Respiratory system · ventilation and gas exchange
Lungs — 20,000 breaths a day
We usually breathe 12–20 times a minute — about 20,000 times a day, mostly without giving it a thought. See how the diaphragm draws air in, how the airways branch as many as 23 times, how oxygen crosses a barrier about 200 times thinner than a sheet of paper, and what actually makes us take a breath.
An educational page — not a substitute for medical advice. The values shown are typical for a healthy adult; in reality they depend on height, sex, age and fitness. The models are simplified.
The lungs have no muscles of their own. When you inhale, the diaphragm contracts (it flattens and moves down) and the external intercostal muscles lift the ribs. The chest gets bigger, and the lungs — “stuck” to it by a thin film of fluid in the pleural cavity — stretch along with it. Pressure in the alveoli drops below atmospheric pressure, and air flows in by itself. Exhaling at rest is passive: the muscles relax and the elastic lungs shrink back like a stretched rubber band. On the right is the same mechanism in the classic bell-jar model — grab the rubber membrane and pull it down.
Grab the membrane handle (or the diaphragm) and pull it down: the volume of the jar grows, the pressure inside drops (Boyle’s law), and the balloons inflate by themselves.
Inhaling — muscle work
The diaphragm does most of the work in quiet breathing. In deep breathing, accessory muscles (e.g. the sternocleidomastoid and scalenes) help by lifting the sternum and upper ribs.
Exhaling — elasticity
At rest, exhaling takes no muscle work. During exercise, coughing or blowing, the internal intercostal muscles and the abdominal muscles join in.
Boyle’s law
At constant temperature p · V = const: as lung volume grows, the pressure of the gas inside falls. A difference of about 1 cmH₂O — roughly 0.1% of atmospheric pressure — is enough to draw in about half a liter of air at rest.
The trachea splits into two main bronchi, each of those splits into two again — and so on. In Weibel’s classic model there are 23 such divisions on average: generations 0–16 only conduct air, while in generations 17–23 alveoli appear and gas exchange takes place. Move the slider and watch the number of airways and their total cross-section grow.
The model is idealized: in a real lung the branching is asymmetric, so different paths have different numbers of generations. The drawing clearly shows the first 11 generations — the later ones are too small.
Spread the alveoli on a tennis court
An escalator made of cilia
Surfactant vs. Laplace’s law
Experiment
Pressure in a spherical alveolus: P = 2T / r (T — surface tension, r — radius). With the same T, the smaller alveolus has the higher pressure and “pushes” its air into the larger one.
T in mN/m. Surfactant is made by type II pneumocytes. Premature babies born before it fully matures can develop neonatal respiratory distress syndrome — they are then given surfactant into the trachea.
Oxygen and carbon dioxide aren’t “pumped” — they cross the thin barrier between the alveolus and the capillary on their own, by diffusion, always from higher to lower partial pressure. The rate is described by Fick’s law: it grows with the area A and the pressure difference Δp, and falls with the barrier thickness d. Change the conditions and see whether a red blood cell manages to “load up” with oxygen before it leaves the capillary.
A red blood cell in a capillary — O₂ pressure over time
Thin line — a healthy person at rest at sea level (for comparison). Vertical line — the moment the red cell leaves the capillary. Oxygen binds to hemoglobin, so the red cell “soaks up” a lot of it before its O₂ pressure rises.
Readings
A spirometer measures how much air we breathe in and out. Hold the inhale button (or the space bar) to breathe in; let go and you breathe out on your own. The second button is a forced exhale. Try it: take the deepest breath you can, then blow out everything until nothing more comes out — you’ll measure your virtual vital capacity. Notice that some air can never be blown out.
A model of a “virtual adult” with typical volumes. A real spirometer can’t see the residual air — it only measures what flows through the mouthpiece.
Lung volumes and capacities
- TV — tidal volume
- IRV — inspiratory reserve
- ERV — expiratory reserve
- RV — residual volume
- IC — inspiratory capacity
- FRC — functional residual capacity
- VC — vital capacity
- TLC — total lung capacity
Values typical for an adult man of average height; in women usually about 20–25% smaller, and they also change with age and height.
Dead space: shallow and fast, or deep and slow?
About 150 mL of every breath stays in the trachea and bronchi (roughly 2 mL per kilogram of body weight) and never reaches the alveoli. So what counts is alveolar ventilation: V̇A = (VT − VD) · f.
Blow out the candles
First take a deep breath (hold “inhale”), then blow with the “forced exhale” button.
Blow result
A sketch of the idea behind the test. In real spirometry you blow out as fast as you can after a maximal breath in; healthy people usually blow out more than 70% of the total in the 1st second. When the airways are narrowed (e.g. in asthma), the air comes out more slowly.
The breathing rhythm is set by the respiratory center in the medulla oblongata (in the brainstem) and sent through the phrenic nerves to the diaphragm. The strongest drive to breathe is not a lack of oxygen but an excess of CO₂: CO₂ acidifies the cerebrospinal fluid, which is sensed by the central chemoreceptors. A drop in oxygen is sensed mainly by the carotid bodies — but they respond clearly only to severe oxygen deprivation. Hold your breath in the simulation and watch what rises and what falls.
A schematic model with approximate values (an adult at rest). Breath-hold times and the moment of the “breaking point” vary a lot from person to person.
Important: never hyperventilate before diving
Fast, deep breathing “washes out” CO₂ but barely increases your oxygen reserve. Underwater, the urge to breathe then comes too late — oxygen can fall so low that a person passes out without warning (so-called shallow water blackout) and drowns. Never practice breath-holding in water without supervision.
Hiccups
A hiccup is a sudden, involuntary contraction of the diaphragm, followed a split second later by the glottis snapping shut — hence the familiar “hic”. It usually goes away on its own.
Yawning
We still don’t know exactly why we yawn. The popular “lack of oxygen” theory didn’t hold up in studies — breathing air with extra CO₂ or pure oxygen didn’t change how often people yawned.
How does it work?
The respiratory system consists of the airways (nose, throat, larynx, trachea, bronchi) and the lungs. The right lung has three lobes, the left has two. Air is warmed, humidified and cleaned in the conducting airways, and gas exchange happens only in the hundreds of millions of alveoli, wrapped in a network of capillaries.
Ventilation
Inhaling is active (diaphragm, external intercostal muscles); quiet exhaling is passive. At rest, the pressure in the alveoli changes by only about ±1 cmH₂O, while the pleural cavity stays at negative pressure: about −5 cmH₂O at the end of an exhale and about −7 to −8 cmH₂O at the peak of an inhale.
Alveoli and surfactant
Gas exchange
Control
The respiratory center in the medulla oblongata sets the rhythm, and the chemoreceptors fine-tune it so that CO₂ in arterial blood stays at about 40 mmHg. That’s why we breathe mainly to get rid of CO₂ — oxygen usually comes along “for free” in plenty.
Simplifications: lung mechanics is a model with a single compliance (about 200 mL/cmH₂O) and a single airway resistance; the bell jar and the chest share the same controls. Pressure at altitude is computed with a barometric formula fitted to measurements in high mountains (J. B. West), and alveolar CO₂ in an acclimatized climber comes from an approximate table. The hemoglobin oxygen-binding curve is the Hill equation. We model surfactant as a surface tension that decreases with the square of the radius. The breath-hold curves are illustrative, not exact physiology.
See also
Did you know…?
- The right lung has three lobes, while the left has only two and a distinct notch — the left side needs room for the heart.
- With every breath out we lose water vapor — usually a few hundred milliliters of water a day (about 300–400 mL, more during exercise and in dry, cold air). That’s why you can “see your breath” in winter.
- Healthy lung tissue floats on water, because some air always stays in the alveoli. Even after the deepest exhale, about 1.2 liters remain in the lungs (the residual volume).
Challenges
- Inflate the balloons in the bell jar“How we breathe” mode: pull the membrane hard enough that at least 900 mL of air flows into the balloons.
- Count the bronchial generations down to the alveoli“Bronchial tree” mode: go all the way down from the trachea to the alveolar sacs (generation 23).
- Climb Mount Everest“Gas exchange” mode: set the altitude of the summit (8,849 m) and see how oxygen exchange drops.
- Measure your vital capacity“Spirometer” mode: the deepest breath in and a full breath out — a result of at least 4,400 mL.
- Beat Laplace’s law with surfactant“Bronchial tree” mode: connect two alveoli of clearly different sizes so that neither collapses and their sizes even out.
Keys
- Q W E R T — switch modes
- How we breathe: P pause breathing, O puncture / seal the pleura, ↑ ↓ membrane (when the drawing has focus)
- Bronchial tree: + − generation (also ← → on the drawing), U spread out the alveoli
- Gas exchange: 1–5 altitude: sea level, Rysy (Poland’s highest peak), La Paz, Mont Blanc, Everest
- Spirometer: hold Space or ↑ — inhale, ↓ — forced exhale
- What controls breathing: B hold your breath / breathe
- Esc — pauses / resumes the current mode’s animation (in the spirometer it also releases the buttons)