How You Breathe (Gas Exchange)
Every breath is a swap driven by pressure. Your diaphragm drops and your ribs swing up and out, so air rushes into ~300 million air sacs called alveoli. There, oxygen (O₂) diffuses downhill into the blood while carbon dioxide (CO₂) diffuses the other way out. Watch the live pO₂ / pCO₂ meters (in mmHg) and the lung-volume trace as red blood cells load O₂ onto hemoglobin and flip from bluish to bright red. Push the Respiration rate slider, hit Exercise to breathe faster and deeper, or Hold breath and see the numbers drift.
What's happening
The Science in Plain Language
1. Breathing is a pump, not a magic suck. Your diaphragm — a dome of muscle under the lungs — contracts and flattens downward, while the rib cage swings up and out. Together they enlarge the chest, so the pressure inside drops below the air outside and air flows in (inhale). When the muscles relax and the elastic lungs recoil, pressure rises and air is pushed back out (exhale). Nothing pulls the air; pressure differences do all the work. At rest you cycle about 12–15 times a minute, moving roughly half a liter (the tidal volume) each breath.
2. The trade happens in the alveoli. Air travels down the windpipe and branching airways to about 300 million tiny sacs called alveoli. Unfolded, their combined surface would cover roughly a tennis court — and each sac is wrapped in a mesh of capillaries so thin that red blood cells pass single-file. Huge surface area plus a wall only about half a micrometre thick is exactly what makes gas exchange fast.
3. Gases move by diffusion — downhill in partial pressure. Each gas has its own pressure, measured in millimetres of mercury (mmHg). Fresh air in the alveolus sits at about pO₂ 100 and pCO₂ 40. The venous blood arriving from your body is the reverse: about pO₂ 40 and pCO₂ 46. Molecules always drift from higher to lower partial pressure, so O₂ diffuses into the blood and CO₂ diffuses out — automatically, no energy required — until the leaving (arterial) blood is nearly in balance with the air at ~pO₂ 100 / pCO₂ 40. Those are the numbers the meters in the animation track.
4. Oxygen rides on hemoglobin — and the curve is S-shaped. Inside each red blood cell, iron-rich hemoglobin grabs the incoming O₂. Oxygen-poor blood is dark and bluish; once loaded it turns bright red — the colour shift you see the cells make as they cross the exchange zone. How full the hemoglobin is (the O₂ saturation, SaO₂) follows a steep oxyhemoglobin dissociation curve: at the arterial pO₂ of ~100 mmHg it is about 98% saturated, yet at the venous pO₂ of ~40 mmHg it is still ~75%. That flat top is a safety margin, and the steep middle is what lets tissues pull off O₂ easily.
5. Exercise turns everything up. Working muscles burn more O₂ and pour out more CO₂. Your brain answers by breathing faster and deeper (minute ventilation can climb from ~7 L/min to well over 60 L/min) and your heart pumps more blood past the alveoli. Because ventilation and blood flow both rise together, healthy arterial pO₂ and saturation stay near-normal even during hard effort — what changes most is the venous blood returning, which comes back far more O₂-depleted (and bluer). Press Exercise to watch the rate, depth, and blood flow all increase.
6. Why you can't hold your breath for long. During a breath-hold no fresh air enters, but your blood keeps unloading CO₂ into the alveoli and pulling O₂ out. Alveolar and arterial pO₂ slowly fall while pCO₂ climbs. Surprisingly, it is the rising CO₂ (sensed as rising acidity), not the falling O₂, that triggers the overwhelming urge to breathe. Hit Hold breath to see the meters drift.
7. Why smoking and scarring hurt exchange. The whole system depends on thin wall + big surface. Smoking (see COPD and asthma) destroys and inflames the delicate walls, merging many small sacs into fewer big floppy ones — less surface, less exchange. Scarring from fibrosis (see idiopathic pulmonary fibrosis) thickens that paper-thin wall, so O₂ has farther to travel and less of it makes it across with each breath. The result of either is the same: breathlessness and low blood oxygen even when air is plentiful.