Learn Polish: Poszkodowany stracił dużo krwi po wypadku. - The injured party lost a lot of blood after the accident.

Poszkodowany is a substantivized participle. It is originally the passive participle of the verb poszkodować (to injure), meaning 'injured'. Here, it functions as a noun meaning 'the injured person' or 'victim'. Because it is structurally an adjective, it declines exactly like one.

If the victim were female, how would this sentence change?

Because poszkodowany is an adjective acting as a noun, it changes to match the gender of the person. For a female victim, it becomes poszkodowana, and the past tense verb must also match: Poszkodowana straciła dużo krwi po wypadku.

Why does krew change to krwi in this sentence?

The word dużo (a lot) is a quantifier, which forces the noun that follows it into the genitive case. Krew (blood) is a feminine noun with an irregular declension, making its genitive form krwi.

Why is the word for accident wypadku here instead of wypadek?

The preposition po requires the locative case when it means 'after' in a temporal sense. The noun wypadek drops its movable -e- when declined, becoming wypadku in the locative.