When you are nervous or scared, your body enters fight-or-flight mode, controlled by the autonomic nervous system. Adrenaline levels rise, your heart starts beating faster, palms get sweaty, and blood flow and muscle tone redistribute to prepare you for action. Your bladder and intestines are controlled by smooth muscles and sphincters.

Stress hormones can make the bladder muscles more irritable and loosen the sphincter. Thus you may feel that your bladder is fuller than it actually is or that you are close to leaking. Similarly, anxiety can change the pattern of contractions in the intestines and sometimes speed up the movement in certain parts, causing cramps and an urgent need to pass a bowel movement.

Many researchers believe that in animals, emptying the bladder or bowels may make the body a little lighter and more agile and may also remove internal distractions so the animal can focus on fleeing or fighting. Even if this argument is not complete, the fact remains that evolution has endured cleaning up waste under intense stress for a long time.

Anxiety also changes the way you pay attention to physical sensations. Sensations from the bladder and bowels that you would normally ignore make it difficult to do so.

In people with chronic anxiety, signals travel back and forth along nerves like the vagus nerve and stress hormones affect visceral sensitivity. This is one reason why irritable bowel syndrome often flares up when a person is stressed: even without an infection, there can be more gas and more cramping.