Veylora
AnatomyBeginner2D simulation~15 min

Respiratory System

You take around 20,000 breaths a day without thinking about it. Each one is driven by muscles changing the size of your chest, and each one brings fresh oxygen to millions of tiny air sacs where it slips into your blood. Watch it happen, take control of the diaphragm, and see how exercise, altitude and asthma change things.

Breathing Click any part to learn about it
t = 0.0 s
Situation

12

Breaths per minute. Adults at rest: about 12–20.

500

Air the muscles try to draw in per breath.

0

Or drag the round handle on the diaphragm up and down.

Now
—
Lung volume
—
Pressure in the lungs (vs. outside air)
—
Air moved
—
Ventilation
—
Blood oxygen saturation
—
Blood carbon dioxide
—

Lung volume over time

Lung volume in litres over the last 20 seconds.

    How to use this simulator

    1. Watch the diaphragm (red curve) move down and the ribs lift as the lungs fill, and compare with the pressure readout.
    2. Look at the alveolus on the right: oxygen (red dots) moves into the blood and carbon dioxide (grey dots) moves out.
    3. Change the Situation. Watch the blood oxygen saturation, the blood carbon dioxide and the lung volume graph.
    4. Turn on Control the diaphragm yourself and drag the round handle (or use the slider) to breathe in and out by hand.
    5. Click any part of the diagram to learn what it does.

    Parts of the respiratory system

    Air enters through the nose or mouth, where it is warmed, moistened and filtered. It travels down thetrachea (windpipe), which is held open by C-shaped rings of cartilage, then into twobronchi, one for each lung. The bronchi branch again and again into narrowerbronchioles, about 23 generations of branching in total, ending in clusters ofalveoli. The lungs sit inside the rib cage, above the dome-shaped diaphragm.

    How breathing works

    The lungs have no muscles of their own. Breathing in (inspiration) happens when thediaphragm contracts and flattens, and the external intercostal muscles pull the ribs up and out. The chest gets bigger, the lungs expand with it, and the pressure inside them falls slightly below the air pressure outside. Air flows in until the pressures are equal.

    Quiet breathing out (expiration) is mostly passive: the diaphragm relaxes and domes upwards, the stretchy lungs recoil, the pressure inside rises slightly above atmospheric, and air flows out. During exercise, internal intercostal and abdominal muscles help push air out faster. The simulator shows the pressure inside the lungs: just one or two centimetres of water below or above atmospheric is enough to move half a litre of air.

    Gas exchange in the alveoli

    The two lungs contain roughly 480 million alveoli with a total surface area of about 70 m², the size of a small apartment. Each alveolus is wrapped in capillaries, and the barrier between air and blood is less than a thousandth of a millimetre thick. Gases move by diffusion, from high to low concentration (more precisely, partial pressure). Oxygen diffuses from the alveolar air into the blood and binds to haemoglobin in red blood cells; carbon dioxide diffuses from the blood into the alveoli to be breathed out.

    Breathing by the numbers

    • Tidal volume: air moved in a normal breath, about 500 mL at rest.
    • Breathing rate: about 12–20 breaths per minute in a resting adult.
    • Minute ventilation: rate × tidal volume, about 6–8 L per minute at rest and over 100 L per minute in hard exercise.
    • Dead space: about 150 mL of each breath stays in the airways and never reaches the alveoli.
    • Oxygen saturation: usually 95–100 % in healthy people at sea level.

    Exercise, altitude and asthma

    Exercise: working muscles produce much more carbon dioxide, so breathing becomes faster and deeper to remove it and to supply extra oxygen. High altitude: air is still 21 % oxygen, but the air pressure is lower, so there is less oxygen in each breath; saturation falls even though breathing speeds up, and over weeks the body adapts by making more red blood cells. Asthma: the airways become inflamed and the muscles around them tighten, raising resistance to airflow. Breathing out is hardest, so air becomes trapped and the lungs stay over-inflated.

    What controls breathing

    Breathing is controlled automatically by the medulla in the brainstem. Chemoreceptors constantly monitor carbon dioxide (and the acidity it causes) in the blood. A rise in carbon dioxide is the main stimulus to breathe harder, which is why you cannot hold your breath for very long.

    Key terms

    Ventilation
    Moving air in and out of the lungs.
    Alveoli
    Microscopic air sacs where gas exchange happens.
    Partial pressure
    The share of the total gas pressure due to one gas, such as oxygen.
    Haemoglobin
    The iron-containing protein in red blood cells that carries oxygen.

    Common misconceptions

    • “The lungs pull air in.” The diaphragm and rib muscles enlarge the chest; the lungs follow.
    • “We breathe out only carbon dioxide.” Exhaled air is still about 16 % oxygen and mostly nitrogen.
    • “Breathing and respiration are the same thing.” Breathing is ventilation; respiration is the chemical process in cells that releases energy.

    Educational model only. The values in this simulator are simplified textbook estimates. It cannot assess breathing problems, asthma or oxygen levels in a real person and is not medical advice. If you or someone else has difficulty breathing, seek medical help immediately. See our educational and medical disclaimer.

    Key takeaways

    • Breathing in: diaphragm contracts and ribs lift, the chest gets bigger, pressure falls, air flows in.
    • Gas exchange happens by diffusion across the thin walls of the alveoli.
    • Ventilation = breathing rate × tidal volume.
    • Lower air pressure (altitude) or narrowed airways (asthma) reduce how well oxygen reaches the blood.

    Model note: lung mechanics use a single resistance and compliance; gas values use the alveolar gas equation and a standard oxygen–haemoglobin curve. Real lungs vary from region to region.

    Knowledge check

    Respiratory System: test your understanding

    Answer each question to get instant feedback and an explanation. Use the simulator to check your thinking.

    8 questions3 basic3 intermediate2 advanced

    0 of 8 answered

    1. Q1BasicWhat happens to the diaphragm when you breathe in?
      Show answer

      Answer: B. It contracts, flattens and moves down

      The diaphragm contracts and flattens, making the chest cavity taller. The lungs expand with it and air flows in.

    2. Q2BasicWhere does gas exchange take place in the lungs?
      Show answer

      Answer: C. In the alveoli

      Alveoli are tiny air sacs with walls one cell thick, surrounded by capillaries, so gases can diffuse across easily.

    3. Q3IntermediateWhy does air flow INTO the lungs when the chest gets bigger?
      Show answer

      Answer: B. A bigger volume lowers the pressure inside the lungs below the outside air pressure

      Gas pressure falls as volume rises (Boyle’s law). Air moves from the higher outside pressure to the lower pressure in the lungs. The simulator shows a small negative pressure while breathing in.

    4. Q4BasicWhich feature does NOT help alveoli exchange gases efficiently?
      Show answer

      Answer: D. A thick layer of cartilage

      Cartilage is found in the trachea and bronchi to keep them open. Alveoli need thin walls, large area and good blood flow for fast diffusion.

    5. Q5IntermediateIn the simulator, why does blood oxygen saturation fall at high altitude even though you breathe faster?
      Show answer

      Answer: B. Air pressure is lower, so each breath contains fewer oxygen molecules and less oxygen diffuses into the blood

      Oxygen is still 21 % of the air, but the total pressure is lower, so the oxygen partial pressure in the alveoli falls.

    6. Q6AdvancedDuring an asthma attack, why does the lung volume baseline creep upwards in the simulator?
      Show answer

      Answer: B. Narrowed airways make breathing out slow, so not all the air leaves before the next breath (air trapping)

      Airway resistance is highest when breathing out, so each breath out is incomplete and air is trapped, leaving less room for fresh air.

    7. Q7IntermediateA person breathes 15 times a minute with a tidal volume of 500 mL. What is their minute ventilation?
      Show answer

      Answer: C. 7.5 L per minute

      Minute ventilation = rate × tidal volume = 15 × 500 mL = 7,500 mL = 7.5 L per minute.

    8. Q8AdvancedWhich gas level in the blood is the main trigger that makes you breathe faster during exercise?
      Show answer

      Answer: B. Rising carbon dioxide

      Chemoreceptors are very sensitive to rising CO₂ (which also makes blood more acidic). This drives faster, deeper breathing.

    References and further reading

    1. Betts, J. G., et al. (2022). Anatomy and Physiology 2e, Section 22.3: The Process of Breathing. OpenStax. openstax.org (opens in a new tab)
    2. Betts, J. G., et al. (2022). Anatomy and Physiology 2e, Section 22.4: Gas Exchange. OpenStax. openstax.org (opens in a new tab)
    3. West, J. B., & Luks, A. M. (2020). West’s Respiratory Physiology: The Essentials (11th ed.). Wolters Kluwer.
    4. Severinghaus, J. W. (1979). Simple, accurate equations for human blood O₂ dissociation computations. Journal of Applied Physiology, 46(3), 599–602.

    This page is an educational model, not medical advice. Found an error? Let us know. See our educational disclaimer.

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