"Her blood boiled when she got her test results back" is a sentence that uses a metaphor to express anger. A metaphor makes a comparison between two objects that seem to differ significantly but also have common traits. Boiling blood is a common metaphor for anger.
Metaphors work because they access figurative meanings in words rather than relying on the literal ones. Obviously, having one's blood literally boil would lead to a fairly rapid death. However, it expresses the way that anger makes one's heart pound and one's entire body fill with an energy that is difficult to control once wrath takes over.