What is one of the main functions performed by the stomach?

The stomach plays a crucial role in mechanical processing as part of the digestive system. Its primary function in this regard involves breaking down food into smaller, more manageable pieces through muscular contractions, known as peristalsis. This mechanical processing is essential because it prepares food for further digestion and absorption in the intestines. Additionally, the stomach secretes gastric juices that aid in chemical digestion, helping to break down proteins and kill harmful bacteria that may be present in food. The combination of these mechanical and chemical processes makes the stomach integral to effective digestion.

The other options do not accurately describe the primary functions of the stomach. Gas exchange is primarily associated with the respiratory system, while waste elimination takes place mainly in the intestines and urinary system. Nutrient absorption is also primarily conducted in the intestines, where digested nutrients are absorbed into the bloodstream. Thus, the mechanical processing performed by the stomach is vital for preparing food for the overall digestive process.