This is where the gums constantly bleed and sometimes blood goes down the throat, will this break the fast? If one spits the blood out, it will not affect the fast. However, if one swallows it and the blood was more than the saliva or equal to it, the fast will break. If blood was only minimal, the fast will not break, unless he felt the taste of the blood when he swallowed it.

لو خرج الدم من بين اسنانه ودخل حلقه يفسد لم يصل الى جوفه.
أما اذا وصل فان غلب الدم او تساويا فسد والا لا إلا اذا وجد طعمه

شامى ص 367 / 3 احكام ص 234

Extracted from A Gift for Ramadhan, Page 59.

A Gift For Ramadhan – New 3rd Revised Edition