How we breathe: diaphragm, ribs and the pressure that moves the air

How to use this simulation

1. Press play and watch one full breath: when the diaphragm turns red and flattens and the ribs swing up and out, the pressure gauge moves below the atmosphere and air dots run down the trachea. When everything relaxes, the pressure goes above the atmosphere and the air runs out.
2. Look at the spirometer trace. Each hill is one breath, and the dashed line marks the tidal volume, the height of the hill.
3. Look at the magnified alveolus: oxygen (green) crosses the thin wall into the blood, carbon dioxide (amber) crosses the other way.
4. Move the breathing-rate and depth sliders and watch the minute ventilation reading, which is rate × tidal volume, change.
5. Choose "Exercise" to see a faster, deeper breath with muscles also working on the way out, then switch the model to the bell jar and compare it with the chest.

Count your breaths for one minute

Sit still, look at a clock and count how many times your chest rises in one minute. Do not try to control it; the moment you pay attention it tends to change, so ask a friend to count if you can. Most people at rest land somewhere between 12 and 18. You have just measured your breathing rate.

Now do the sum. If you breathe 15 times a minute and each breath moves about 500 mL of air, you move 7.5 litres of air in and out every minute. One breath takes about 4 seconds, 2 in and 2 out. That is about 21,600 breaths and 10,800 litres of air in a day, and you never once decided to do any of it. Your brain runs it in the background, like a heartbeat.

This page is about how that happens. The surprise is that your lungs have no muscles at all. They cannot pull air in. What happens instead is a mechanical trick: muscles change the size of the box the lungs sit in, and air simply follows the pressure. Once you see the trick, every exam question on this topic becomes a short chain of "so what happens next?".

Breathing and respiration are not the same thing

Textbooks use two words that look alike, and mixing them up is the most common way to lose marks on this chapter.

Breathing (also called ventilation, or in Bangla শ্বাসক্রিয়া) is the physical movement of air into and out of the lungs. It is a mechanical process done by muscles and bones, and it is what this animation shows. Respiration (শ্বসন) is a chemical process that happens inside every living cell: glucose is broken down, using oxygen, to release energy for the cell. Breathing is how oxygen reaches the blood and carbon dioxide leaves it; respiration is why the body needs oxygen at all.

The airway: from nostril to alveolus

Air follows a fixed route. It enters through the nostrils (or the mouth), passes the throat and the voice box, and goes down the trachea, the windpipe. The trachea is held open by C-shaped rings of cartilage, so it cannot collapse when the pressure inside it drops. You can feel these rings if you press gently on the front of your neck.

At the bottom, the trachea divides into two bronchi, one for each lung. Each bronchus branches again and again into narrower tubes, the bronchioles, like a tree turned upside down. The smallest bronchioles end in tiny air sacs called alveoli (singular: alveolus), clustered like bunches of grapes. There are hundreds of millions of them, and this is where the blood actually meets the air.

The lining of the nose and trachea does a second job. Hairs and sticky mucus trap dust and germs, and tiny hair-like cilia sweep the mucus up towards the throat to be swallowed or coughed out. The air is also warmed and moistened on the way, so the delicate alveoli receive clean, warm, damp air.

• Nostrils: filter with hairs and mucus, warm and moisten the air.
• Trachea: windpipe held open by cartilage rings, lined with cilia.
• Bronchi: two branches, one into each lung.
• Bronchioles: ever narrower branches.
• Alveoli: thin-walled air sacs where gases are exchanged.

The chest as a box with a movable floor

Picture the chest cavity (thorax) as a box. The back wall is the spine. The front wall is the sternum. The sides are made of ribs, twelve pairs of curved bones that join the spine at the back and, for most of them, the sternum at the front. The roof is the base of the neck. The floor is a dome-shaped sheet of muscle: the diaphragm, which separates the chest from the abdomen. In the simulation we draw six pairs of ribs to keep the picture clear.

Between each pair of neighbouring ribs run two thin layers of muscle, the intercostal muscles (inter = between, costal = ribs). The external intercostals have fibres that slope down and towards the front, like your hands slid into your pockets. The internal intercostals lie underneath with fibres sloping the other way, so the two layers form a crisscross. When the external ones contract, they pull each rib towards the one above it, and since the top ribs are held steady the whole cage swings up and out. Look for the red X marks between the ribs in the simulation.

The lungs sit inside this box, wrapped in a double-layered membrane called the pleura. A thin film of fluid between its two layers lets them slide over each other, and also makes the lung surface cling to the inside of the chest wall, like two wet glass plates that slide but do not pull apart. This is why the lung follows the chest wall when the wall moves. If air leaks into that space, the clinging is lost and the lung can collapse.

The law behind it all: Boyle's law

Robert Boyle found that for a fixed amount of gas at a constant temperature, pressure and volume are inversely related. Make the container bigger and the gas spreads out, so there are fewer molecules hitting each square centimetre of wall every second, which is lower pressure. Squeeze the container smaller and the pressure rises. Think of a bicycle pump with your finger over the nozzle: push the handle in and you feel the pressure build.

Now the second half of the idea. Air always flows from a region of higher pressure to a region of lower pressure, never the other way. So if the pressure inside your lungs is even slightly lower than the pressure of the air outside, air will flow in through the airway until the two are equal. If it is slightly higher, air flows out. That's all breathing is.

The muscles' only job is to change the volume of the chest. Pressure and flow follow automatically.

Inhalation, step by step

Inhalation (inspiration, in Bangla প্রশ্বাস, in Hindi अंतःश्वसन) is always active: it needs muscles to work. Here is the whole sequence, in the order you can watch it in the simulation.

Exhalation, step by step

Exhalation (expiration, নিঃশ্বাস, उच्छ्वसन) at rest needs no muscle work at all. It is passive. The lungs are elastic, like a stretched balloon, and once the muscles stop pulling they spring back on their own. That's why quiet breathing costs so little energy.

How big is "lower pressure"? The numbers

The atmosphere presses with about 760 mm Hg at sea level. During quiet breathing the pressure in the lungs differs from this by only about 1 mm Hg: a little under on the way in, a little over on the way out. That is why the gauge in the simulation is measured relative to the atmosphere, with zero in the middle. At the default settings of 15 breaths a minute and 500 mL, the model's gauge swings between about −1.0 and +1.0 mm Hg.

It is a tiny difference, but air is light and the airway is wide, so a tiny difference is all it takes. The pressure is largest when the air is flowing fastest, in the middle of each half-breath, and falls to zero at the very end of each, when the flow stops. Notice that at the end of inhalation the lungs are fullest but the pressure gauge reads zero. Pressure drives flow; it does not mean the lungs are full.

The bell jar model: a classic diagram

Almost every school textbook has a drawing of a bell jar with balloons inside. It is the cheapest way to see Boyle's law at work, and exam questions on it are very common. Pick "Bell jar model" in the simulation to see it move.

The parts are a one-to-one map of the human chest. A glass bell jar stands for the chest cavity. A Y-shaped glass tube through a cork at the top stands for the trachea and the two bronchi. Two balloons tied to the arms of the Y stand for the lungs. A sheet of stretched rubber closing the bottom of the jar stands for the diaphragm, with a knob in its middle you can pull.

Pull the rubber sheet down. The volume inside the jar grows, so the pressure of the air in the jar falls below the atmospheric pressure outside. The air outside, now at higher pressure than the space around the balloons, rushes in through the Y-tube and the balloons inflate. Let the sheet go back up: the volume shrinks, the pressure in the jar rises, and the balloons deflate as the air is squeezed out. In the simulation, notice that the dots of air in the jar thin out while the sheet is pulled down.

Lung volumes and the spirometer

A spirometer is an instrument that records how much air you move in and out. The trace it draws goes up with each inhalation and down with each exhalation, so a quiet breather draws a regular wave, like the one on the right of the simulation.

The height of one wave is the tidal volume: the volume of air moved in or out in one normal breath. At rest it is about 500 mL in a healthy adult. The name comes from the tides, since air washes in and out and the lungs never empty completely. In the simulation the dashed line marks the tidal volume; slide the depth control and the dashed line moves with it.

Textbooks name other volumes for a spirometer trace taken with deliberately deep breaths. The inspiratory reserve volume is the extra air you can still breathe in after a normal inhalation. The expiratory reserve volume is the extra air you can push out after a normal exhalation. The residual volume is the air that stays in the lungs even after the hardest exhalation; it keeps the alveoli from collapsing. The vital capacity is the largest volume you can move in one breath, adding the tidal volume to both reserve volumes. These deep-breath values vary a lot between people, so in this page we only use the tidal volume.

Gas exchange: what happens at the alveolus

The magnified panel shows the real purpose of all this movement. Each alveolus is a tiny sac with a wall one cell thick, wrapped in a net of capillaries whose walls are also one cell thick. The two walls are so close that gases cross between air and blood in a fraction of a second. The wall is kept moist, which lets the gases dissolve as they cross.

The crossing is by diffusion: molecules move from where they are crowded to where they are scarce. The air in the alveolus is rich in oxygen and poor in carbon dioxide. The blood arriving in the capillary has come back from the body, so it is poor in oxygen and rich in carbon dioxide. Oxygen therefore diffuses from the air into the blood, and carbon dioxide diffuses from the blood into the air. Fresh breaths keep topping up the oxygen and sweeping out the carbon dioxide, which keeps the differences large. That is the second reason breathing has to be continuous.

In the blood, oxygen is picked up by haemoglobin inside red blood cells and carried to the heart, which pumps it round the body. Most carbon dioxide travels the other way dissolved in the plasma, as bicarbonate. In the simulation the capillary turns from blue (oxygen-poor) to red (oxygen-rich) as it passes the alveolus.

Air that goes in is not the same as air that comes out. The textbook figures, all approximate, are in the table. Because the exhaled air carries about 100 times as much carbon dioxide as the inhaled air, and 5 percentage points less oxygen, the lungs clearly keep a part of what they receive.

Rate and depth: resting breathing and exercise

A healthy adult at rest breathes about 12 to 18 times a minute. Minute ventilation is the total volume of air moved in one minute. It equals the rate times the tidal volume, so that range gives 6.0 to 9.0 L/min with 500 mL breaths.

When you run up the stairs, your muscle cells respire faster. They use more oxygen and make more carbon dioxide, and the blood carries the extra carbon dioxide back to the lungs. The brain has a breathing centre in the medulla of the brain stem that watches the carbon dioxide level in the blood. When it rises, the centre sends signals faster and stronger: you breathe faster (higher rate) and deeper (higher tidal volume), and the minute ventilation shoots up. The main trigger is the rise of carbon dioxide, not the fall of oxygen, which many students expect.

The "Exercise" button in the simulation uses 25 breaths a minute and 1,500 mL, which gives 37.5 L/min. These are illustrative values chosen to make the change obvious, not a medical standard: how much a body increases breathing depends on the person and the effort.

Try these in the simulation

Each experiment takes under a minute and makes one idea stick.

Solved problems

Every answer is worked from the simulation's own model, so you can check it against the readings panel.

Common mistakes

Most lost marks on this topic come from a handful of slips. Check each one before you hand in.

• Saying the lungs pull air in. The lungs have no muscle; the diaphragm and rib muscles change the chest volume and the pressure difference moves the air.
• Writing that breathing and respiration are the same. Breathing is mechanical air movement; respiration is the release of energy from glucose in cells.
• Saying the diaphragm moves up during inhalation. It contracts, flattens and moves down.
• Saying the ribs move down and in during inhalation. They move up and out.
• Mixing up the pressure: during inhalation the lung pressure is lower than the atmosphere, not higher.
• Calling quiet exhalation active. At rest it is passive: the muscles relax and the elastic lungs recoil.
• Believing that we breathe out only carbon dioxide. Exhaled air still holds about 16% oxygen and 78% nitrogen.
• Drawing the bell jar sheet moving up when inhaling. It is pulled down.
• Forgetting units in calculations: convert mL to litres before giving minute ventilation in L/min.

Breathing in real life and health

Everything above is a normal, healthy lung. Many everyday health topics are about what happens when one link in the chain goes wrong. This is textbook-level background, not medical advice; a doctor is always the right person to ask about a real person.

Asthma is a long-term condition in which the airways become inflamed and narrow, so air is harder to move through them and breathing out is especially hard. Medicines taken from an inhaler, prescribed by a doctor, relax and open the airways.

Smoking harms the whole system. The tar and chemicals in tobacco smoke damage the cilia, so mucus and germs are no longer swept out. Over years the airways become inflamed and the thin alveolar walls can break down, leaving fewer, larger air spaces and much less surface for gas exchange. Carbon monoxide in the smoke also attaches to haemoglobin and reduces the oxygen the blood can carry.

Air pollution is a matter of daily life in many cities, including Dhaka in the dry season, when dust and smoke particles are breathed deep into the airway. Masks, cleaner fuel and less burning of waste all reduce the load on the airway.

The same idea of pressure and volume lies behind rescue breathing. Exhaled air still contains about 16% oxygen, enough that breaths given by a trained rescuer can help a person who has stopped breathing. Anyone can learn this and chest compressions in a certified first-aid course, and it is best learned from a real instructor rather than from a page. In an emergency, always call the local emergency number first.

People at high altitude breathe faster because each gulp of thin air carries less oxygen. And when you hold your breath, the urge to breathe comes mainly from carbon dioxide building up in the blood, not from running out of oxygen.

Exam corner

This topic produces predictable questions at every level. Practise these until you can write them without looking.

Definitions: breathing, respiration, inspiration, expiration, tidal volume, minute ventilation. Diagrams: the human respiratory system with trachea, bronchi, lungs, alveoli and diaphragm labelled; the bell jar model with its four parts labelled; a close-up of an alveolus with its capillary and arrows for oxygen and carbon dioxide. Always draw arrows for the direction of movement.

Short answers: why the lungs do not collapse (the pleura); why the trachea has cartilage rings; why the alveoli have thin walls and a rich blood supply; what the bell jar balloons and rubber sheet represent. Compare inhalation and expiration in a table with the rows diaphragm, ribs, chest volume, lung pressure and air movement.

Calculations: minute ventilation = rate × tidal volume (convert mL to litres), the rate from a 30-second count, and the tidal volume from a spirometer. Write the formula first, substitute the numbers with units, and give the answer with units.

For board exams in India (CBSE and NCERT), this is covered in the Class 10 Science chapter on Life Processes and in more detail in Class 11 Biology, "Breathing and Exchange of Gases", where the lung volumes (inspiratory reserve volume, expiratory reserve volume, residual volume, vital capacity) are also asked. For GCSE, IGCSE and similar courses, expect the bell jar, the gas-composition table and the effect of exercise.

One-screen revision

Read this the night before; every line is a likely question.

• Breathing is mechanical (air in and out); respiration is chemical (glucose and oxygen make energy in cells).
• Airway: nostrils, trachea, bronchi, bronchioles, alveoli.
• Inhalation: diaphragm contracts and flattens, external intercostals lift the ribs, volume up, pressure down, air in.
• Exhalation at rest: muscles relax, diaphragm rises, ribs fall, volume down, pressure up, air out.
• Forced exhalation uses internal intercostals and abdominal muscles.
• Boyle's law: pressure × volume is constant, and air flows from high to low pressure.
• Bell jar: jar = chest, Y-tube = airway, balloons = lungs, rubber sheet = diaphragm.
• Tidal volume ≈ 500 mL at rest; minute ventilation = rate × tidal volume = 15 × 500 = 7.5 L/min.
• Inhaled air ≈ 21% O₂ and 0.04% CO₂; exhaled air ≈ 16% O₂ and 4% CO₂.
• Gas exchange is by diffusion across a wall one cell thick: O₂ into the blood, CO₂ out.

Frequently asked questions