Acute cholecystitis, or inflammation of the gallbladder, usually comes about because of a gallstone being lodged in the cystic duct. The cystic duct is the one that leaves the gallbladder and connects to the common bile duct.
So let’s say this person’s gallbladder’s got a few gallstones in it, and they go to eat a hamburger, the small intestine secretes cholecystokinin, sometimes shortened to CCK, into the blood where it makes it’s way to the gallbladder, and signals it to squeeze out some bile to give it a hand with digestion of that hamburger. The gallbladder contracts and one of these stones gets lodged right in the cystic duct, which blocks bile flow...now what? Well this person probably start experiencing some pain, specifically midepigastric pain, which happens because the gallbladder’s trying to squeeze on a blocked duct...and just like if you squeezed a partly filled balloon with the end blocked off, it physically stretches out and irritates the nerves in the gallbladder and duct. This can also lead to nausea and vomiting, which can last for long periods of time. And as the gallbladder squeezes more and more, the stone might get even more stuck, and at this point the bile, being stuck in the same place, or in a state of stasis, becomes a kind of chemical irritant, and causes the mucosa in the walls to start secreting mucus and inflammatory enzymes, which results in some inflammation, distention and pressure buildup.
At this point, there might also start to be some bacterial growth, most commonly E coli which is all over the gut, but also Enterococci, Bacterioides fragilis, and Clostridium, which can also be found there. As it sort of balloons up, the pain might start to shift to the right upper quadrant, and it’ll be this kind of dull, achy pain that can even radiate up to the right scapula and shoulders. After a while, bacteria starts invading into the gallbladder wall and eventually through the wall, causing peritonitis, inflammation of the peritoneum, which can cause what’s called rebound tenderness, where pain is brought on when pressure is actually taken off the belly rather than when it’s applied.