I believe the answer is carbon dioxide. Please let me know if i can help with anything else
Sensory receptors?? I believe that is the answer but you may have wanted a more in depth explanation, if so, my bad.
A. Vili in stomach. And this is because of gluten.
Answer - Hypothalamus
Reasoning - In that area which is the hypothalamus it has concentrated nerve areas where its function can be (autonomic) such as temp, homeostatic, and basic remembrance. Just like what your reading now your automatically recalling what your reading.