A series of molecular signals mediated by hypoxia-inducible factor (HIF1) in response to lowered oxygen levels (hypoxia). Under hypoxic conditions, the oxygen-sensitive alpha-subunit of hypoxia-inducible factor (HIF)-1 dimerizes with a HIF1-beta subunit (also called ARNT or aryl-hydrocarbon-receptor nuclear translocator), translocates to the nucleus and activates transcription of genes whose products participate in responding to hypoxia.