Digestive System

The digestive system refers to the network of organs that breaks down food, absorbs nutrients and water, and eliminates waste. It comprises the gastrointestinal tract (mouth, esophagus, stomach, small intestine, and large intestine, ending at the anus) and the accessory organs (salivary glands, liver, gallbladder, and pancreas), which contribute enzymes, bile, and hormones. Through mechanical, chemical, enzymatic, and microbial action, the system converts food into components absorbed into the blood and lymph to support energy, growth, repair, immune function, and physiological regulation, while eliminating undigested waste.

The digestive system also houses the enteric nervous system (often called the “second brain”) and the gut microbiota, and communicates bidirectionally with the central nervous system via the vagus nerve and the microbiota-gut-brain axis. This makes it a central substrate for nutritional psychology: psychological states such as stress, emotion, and sensory experience modulate digestive function, and digestive signals in turn influence mood, cognition, appetite, and behavior.