Final answer:
CCK is a hormone secreted in the duodenum in response to the presence of fats and proteins, targeting the pancreas and gallbladder to release digestive juices and bile, aiding in the digestion of substrates.
Step-by-step explanation:
The sources, substrates, and actions of cholecystokinin (CCK), a hormone integral to food digestion, are vital to understand its role in the gastrointestinal system. CCK is secreted by specialized epithelial cells called endocrinocytes located in the mucosal epithelium of the small intestine, particularly the duodenum. The presence of proteins and fats in food acts as the stimulus for the secretion of CCK. Once released, CCK targets organs such as the pancreas and the gallbladder, triggering a series of digestion-aiding responses.
The site of secretion of CCK is the duodenum. The main target organs and the subsequent effects of CCK include the pancreas, which is stimulated to release enzyme-rich pancreatic juices, and the gallbladder, which contracts to release bile into the duodenum. The bile salts from the gallbladder help to emulsify fats, thus aiding in their digestion. Moreover, CCK also stimulates the pancreatic secretion of lipase, an enzyme essential for the breakdown of fats.
The actions of CCK are crucial for the regulation of digestive secretions and the proper breakdown of substrates such as fats and proteins. It also plays a role in slowing gastric emptying, coordinating the activities of the digestive tract to ensure efficient nutrient absorption.