Tukuk-OSBreathing is inflation by vacuum: the diaphragm — a dome under your lungs — contracts downward, the ribcage spreads, chest pressure drops below atmospheric, and the outside air simply shoves itself in. No muscle “pulls air”. Exhaling at rest is mostly elastic recoil: your lungs are stretched-rubber springs that snap back. Rest rate is 12–20 breaths a day-and-night without a single decision, run automatically by a brainstem rhythm generator that mostly listens to one signal: CO₂ levels. Surprisingly, the urge to breathe comes from carbon dioxide waste, not oxygen shortage.
Inside, the airway tree branches some 23 times until tubes are too thin to blow air through, and gas moves by diffusion alone across the final stop: the alveoli, several hundred million tiny sacs whose folded total exchange surface is commonly estimated at 50–100 square metres — the floor area of a small flat, packed into your chest. Walls one cell thick touching capillary walls one cell thick: oxygen crosses both in under a second. The surface stays wet (otherwise the sacs collapse like wet plastic bags) and is lined with a detergent — surfactant — that keeps the water from pinching them shut; premature babies die without it, and giving it is one of medicine’s quiet miracles.
The exhaled gas is the receipt: about 16% oxygen (down from 21%), 4% CO₂ (up from 0.04%), saturated with water — the visible cloud of a cold-morning breath. The leftovers matter: rescue breathing works precisely because your waste breath still holds useful oxygen, and every litre of exhaled air is carrying away metabolised water and the spent carbon of the food you burned.