Cyanide poisoning kills in minutes despite normal blood oxygen. Cyanide blocks complex IV — electrons can't reach oxygen, the whole chain jams, proton pumping stops, ATP synthesis collapses. Blood stays oxygenated (arterial and venous both bright red) but cells can't use it — histotoxic hypoxia: oxygen present, unusable. Antidotes (nitrites, hydroxocobalamin) pull cyanide off the enzyme. The lesson: breathing delivers oxygen, but the ETC spends it — poison the spender and oxygen becomes decoration.