Normal pH levels for the human stomach range between 1 and 5, according to About.com. These numbers indicate high acidity, which is because the stomach produces hydrochloric acid to help it digest food. The acid is needed because many enzymes, especially those that break down proteins, work best in an acidic environment.
Some people wonder why the stomach doesn't digest itself, states HowStuffWorks. A healthy stomach doesn't digest itself because it protects itself. The stomach has an outer layer called the serosa, followed by two muscle layers that do the mechanical work of moving the food along, and then the mucosa, or inner layer. This layer is made up of epithelial cells, g-cells and parietal cells. The parietal cells actually make the hydrochloric acid, while the g-cells make a hormone called gastrin. It's the epithelial cells that protect the stomach by bathing it in a solution of alkaline bicarbonate.
The normal volume of stomach fluid is between 20 and 100 milliliters, claims MedlinePlus. Problems arise when the stomach makes too much acid, which can lead to stomach ulcers and may be the result of Zollinger-Ellison syndrome. This is a disorder in which tumors that may or may not be malignant make too gastrin. Gastrin in turn causes the stomach to make too much acid.