The human organ primarily responsible for digestion is the stomach. The stomach plays a vital role in breaking down food through mechanical and chemical processes. Once food is ingested, it is mixed with gastric juices, containing hydrochloric acid and digestive enzymes, which help to degrade proteins and churn food into a semi-liquid substance known as chyme. This process allows nutrients to be absorbed in the small intestine following digestion in the stomach. Overall, the stomach is essential for effective digestion and nutrient assimilation.