The stomach is a muscular sac. It's about the size of a small melon that expands when you eat or drink. It can hold as much as a gallon (about 4 liters) of food or liquid. Once the stomach grinds the food, strong muscle contractions called peristaltic waves push the food toward the pyloric valve. The pyloric valve leads to the upper part of the small intestine, called the duodenum.
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