In "21 Grams", what organ does Paul need?

In “21 Grams,” Paul needs a heart transplant because his failing heart is expected to give him less than a month to live.

Paul Rivers, played by Sean Penn, is a mathematics professor suffering from severe heart disease. His wife Mary hopes they can still have a child, but Paul’s immediate medical crisis is finding a suitable organ donor. The film’s intertwined storylines connect Paul to Cristina Peck after her husband and children are killed in a hit-and-run accident.

Cristina’s husband becomes the organ donor whose heart is transplanted into Paul. That literal connection intensifies the film’s themes of grief, guilt, identity, and the idea that one person’s life can continue inside another. The transplant initially gives Paul a chance to recover, but his body later begins rejecting the new heart.

A common mix-up is the film’s title: “21 Grams” refers to a disputed early-20th-century experiment about the supposed weight of the soul, not the weight of a human heart or the amount of tissue Paul needs.