It is written in Durr-ul-mukhtar:
Vomiting a mouthful of bile, black blood clots, food, or water breaks wudu’. [Their coming out of the mouth breaks wudu’.] (The section dealing with wudu’)
What breaks wudu’ is filthy substance’s coming out of the body, not filthy substance itself. Passing gas breaks wudu’not because gas is considered impure (najs) but because it exits from the place of impurity (najasat). (Ibn Abidin)
It is understood from these statements that blood and other impure things do not break wudu’ unless they come out of the body. The wudu’ of a person with hemorrhoids bleeding will not be nullified unless blood oozes out. To decide whether one’s wudu’ has been nullified or not, the criterion to be used is whether impurities (najasat) have come out of the body.
It is written in Durar wa’l Ghurar:
Wudu’ is nullified if blood or pus predominates over saliva or equals it. It is not nullified if saliva predominates. (The section dealing with the nullifiers of wudu’)
[What is meant by these statements is blood’s coming out of the mouth. No fiqhbook says that swallowing it breaks wudu’.]
Swallowing the blood in the mouth breaks neither wudu’ nor salat. It is written in al-Fatawa al-Hindiyya:
If one, while performing salat, swallows the blood in between one’s teeth, it does not break one’s wudu’, even if the blood predominates over the saliva. While performing salat, if one, deliberately or absent-mindedly, eats or drinks something, one’s salat becomes nullified. However, if one who is offering salat swallows the blood oozing from in between one’s teeth, one’s salat is not nullified if the blood is not a mouthful. The same is written in the books Fatawa al-Kadihan and Khulasa. (The section dealing with the nullifiers of wudu’)
If the blood coming out of the mouth equals saliva or predominates over it, wudu’ is nullified. (Halabi as-saghir, Ni’mat-i Islam) [What is mentioned here is blood’s coming out of the mouth, not its accumulation in the mouth.]
What breaks wudu’ is not the accumulation of blood in the mouth but its coming out of the mouth. As the inside of the mouth, in terms of wudu’, is deemed an inner limb, bleeding from gums will not break wudu’unless blood comes out of the mouth. It does not break wudu’ either even when one swallows the blood.