The organ in the human body primarily involved in digestion is the stomach. Located between the esophagus and the small intestine, the stomach is responsible for breaking down food using a combination of muscular contractions and gastric juices, which include hydrochloric acid and digestive enzymes. This process transforms food into a semi-liquid mixture called chyme. The stomach's acidic environment activates enzymes that aid in protein digestion and helps to kill harmful bacteria. After the stomach, chyme moves into the small intestine, where further digestion and nutrient absorption occur, making the stomach a crucial part of the digestive system.