Fainting. Simple maneuvers could keep you from losing consciousness if you're feeling faint.
D001794:Blood Pressure; D006801:Humans; D007537:Isometric Contraction; D007866:Leg; D009119:Muscle Contraction / Q000502:physiology; D013575:Syncope / Q000517:prevention & control* / Q000523:psychology; D014474:Unconsciousness / Q000517:prevention & control