In "Gulp: Adventures on the Alimentary Canal," Mary Roach discusses an unusual but serious risk associated with defecation: pulmonary embolism. This condition can occur when a blood clot, previously lodged in a major blood vessel, becomes dislodged as the individual relaxes during a bowel movement. The sudden surge of blood flow can lead to this dangerous situation, which may result in sudden death.

Roach highlights the intriguing connection between everyday bodily functions and severe medical outcomes, emphasizing how something as routine as defecation can carry unexpected risks. The phenomenon illustrates the complexities of the human body and underscores the importance of understanding the potential dangers associated with seemingly benign activities.