Final answer:
Secretin levels increase in response to acid in the duodenum, as it stimulates the pancreas to produce bicarbonate to neutralize the acid, working in tandem with cholecystokinin.
Step-by-step explanation:
In response to the acid in the duodenum, the correct statement is that the blood levels of secretin increase.
Secretin is a hormone that stimulates the pancreas to produce an alkaline bicarbonate solution, which neutralizes the acidic chyme that enters the duodenum from the stomach.
When acidic chyme is present, secretin works alongside cholecystokinin (CCK), which additionally aids in stimulating the pancreas and causing the gallbladder to release bile.
It's important to note that gastrin, another hormone, increases the secretion of gastric acid and is not linked to the neutralization of acid in the duodenum.
Hence, the blood levels of gastrin would not increase in response to acid already in the duodenum.