard exercise.
Key takeaway: more CO₂ → brainstem speeds and deepens breathing.The heart sits slightly to the left in the chest, so the left lung is a little smaller and has a hollow, the cardiac notch, where the heart fits. The right lung has upper, middle and lower lobes; the left lung has only upper and lower lobes. Each lobe has its own bronchus and blood supply.
Key takeaway: the left lung makes room for the heart.The bigger airways must stay open as the pressure inside changes during breathing, so C-shaped rings or plates of cartilage stop them collapsing. The open part of each C faces the esophagus so food can pass behind it. The bronchioles are tiny and are held open by the elastic lung around them; instead of cartilage they have smooth muscle that can widen or narrow them to control airflow — which is why they narrow in asthma.
Key takeaway: cartilage keeps the big airways open; smooth muscle controls the small ones.