SHAFAQNA – The Grand Ayatollah Sistani answered question about nose bleed and the blood entering the throat at the time of fasting.
Question: When nose bleed happens, if a person looks up for the blood to stop, the blood may enter the throat;
a) Can this act make fasting void (Baatel)?
b) Does it make a difference the blood enters the mouth or passes the throat?
The Grand Ayatollah Sistani: If not certain that blood is entering the throat and if it enters by accident, there is no problem; despite (the fact that) small amount of eating and drinking can void the fasting.
Source: PERSIAN SHAFAQNA