The primary substance in the human digestive system is food, which is broken down into nutrients that the body can absorb. The digestive system includes various organs such as the stomach, intestines, liver, and pancreas, which work together to process food. Enzymes and acids in the stomach begin the breakdown of food, while the small intestine absorbs essential nutrients like carbohydrates, proteins, and fats. The indigestible parts then pass to the large intestine, where water is reabsorbed and waste is formed. This complex process is essential for providing the energy and building blocks necessary for the body's growth and maintenance.