Your stomach contains a powerful mix of acid, enzymes, mucus, salts, hormones, gas, and whatever food or drink you’ve recently consumed. Even when it’s completely empty, it still holds about 2.5 ounces of gastric juice, a clear fluid that’s almost as acidic as battery acid. It’s a surprisingly complex environment designed to break food into a form your body can absorb.
Gastric Juice: The Baseline Fluid
Whether or not you’ve eaten recently, your stomach produces gastric juice continuously. About 99% of this fluid is water. The remaining 1% is what makes it remarkable: hydrochloric acid (0.4% to 0.5%), digestive enzymes, a thick layer of protective mucus, and dissolved salts of calcium, sodium, and potassium.
The hydrochloric acid is the dominant player. On an empty stomach, the pH sits between 1.0 and 2.5, which is acidic enough to dissolve a thin piece of metal over time. After a meal, the pH rises to somewhere between 3 and 7, depending on what you ate and how much. A large, protein-heavy meal buffers more of the acid than a light snack would.
That acid serves two jobs. It breaks apart the chemical bonds in food, and it kills most bacteria and other pathogens that ride in with whatever you swallow. The stomach lining protects itself from this acid with a thick coating of mucus and a glycoprotein called mucin. When that barrier fails, you get ulcers.
Enzymes That Break Down Food
Your stomach produces two main digestive enzymes. Pepsin handles protein. It chops long protein chains into shorter fragments so your small intestine can finish the job. Pepsin only works in a highly acidic environment, which is why the stomach keeps the pH so low.
Gastric lipase handles some fat digestion. It starts breaking down triglycerides (the main type of fat in food) into smaller molecules, though most fat digestion happens later, in the small intestine, with help from bile and pancreatic enzymes. The stomach’s contribution is relatively small but gets the process started, especially for the fats in dairy products.
Hormones and Signaling Molecules
Your stomach isn’t just a bag for digestion. It’s also an endocrine organ that produces hormones affecting your whole body. The most well-known is ghrelin, often called the “hunger hormone.” When your stomach is empty, ghrelin levels rise and signal your brain that it’s time to eat. Once you eat, ghrelin drops.
Ghrelin does more than trigger hunger. It helps move food through the digestive tract, influences insulin release, and prompts your pancreas to release glucagon, a hormone that raises blood sugar. So even between meals, your stomach is actively communicating with your brain, pancreas, and intestines.
The stomach lining also produces a protein called intrinsic factor, which binds to vitamin B12 in your food and carries it to the small intestine for absorption. Without intrinsic factor, your body can’t absorb B12 properly, which leads to a specific type of anemia and nerve damage over time. This is why people who’ve had stomach surgery or certain autoimmune conditions often need B12 injections.
Gas in the Stomach
There’s always some gas in your stomach, and most of it comes from swallowed air. Every time you eat, drink, chew gum, or even swallow saliva, small amounts of nitrogen and oxygen enter the stomach. Eating or drinking quickly brings in more. This gas is odorless and accounts for most stomach gas overall.
The gas mixture in the digestive tract is primarily carbon dioxide, oxygen, nitrogen, hydrogen, and sometimes methane. The stomach’s gas is mostly swallowed nitrogen and oxygen, while the carbon dioxide, hydrogen, and methane tend to be produced lower in the gut, where bacteria in the large intestine ferment undigested carbohydrates. A burp is your stomach releasing that swallowed air back up before it moves further along.
What Happens to Food Inside
When food arrives in the stomach, it’s still in roughly the form you chewed it into. Over the next two to five hours, the stomach’s muscular walls contract in rhythmic waves, churning the food and mixing it with gastric juice. The combination of acid, enzymes, and physical grinding transforms solid food into a semifluid paste called chyme.
Chyme is highly acidic, with a pH around 2, and looks nothing like the food you ate. It has the consistency of a thick soup. The stomach releases chyme into the small intestine in small, controlled squirts through a muscular valve called the pyloric sphincter. It doesn’t dump everything at once because the small intestine can only handle so much acidity and volume at a time. Once chyme enters the small intestine, bicarbonate from the pancreas neutralizes the acid so intestinal enzymes can work.
The rate at which your stomach empties depends on what you ate. Liquids pass through fastest. Simple carbohydrates move quicker than proteins, and fats take the longest. A fatty meal can keep the stomach working for four hours or more, which is why high-fat foods feel “heavy” and keep you full longer.
Stomach Size and Capacity
An empty adult stomach holds about 2.5 ounces and is roughly the size of a fist. But the organ is remarkably stretchy. It can expand to hold about 1 quart of food and liquid, roughly 16 times its resting volume. Stretch receptors in the stomach wall signal your brain as it fills, which is one of the mechanisms behind feeling full.
Your stomach doesn’t permanently stretch from large meals, despite the common belief. It returns to its resting size once it empties. However, people who consistently eat very large portions may become less sensitive to those stretch signals over time, making it harder to feel satisfied with a normal-sized meal.