The poem by the Venezuelan writer that Paul (Sean Penn) quotes when he's in the restaurant with Cristina (Naomi Watts) is "La Tierra Giró para Acercarnos" by Eugenio Montejo.
During the scene where Jack Jordan tries to hang himself, the over-weight prisoner who saves him has a large tattoo of the name "Maria Eladia" on his chest. This is Alejandro González Iñárritu's wife, to whom the film is dedicated.
The title comes from the work of Dr. Duncan MacDougall of Haverhill, Massachusetts in the early twentieth century. MacDougall weighed dying subjects, in experiments of extremely dubious scientific value, believing that he was proving that the soul was material and measurable. His results in fact varied wildly.
When Paul finds the photos of Cristina and her husband in a book, the book he is looking at is "Cruising Paradise", a collection of 40 stories about solitude and loss by playwright-actor Sam Shepard.
The story in Cruising Paradise that the pictures are marking is called "The Hero is in His Kitchen". It revolves around a payphone conversation between two men, one in Tucson, Arizona and the other in Los Angeles, California. One of the last things said on the last page of the story (the one the pictures are marking) is "Don't buckle under the weight of a heavy heart!"
The Spanish-language dedication to María Eladia at the end, "pues cuando ardió la pérdida, reverdecieron sus maizales," translates literally as "for when the loss burned, their cornfields became green again."
Katrin Cartlidge originally cast as Marianne but she died shortly before filming was to start. Melissa Leo, who had previously auditioned for the role, was invited to audition again and won the part.
In the scene where Paul tells Cristina that he got her husband's heart implanted, he says that the surgical intervention took place October 11th. 10 plus 11 equals 21.