Postural syncope occurs when the body cannot supply enough blood to the brain in the upright position because of low blood pressure.直立时因血压过低导致大脑供血不足时会出现直立性晕厥。The blow all but knocked him unconscious.这一击几乎把他打得晕厥过去。It was a very hot day, and I suddenly felt faint and sick.那天很热，我忽然感觉晕厥不适。