Katara asks Sokka and Aang to get her medical texts. Water bending heals, as does she, and neither of them are surprised. Toph may suspect, but Toph keeps their uneasy peace and says nothing.
She learns muscle and sinew, saliva and bile, tears and vitreous humor. She traces the paths of veins through the human body, first on paper, then with brush and ink, and finally, with her mind. At first, she is too cruel for the wisp-thin capillaries, and she sports large purple bruises for days. As they fade to mottled green and yellow, she memorizes the workings of the heart. She touches the brain only when the bruises have stopped blooming on her body.
She can stop Sokka's nosebleeds with a wave of her hand, apply an unseen tourniquet to spurting arteries, even draw water from lungs. She can also wring every cell in the human body dry in less than a second.
Seventy percent of the human body is water, the old lady told her, and Katara will never forget.