Plain English Meaning
Not enough blood is reaching part of the body, so that part isn't getting the oxygen it needs — even if the blood itself has plenty of oxygen in it.
Full Definition
A reduction or stoppage of blood flow to a tissue or organ, resulting in an inadequate supply of oxygen and nutrients to that area. In aviation medicine, ischemia is associated with stagnant hypoxia, where blood — though normally oxygenated — fails to circulate properly to the tissues that need it.
Context Anchor
Seen in discussions of stagnant hypoxia, where oxygen may be in the blood but poor blood flow keeps enough of it from reaching the tissues.
Derivation
From Greek 'ischein' (to hold back) and 'haima' (blood) — literally 'holding back of blood.' The origin captures the core idea: blood flow is restricted or held back from reaching the tissue.
Why Pilots Care
In stagnant hypoxia, ischemia reduces oxygen delivery to the brain and muscles, which can quickly degrade alertness and .