You should stop the procedure as soon as possible. The medical staff will be prepared to intervene, depending on the level of your blood pressure, they might give you an infusion, or some medicament.
The blood donation must immediately cease. The person must lie on their back with their legs above their heart. Also, they should drink juice to replenish electrolites.
If someone does faint, lay the person on their back and elevate their legs about 12 inches. If they are unconscious more than one minute seek medical help.
