Video narration
If your breath has four basic volumes, why four capacities? TV, tidal volume, is about 500 milliliters in an ordinary breath. IRV, inspiratory reserve volume, is extra air you can inhale: about 3,000. ERV, expiratory reserve volume, is extra air you can exhale: about 1,100. RV, residual volume, is air left in your lungs afterward: about 1,200. Each volume is one amount; a capacity combines selected amounts from that same breath. Capacities aren't extra air amounts. They're different sums of the same four volumes. IC, inspiratory capacity: TV plus IRV, about 3,500 milliliters. FRC, functional residual capacity: ERV plus RV, about 2,300. VC, vital capacity: IRV plus TV plus ERV, about 4,600. TLC, total lung capacity: IRV plus TV plus ERV plus RV, about 5,800. These are approximate adult examples; values vary. So why four? Because each adds different parts of your breath: some count air you can move, others include what remains.