She has more blood than a normal body would. Her cardiovascular system in general is presumably beyond the bounds of what occurs in ordinary humans.
She grew that herself, rather than having it grafted onto an already complete body, so she adjusted to it. It's similar to how people who live at high altitudes can easily handle conditions that would cause others to pass out.
Consider two humans in a sci-fi scenario: One grew up on the moon, and the other grew up on a planet with three times more mass than Earth. The heavy one will be very stocky with thick bones, strong muscles, and a powerful cardiovascular system. The moon one will have a light body suited to flying around in low gravity. If these groups weren't aware of each other's existence, the moon people would believe the heavy people's physical traits to be impossible for humans.