Note that the urge to breathe is triggered by rising carbon dioxide levels in the blood and not by the reduction of oxygen. The body can actually detect low levels of oxygen but this is not normally perceptible prior to blackout. Persistently elevated levels of carbon dioxide in the blood, hypercapnia (the opposite to hypocapnia), tend to desensitise the body to carbon dioxide, in which case the body may come to rely on the oxygen level in the blood to maintain respiratory drive. This is illustrated in the scenario of type II respiratory failure. However, in a normal healthy person there is no subjective awareness of low oxygen levels.