The stomach, somewhat like a rotating cement mixer on a truck, acts on food mechanically and chemically, and also serves as a reservoir. The stomach is an elastic organ, divided into three sections: the cardiac, or the upper, portion; the fundus, or the large, rounded, central portion; and the pylorus, or the lower outlet.
As food first enters the stomach, it spreads toward the outer edge; food swallowed later moves toward the center. This enables the mass of food to maintain the proper alkaline or neutral pH that allows the salivary digestion that started in the mouth to continue. Meanwhile, the outer layers are being digested by acid gastric juices. Here and elsewhere in the digestive system, the pH (acid or alkaline quality) of the food’s surroundings works to facilitate certain digestive processes and retard others. Without this important control, digestion would not be possible.
The stomach moves in rhythmic, muscular contractions and eventually combines all of the food with the gastric juices, forming a semisolid mixture known as chyme. The gastric juice contains hydrochloric acid, which creates an acidic medium to aid in the splitting of proteins and to destroy microorganisms and protect the body from food-borne infection; mucin, which acts as a lubricant to help move food through the digestive tract, and protects the tract itself; and the enzymes pepsin and gastric lipase, which help to split protein and fat molecules, respectively.
The amount of gastric juice present in the stomach will be increased by smelling, tasting, or simply thinking about food, but secretions will be inhibited by emotions such as anger or fear or by repulsive sights or odors.
On the average, foods remain in the stomach for about 4 hours. The actual transit time, however, depends on the type of food eaten, and some foods can leave the stomach in 1/2 hour whereas other may take as long as 7 hours. Carbohydrates take the least time to be processed; proteins, slightly more; and fats require the longest digestion time – which explains their high satiety value, or feeling of “fullness.” The more food one consumes, the longer it will take to process this food in the stomach. Once the work of the stomach is finished, the chyme is flushed by peristalsis out of the stomach through a valve called the pyloric sphincter and into the duodenum, the upper section of the small intestine.