Which structure is not part of the digestive system?

The kidneys are not part of the digestive system; rather, they are integral components of the urinary system. Their primary functions include filtering blood to remove waste products and excess substances, which are excreted as urine. In contrast, the mouth, stomach, and large intestines are all critical structures within the digestive system, where food intake, digestion, absorption of nutrients, and elimination of waste occur. The mouth begins the digestive process through mechanical breakdown and enzymatic action, the stomach further breaks down food and begins the digestion of proteins, while the large intestines absorb water and electrolytes, ultimately preparing waste for excretion. Therefore, the kidneys do not contribute to the processes involved in digestion.