The human stomach is a muscular bag located in the upper middle of the abdomen. When empty, the stomach flattens out, leaving a small air bubble in the upper portion, and with the walls of the main body and lower portion of the stomach more or less touching one another. The lining is thrown into folds that disappear as the stomach distends with food.
Ingested food and liquids pass down the esophagus to enter the stomach. The esophagus has a sphincter muscle at its junction with the stomach that can contract to slow or control the entry of food into the stomach. There are no glands controlling this process, however there are pressure sensors involved.
Gastric (stomach) emptying is a complex process and varies according to the nature of the ingested food.
In the case of ingestion of a large quantity of water, there is no need for the water to remain in the stomach for processing, and it will pass rapidly through the stomach into the duodenum and small intestines.
A Big Mack, on the other hand, is handled differently from the water. Pressure sensors will promote relaxation and expansion of the stomach and delay emptying. Glands in the duodenum and small intestine will relay information back to the stomach to modulate the stomach’s contraction and emptying. Like the esophagus, the stomach has a sphincter muscle at its outlet that helps control the rate of gastric emptying. The food will remain in the stomach longer than the water in order for the gastric pepsin and acid to begin digesting the food.
The first reference refers to the dog’s stomach, but is applicable to humans. The second gives expanded information on human gastric emptying.