If the fasting person detects the taste of blood in his throat

If the fasting person
detects the taste of blood in his throat, that does not affect him even if
he swallows it, but if it comes up to the mouth then he swallows it, he
breaks his fast. 

The same is said concerning
phlegm, sputum and everything that may appear in the throat. 

Shaykh Ibn ‘Uthaymeen (may
Allaah have mercy on him) said: I should point out with regard to the issue
of sputum and phlegm, because some fasting people make it too hard for
themselves, and if they notice that in the deepest part of their throats,
they try very hard to expel it, and this is a mistake, because phlegm and
sputum do not spoil a person’s fast unless they reach his mouth then he
swallows them, in which case it breaks the fast according to some scholars,
but according to others that does not break the fast either. 

But that which is in his
throat and goes down into his stomach does not break his fast, even if he
feels it, so he should not tire himself out by trying to expel that which is
in his throat of these things. End quote from Fataawa al-Shaykh Ibn
‘Uthaymeen (19/356).