I say, and with God's success: Wudu (ablution) is invalidated by what comes out from other than the two passages: if it is impure and flows to a place that must be purified, whether in wudu or ghusl (full ablution). Flowing is a condition for what comes out from other than the two passages; because under every skin there is blood and moisture, and in what does not flow, it is apparent but not outside, unlike the two passages; because whenever it appears, it is considered to be transferred and thus is outside. The impure substances include: spilled blood, pus - the thick white substance that is not mixed with blood - and serum - the thin liquid from a wound mixed with blood - these invalidate wudu. However, mucus, tears, saliva, and sweat do not invalidate it; due to their purity. Refer to: Fath Bab al-'Inayah 1: 61, Hawaashi Multaqata 'ala al-Naqayah p. 4, and al-Ikhtiyar li Ta'leel al-Mukhtar 1: 16, and God knows best.