The stomach, located in the upper abdomen from the left hypochondriac to umbilical regions, connects to the esophagus at the cardia and continues through the fundus, body, antrum, and pylorus. The outflow of digested food is regulated by the pyloric sphincter. The stomach has lesser and greater curvatures, with mucosal folds along the greater curvature to aid digestion. It serves as a temporary storage site, initiates protein digestion using gastric acid, kills bacteria, and mixes food through peristalsis before gradually releasing it into the duodenum. The duodenum is the first 25 cm of the small intestine, C-shaped around the pancreas, and divided into four parts: bulb, descending, horizontal, and ascending. It is retroperitoneal and receives bile and pancreatic juice via the major duodenal papilla, which are essential for digesting proteins, fats, and carbohydrates. The stomach and duodenum walls share four layers: mucosa (for digestion and absorption), submucosa (containing blood vessels and nerves), muscularis (for contraction), and either serosa or adventitia depending on their location. Hormones play key roles: gastrin from the stomach stimulates gastric acid and enzyme release, while secretin and cholecystokinin (CCK) from the duodenum regulate pancreatic and gallbladder secretions to neutralize acid and aid digestion.