Rashi on Leviticus 17:14/רש"י על ויקרא י״ז:י״ד

1דמו בנפשו הוא means its blood represents its life (במקום הנפש), in as much as its life is depending on it.

2כי נפש כל בשר דמו הוא means the life is (is identical with) the blood; דם and בשר are masculine, נפש is feminine.