The stomach is where the true work of digestion begins – everything up until now has been more or less preparation. Assuming you thoroughly chewed what you ate, the stomach receives a well-mashed bolus of food mixed with saliva to start working on.

Anatomically, the stomach is a muscular sac that collapses when it’s empty and is divided into three portions: the fundus or upper portion, the body or middle portion and the antrum or lower portion.

Some of the true marvels of the human body are demonstrated in the stomach. It secretes a very strong acid with a pH of only 0.8 (hydrochloric acid or HCl); yet at the same time, it produces a thick, alkaline mucus to protect its lining. In addition, this part of the digestive process requires constant communication between the stomach, the brain, and eventually the first part of the small intestine, along with a cascade of reactions.

When food first hits the stomach from the esophagus, signals are sent back to the brain, which in turn sends signals back to the stomach, telling it to relax so it can expand and receive the incoming food. The stomach can expand to hold up to about 1.5 liters. After it begins receiving food, the muscles of the stomach also begin their churning action to mix the food with the digestive juices that are being secreted.

The arrival of food also causes gastrin, a hormone, to be secreted, which in turn triggers the release of gastric juice. Gastric juice contains both the aforementioned HCl and a substance called intrinsic factor; this factor is necessary for your body to absorb vitamin B12 through the GI tract.

The saliva that was secreted in the mouth continues working on the starch in your food for about an hour after it enters the stomach, breaking down up to 30-40% of it. Lingual lipase (a fat-digesting enzyme) from the saliva also continues to work on fats, digesting a small amount of the triglycerides in your food.

What the stomach really acts on, though, is protein. It achieves this through both acid and pepsin, a proteolytic enzyme. Actually, the stomach produces pepsinogen—a proenzyme or zymogen—an inactive form that must be converted to pepsin to start digesting. And it’s acid in the stomach that promotes conversion from the inactive pepsinogen to the active pepsin. Pepsin—as most enzymes—is pH sensitive and is active only within a fairly narrow range. It works best in a range of 1.8 to 3.5, and once the pH gets above 5, pepsin is virtually inactive.

Meanwhile, the muscular contractions—or constrictor waves—continue in the stomach, both mixing and gradually moving food from one end of the stomach to the other, preparing it for entry into the small intestine. These waves serve to blend, mix and mash the food with gastric juice and digestive enzymes, creating a semi-fluid mixture called chyme.

As with the esophageal sphincter at the top end, the bottom of the stomach is bordered by the pyloric sphincter, another tightly controlling band of muscle. The mixing waves intensify as the chyme moves closer and closer to this opening.

Conversely, the closer the chyme gets, the more the pyloric sphincter tightens, creating even more of a resistance to the passage of chyme into the small intestine. This tightening tends to result in a backsplash effect, causing the chyme to become even more thoroughly mixed as it approaches the opening. Chyme must be in an almost liquid state before it is allowed to pass through the pyloric sphincter.

Other than protein digestion and the remaining action of the salivary enzymes, very little actual digestion takes place in the stomach. A small amount of gastric lipase is produced for fat digestion, but its action is limited to only certain types of fat, like butterfat. Both digestion and absorption are primarily functions of the small intestine. Only a few compounds—such as alcohol, aspirin and some other NSAIDs (non-steroidal anti-inflammatories)—are absorbed in the stomach.

The entire mixing process in the stomach can take several hours, but usually within 2 to 4 hours, the food—now chyme—has completely left the stomach and entered the small intestine.