The Stomach · StomachLab

Where is the stomach

The human digestive system is made up of the digestive tract and the digestive glands. From top to bottom the tract runs through the mouth, pharynx, esophagus, stomach, small intestine, large intestine and anus; the glands are the salivary glands, gastric glands, liver, pancreas and intestinal glands.

The stomach is the widest part of the digestive tract. It sits in the upper left of the abdomen, right under the diaphragm, mostly hidden behind the left ribs. Above, it joins the esophagus; below, it leads into the first part of the small intestine, the duodenum.

Click “Follow a bite” to trace a mouthful of food through the whole tract. Drag to rotate, scroll to zoom, and hover over an organ to see its name.

Parts of the stomach

The stomach is like a curved bag. Its entrance is the cardia, which joins the esophagus; its exit is the pylorus, which opens into the duodenum.

The part bulging up and to the left above the level of the cardia is the fundus, the large middle part is the body, and the section near the pylorus is the pyloric part. The short, concave upper right edge is the lesser curvature; the long, convex lower left edge is the greater curvature. Blood vessels supplying the stomach run along both curvatures.

Drag “Fullness”: when empty the stomach shrinks to only about 50 mL and its lining has many folds (rugae); after a big meal it can stretch to about 1.5 L and the folds are smoothed out. Cut open the front wall to see them.

Layers of the stomach wall

Drag “Peel” and the stomach wall opens up layer by layer like steps. From inside to outside:

Mucosa: the innermost lining. Its surface is a layer of epithelial tissue that protects and secretes, and it holds many gastric glands that produce gastric juice.Submucosa: mainly connective tissue with blood vessels and nerves; it connects, supports and nourishes.Muscle layer: the thickest layer, made of smooth muscle (muscle tissue) in oblique, circular and longitudinal sheets that churn and push the food. Between them lies nervous tissue, which receives stimuli, conducts impulses and controls the stomach's movements.Serosa: a smooth thin outer membrane that reduces friction with the surrounding organs.

Click a layer to see its microscopic structure. The stomach is made of epithelial, connective, muscle and nervous tissue joined in a set order and carrying out digestion together, so it is an organ; together with the other digestive organs it forms the digestive system.

Peristalsis and emptying

Food stays in the stomach for several hours. The muscle layer contracts rhythmically, forming rings of contraction that travel from the middle of the body toward the pylorus about 3 times a minute. This is peristalsis.

Peristalsis mixes the food thoroughly with gastric juice and grinds it into a soupy chyme. When a ring reaches the pylorus, only very small particles pass into the duodenum; larger pieces are squeezed back into the stomach to be ground further. So the stomach can store food for a while and release it into the small intestine a little at a time.

Compare different foods: thin porridge empties fastest, rice and lean meat take longer, and fried food takes longest. An ordinary mixed meal needs 4–6 hours to leave the stomach.

The front wall is cut open here; the particles and the pyloric opening are enlarged and time runs faster.

Gastric juice and protein

This is an enlarged model of a small piece of stomach lining. The tiny pits covering the top are gastric pits, and gastric glands open into each one. The glands have two main kinds of cells: parietal cells (pink) secrete hydrochloric acid, and chief cells (purple-blue) secrete pepsinogen, which becomes active pepsin in the acid. An adult secretes about 1.5–2.5 L of gastric juice a day.

Pepsin (teal) cuts long protein chains into shorter polypeptides. This is the initial digestion of protein; the polypeptides are only fully broken down into amino acids in the small intestine. The stomach hardly digests starch or fat.

Adjust the pH and temperature: pepsin works best in strong acid at about pH 2 and near body temperature; if the acid weakens or it gets too hot, it stops working or even denatures. The mucus layer on the lining holds back the acid and enzymes, so the stomach does not digest itself. The stomach can also absorb a little water and alcohol.

In-class testing