Food leaves your stomach soaked in acid. S cells sense it and send out secretin, which calls for a juice that calms the acid down.
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S cells live in the lining of the first part of your small intestine. That's right where food comes out of the stomach.
An S cell is tall and thin. Its top senses what is passing by. Its bottom holds tiny bubbles of a hormone called secretin.
Secretin was the first hormone scientists ever discovered, in 1902. Soon after, they made up the word hormone!
Stomach acid is great in the stomach, but the small intestine needs things calmer. When S cells sense acid, they send secretin into the blood.
Secretin tells the pancreas to send out a watery juice full of bicarbonate, a bit like baking soda. It calms the acid so the next helpers can do their job.
Scroll up to the 3D model and press Send secretin to watch the hormone go into the blood.