expression, ichidan verb
have a nosebleed; bleed from the nose
Common everyday expression. The subject is the person, and the nosebleed is described as coming out.
乾燥すると鼻血が出やすくなる。
When it's dry, I tend to get nosebleeds easily.
さっきから鼻血が出ていて止まらない。
My nose has been bleeding for a while and it won't stop.
鼻血 is the noun 'nosebleed', while 鼻血が出る is the full phrase meaning 'to have a nosebleed'.
Compound of 鼻血 (nosebleed) + が (subject marker) + 出る (to come out). The reading はなぢ reflects rendaku (sequential voicing) on 血.