Fig 15AlveolusHover ↔ exercise · click/F fibrosisrest · 0.75 s · out 100 mmHg, 97%
Red cells arriving, ~75% saturatedLeaving, ~97%Alveolar wall in section; collagenMonitor: capillary PO₂ vs timeOne human alveolus at true scale (ruler: 1 tick = 10 µm), about 200 µm across, cut at its equator; the 0.6 µm blood–air barrier is drawn 3× thick. A capillary runs inside the wall; in life a dense net covers most of the outer surface, as on the hull here. Red cells, 7.5 µm across and one per ~12 µm of capillary, enter at a PO₂ of 40 mmHg and meet alveolar air at 100 mmHg. Oxygen diffuses across the barrier and binds haemoglobin; capillary PO₂ reaches the alveolar value in about 0.25 s, a third of the 0.75 s a cell spends here at rest, so even at exercise (0.25 s) the blood leaves full. Thicken the barrier (fibrosis, ~3 µm) and diffusion slows about fivefold: still fine at rest, but at exercise cells leave before they are loaded. Flow is shown 4× slower; the capillary path stands for the whole transit, which in life crosses two or three alveoli.