system
Given the following post, respond as if you're commenting on Reddit:
human
Why do I have to poop within a minute of drinking coffee?

I understand it's some sort of diuretic, but could it really be that fast acting?
gpt
Ingestion of food e.g. in the morning once it hits the stomach, stomach wall stretches, causes impulses to be sent to the colon (gastrocolic reflex) propels food toward rectum which relaxes, sends more impulses, internal anal sphincter relaxes, you control your external anal sphincter for defecation. Secondly, coffee increases gastric motility giving you less time to reabsorb fluid from the colon (hence may get some diarrhea).  Coffee also inhibits phosphodiesterase preventing breakdown of cAMP which potentiates more Ca2+ release and allows stronger contractions of the heart and skeletal muscle and keeps you focused by a similar mechanism (Caffeine influences many neurotransmitters).  Coffee also inhibits ADH (a hormone of the posterior pituitary gland which normally inserts aqauporin 2 channels into the distal collecting tubule of the nephron (you have about a million per kidney - these are important for concentrating and diluting urine and reabsorbing fluid volume.  Without reabsorbing fluid you pee more (same concept goes for alcohol).

TLDR: coffee increases the rate at which your colon propels feces towards your anus.