Fig. 1

Experimental chamber and environmental measures in the chamber during hypoxic conditioning. a The chamber could contain two rabbits caged independently. Chamber O₂ level (F_IO₂) was monitored continuously with an O₂ sensor (OS). If F_IO₂ values were > 10% or < 8.9%, then the O₂ sensor triggered a relay controller to activate a solenoid valve to admit either N₂ (S1) or air (S2), respectively, and another (S3) to exhaust chamber gas (arrows on the gas lines indicate direction of gas flow). A safety release valve (R) opened if the chamber pressure increased to 15 mmHg ≥ atmospheric pressure. Chamber gas was mixed continually by fans (F, n = 4). b Every time the F_IO₂ reached about 8.7% (upper panel) the system injected compressed air, increasing F_IO₂ up to around 9.8% (upper panel) and the pressure in the chamber near 15 mmHg (lower panel). After the initial pressure increase, the pressure fell (inset) with an initial fast exponential decay due to the exhaust valve and a second slower decay due to pressure leak through the lid of the chamber. c Temperature recordings of 48 h periods in different experiments (n = 7). No temporal pattern of temperature variation was observed in the chamber