Someone has hypotension when his blood pressure drops down below normal levels, which is 120 - 70mm for the sistolic and diastolic respectively. Symptoms are somnolence (sleepiness), fatigue, and headache, but if the blood pressure is too low, one feels dizzy, cold, with the vision getting blurry or cloudy and then faints, falling into a state of unconsciousness. If someone passes out due to hypotension, have him lie supine (on his back) and lift his legs at 45º; when he wakes up, have him drink coca cola or coffee until the ambulance arrives. Extreme hypotension is caused by traumatism, in which case it is called hypovolemic shock, which is an emergency because the patient's heart might stop beating.
Hypotension is caused by anemia, sudden drop in sugar levels (as in diabetes), malnutrition, dehydration (due to excessive transpiration, as in high fever, and diarrhea), hemorrhage (bleeding), and a hypovolemic shock etc.