فإنه دم عرق انفجر
For it is blood from a ruptured vessel.
In the account of istihadah, the nature of bleeding is distinguished from menstruation. Not every appearance of blood carries the same consequences for prayer and purification.
The causal issue is whether the operative attribute is simply visible blood or the legally relevant kind and source of the bleeding. Extending a rule to wounds or nosebleeds also requires attention to separate evidence and school-specific principles.