Oxygen in the traced cell to spare 100% 50% 00.250.50.75 s

Resting: each red cell spends about three-quarters of a second beside the air sacs, and is full within the first third of it.

Pick an effort · watch the time to spare shrink
Oxygen-rich bloodOxygen in the airAir sac wallEvery breath refills hundreds of millions of tiny air sacs, packed like foam, each wrapped in a net of capillaries so narrow that red cells pass in single file. Oxygen crosses a wall far thinner than a sheet of paper into each passing cell, and carbon dioxide crosses the other way.At rest a cell spends about 0.75 s beside the air and is fully loaded within the first quarter-second, so most of its trip is time to spare. All out, the heart pumps nearly five times as much blood; more capillaries open, but each cell still passes in about 0.29 s, and it arrives emptier (31% instead of 75%). In healthy lungs it still leaves about 97% full. The lungs are rarely what limits VO₂max; the heart's output usually is.Typical healthy adult at sea level, drawn far larger than life: an air sac is about a quarter of a millimetre across. One capillary stands for the several a cell passes. Lung disease, altitude and some very fit athletes at their peak can leave blood short of full; not shown. Blue and red follow the textbook convention.